Tire Pressure Calculation

[joecool911]

All 65 MPH from Riggins to Grangeville but very little traffic so I set the cruise and relaxed. As for tire pressure I need to tweak them a bit. Front: 4400 / 2 = 2200 / 3042 = 0.72 x 80 = 57.8 PSI Rear: 3080 / 2 = 1540 / 3042 = 0.50 x 80 = 40.4 PSI (so I'm 3 PSI off...)

can you expand on your formula for tire pressure?

[AH64ID]

Tires are rated to carry 0 lbs at 0 psi and their max weight and pressure is on the sidewall. It's a linear equation to go from 0 to max load. So if a tire is rated to carry 3042lbs at 80 psi and you have 1540 lbs on the tire then the tire is at 50.6% of its rated load. 50.6% of 80psi is 40.5, or 41 psi of air required. That is the minimum air required to carry the load, typically the rolling resistance is reduced if you increase air above that point. The ideal air pressure is as much as it can take and not effect ride/traction/wear. My rear tires only need 41 psi but I run about 45-55 when empty.

[Mopar1973Man]

AH64ID is right is very linear in pointing out the pressure for weight. But where it comes in handy is getting the right pressure for the tires from the get go. Like the owners manual might state some numbers that are close. But as you buy different tires, different sizes, different load ranges, etc each and everyone needs a bit of tuning for optimal tread wear and traction.So I'll break it down for you...(Axle weight / 2) / Tire Weight capacity x Tire Max Pressure = Inflation pressure.So say my front axle weighs 4,400 pounds. I need to divide that in half to get the weight at both tires. So that gives me 2,200 pound on each front tire. Now looking at the capacity of the tire which is stamped on all tires you need the max weight number which typically is 3,042 pounds. So you would divide 2,200 by 3,042 and get a percent (decimal) as a return. In this example it would be 0.72 . This is the percent of load on the tire in weight from 0 to 1 (0% to 100%). Then you would take the maximum inflation number which is typically 80 PSI on a Load range E tire. Now you would multiply 0.72 by 80 and get your inflation value. 57.8 PSI.Like other example of tires I've Ran...235/85 R16 - Load Range G (3750# @ 110 PSI)4400 / 2 = 2200 / 3750 = 0.58 x 110 = 64.5 PSI265/75 R16 - Load Range E (2415# @ 80 PSI)4400 / 2 = 2200 / 3415 = 0.64 x 80 = 51.5 PSISo you can't use the same inflation from one rating of tire to another they all vary just a bit. Even look at the difference from the tires I just listed there is a slight variation to the pressure numbers.

[dorkweed]

How do you guys know that the relationship between "load" and "tire pressure" is LINEAR???? I do know of Mike's figures and all, but I've never heard from "real tire guys" about this. I run 70psi in all 4 all the time............get good mileage.............and good tire wear. Just asking, not arguing.

[AH64ID]

I have seen several tire inflation charts by several manufactures and they are near linear, or more weight than a linear equation giving you a safety margin if you run linear. Linear is simply a quick equation that will provide you with enough air. http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf http://www.michelinrvtires.com/michelinrv/tires-retreads/load-inflation-tables.jsp http://toyotires.com/tires-101/load-and-inflation-tables

[Mopar1973Man]

You can over-inflate your tire to improve your MPG numbers but now your tire wear becomes more in the centers and less on the shoulders. Traction on dirt roads is also reduced and ride is rougher. Using the math gives you a middle of the road for the weight you are now currently carrying. Good ride, good tire wear, good MPG numbers. It handles its weight just fine, traction on dirt roads is OK. Under-inflation you gain good amount of traction in loose dirt and on snow. But take a risk of blow out if hauling heavy. Then the tire wears more on the shoulders than in the centers. Usually the truck feels squirrelly and mushy. Between using a tread depth gauge and the math I can tune the pressures to get the best ride and tread life from the tire. Typically my MPG's are unaffected by using the formula for pressures. Being I'm now capable of reaching mid-20's when I want to with a empty truck.Ok lets use AH64ID table...245/75 R16 tire in the table list at 35 PSI at 1,700 pounds.Formula now...1700 / 3042 = 0.55 x 80 PSI = 44 PSI1865 / 3042 = 0.61 x 80 PSI = 49 PSI2030 / 3042 = 0.66 x 80 PSI = 53 PSI2205 / 3042 = 0.72 x 80 PSI = 58 PSI2335 / 3042 = 0.76 x 80 PSI = 61 PSI... I think you see a difference.

[AH64ID]

You can over-inflate your tire to improve your MPG numbers but now your tire wear becomes more in the centers and less on the shoulders. Traction on dirt roads is also reduced and ride is rougher.

The increased wear is really only tires that are not on the proper size rim. If the rim/rubber are matched you should be able to run extra pressure and still wear smooth. My 255/80's can run max pressure and run a flat tread with even wear between the center and sides, but my 265/70's can only go to about 60 psi empty and my 285/70's were only good to 50 psi with an empty bed. This is all mounted on my OEM 17x8" rims. About 4500 lbs on either axle and I can run the 265's at max pressure and the 285's took 5K lbs or more.

[Mopar1973Man]

I'm running 16" x 7" rims on both the 96 and the 02 both trucks with 235's now. Still if I over inflate on either truck I can measure a difference in tread wear from center to shoulder. You can see it a bit more with the eyes after running a dirt road. That's what got me think about the tire pressure vs. weight.

[AH64ID]

I'm running 16" x 7" rims on both the 96 and the 02 both trucks with 235's now. Still if I over inflate on either truck I can measure a difference in tread wear from center to shoulder. You can see it a bit more with the eyes after running a dirt road. That's what got me think about the tire pressure vs. weight.

Interesting you have that issue with 235's on a 7" rim. I use the dirt road dust test as well.

[dorkweed]

I put on about 22,000+ miles a year..............averaged for the last few years. Probably 3-5,000 miles is on gravel/dirt roads, lanes, driveways etc. towing my work trailer or another trailer full of hunting gear. Probably half of the total miles are "empty" driving also. So how do I get the mileage out of my tires that many here cannot?????:shrug:Based upon the threads I read here and elsewhere; and sometimes reply to here and elsewhere, it'd seem that it doesn't matter what you do with "stock sized" tires!!! Many of the posts I read are about guys that cannot get the tire wear I do.........driving the same amount of miles. Most of them are running bigger than normal sized tires.Flame suit is standing readyl:smart:

[AH64ID]

I put on about 22,000+ miles a year..............averaged for the last few years. Probably 3-5,000 miles is on gravel/dirt roads, lanes, driveways etc. towing my work trailer or another trailer full of hunting gear. Probably half of the total miles are "empty" driving also. So how do I get the mileage out of my tires that many here cannot????? Based upon the threads I read here and elsewhere; and sometimes reply to here and elsewhere, it'd seem that it doesn't matter what you do with "stock sized" tires!!! Many of the posts I read are about guys that cannot get the tire wear I do.........driving the same amount of miles. Most of them are running bigger than normal sized tires. Flame suit is standing readyl:smart:

IIRC your work trailer is small, and easy on wind. Quite the opposite of my camper. I also don't think your average tow involves any 6-8% grades, but I could be wrong. I don't recall the last time I towed more than 20 miles and didn't hit at least one grade over 5%. The gravel roads are the same, it's rare to not hit a gravel road without a grade. That kind of driving just eats tires when they are at 90-100% of max loading and have a decent amount of torque applied to them. Larger tires seem to last longer, simply because they have more rubber. My summer tires are OEM take-offs from a 2011, Michelin LTX 265/70/17. They currently have about 18K miles on them and are down to approx 38% useable tread. At the current wear rate, which should slow as they get down to 5/32", I will only get 31K miles out of the tires. I have towed 10,534 of the 19,344 miles on them. My winter tires are 255/80/17 BFG KM2's. I do a 5 tire rotation and average all 5 for tread. I have 16,834 miles on them, 2938 of towing (it was the first year of the tires and I am sure that accelerated the wear). At their current wear rate I will get 31K miles out of them before they hit 2/32. With rare exception I don't even tow with them any more and like I said that's 5 tires. I don't accelerate hard (occasionaly at higher speeds to test tunes, but nothing abusive or common), don't do burnout's, etc. I just drive on a lot of gravel with both sets, and tow on a lot of gravel with the OEM's. So how do you get the mileage? My guess is your towing is light, your gravel use is light, and you don't experience many steep grades with decent weight. Doing some really quick math, I am putting over 800 ft/lbs to the ground from 1800-2400 rpms. I can just maintain speed on a 6-7% grade in 6th at 67 mph. So that's all 800 ft/lbs, that's a lot of twisting force on the tires, especially when they are near tire weight rating. If I drop to 5th and slow to 60 the load drops, but I gain gearing to increase wheel torque, so I bet I am still pushing 750 ft/lbs plus at the tires. I suspect the new Dodge's with 850ft/lbs of flywheel torque will eat tires when towed heavy with more than their predecessors.

[Wild and Free]

This is another thread subject I have to laugh at here. All of the "Science and formulas" are fine but are for reference and a starting point only.In the real world every tire manufacturer rubber compounds are different as well as the biggest factor of all I see absolutely nobody mention here and that is tire temperatures which is affected by ambient temps , load on tire, speed, pressure, conditions the tire is being used in and again the tire itself.If you all check your pressures at a set ambient temp with cold tires meaning the vehicle has not moved for 8 hours and is equalized with ambient temps then load them up and run for 20 miles and then check and see what your pressures are again, on average I see between 5-10 psi increase when the tires are hot.I have been through the Michelin Off Road Tire service Training program in Greenville SC for a week and work with large mine equipment tires on a daily basis so I know a thing or two about this, BTW we have tires that cost in the neighborhood of $100K each so getting max life from them is critical. Just saying........................

[dorkweed]

IIRC your work trailer is small, and easy on wind. Quite the opposite of my camper. I also don't think your average tow involves any 6-8% grades, but I could be wrong. I don't recall the last time I towed more than 20 miles and didn't hit at least one grade over 5%. The gravel roads are the same, it's rare to not hit a gravel road without a grade. That kind of driving just eats tires when they are at 90-100% of max loading and have a decent amount of torque applied to them. Larger tires seem to last longer, simply because they have more rubber. My summer tires are OEM take-offs from a 2011, Michelin LTX 265/70/17. They currently have about 18K miles on them and are down to approx 38% useable tread. At the current wear rate, which should slow as they get down to 5/32", I will only get 31K miles out of the tires. I have towed 10,534 of the 19,344 miles on them. My winter tires are 255/80/17 BFG KM2's. I do a 5 tire rotation and average all 5 for tread. I have 16,834 miles on them, 2938 of towing (it was the first year of the tires and I am sure that accelerated the wear). At their current wear rate I will get 31K miles out of them before they hit 2/32. With rare exception I don't even tow with them any more and like I said that's 5 tires. I don't accelerate hard (occasionaly at higher speeds to test tunes, but nothing abusive or common), don't do burnout's, etc. I just drive on a lot of gravel with both sets, and tow on a lot of gravel with the OEM's. So how do you get the mileage? My guess is your towing is light, your gravel use is light, and you don't experience many steep grades with decent weight. Doing some really quick math, I am putting over 800 ft/lbs to the ground from 1800-2400 rpms. I can just maintain speed on a 6-7% grade in 6th at 67 mph. So that's all 800 ft/lbs, that's a lot of twisting force on the tires, especially when they are near tire weight rating. If I drop to 5th and slow to 60 the load drops, but I gain gearing to increase wheel torque, so I bet I am still pushing 750 ft/lbs plus at the tires. I suspect the new Dodge's with 850ft/lbs of flywheel torque will eat tires when towed heavy with more than their predecessors.

John, I don't disagree with you at all. I'm kinda just doing a SWAG here based upon tire posts in the far past and now the near past based on forum threads here!!! The OEM Michelins on my '04.5, I changed out at 81,000...............not because I had to..........because I had a flat, and I couldn't see patching a tire with that many miles on it........................So I bought the Cooper Discoverer ATR's and got 75,000+ miles on them..................................and I only replaced them because Cooper had a Sale/Rebate going on at the time..................................I now have over 38,000 miles on my Cooper Discoverer AT3's and expect to get another 38,000 miles out of them at least. Heck; on my '99 with the OEM Michelins..............I got 125,000 on them!! And I was "harder" on those tires than any others I've driven on since!!! I say that because of the roads I was driving on at the time.......................mostly highway...................but brutal expansion joints...................................spaced so that the truck+trailer bounced like a "***** in your dreams"!!!!! I had a female "apprentice" at the time driving my '99............she used to tell me the it hurt her "boobs" going up and down those roads, due to the bouncing. Joe

[AH64ID]

This is another thread subject I have to laugh at here. All of the "Science and formulas" are fine but are for reference and a starting point only.

Absolutely, it a great place to start. I have never done the math and set to that exact psi, I use it as a minimum. I have yet to see an "official" tire inflation chart that has weight at "x" pressure be lower than linear.

In the real world every tire manufacturer rubber compounds are different as well as the biggest factor of all I see absolutely nobody mention here and that is tire temperatures which is affected by ambient temps , load on tire, speed, pressure, conditions the tire is being used in and again the tire itself.

Another big one is elevation. Filling to 80 psi cold at my house, 2710', and going up to 6,000 feet and checking the pressure after a tow will often yield 95 psi hot and 83-85 psi cold.

I check and adjust my pressure regularly when they are cooled, and I prefer to check them before the heat of the day as well.

If you all check your pressures at a set ambient temp with cold tires meaning the vehicle has not moved for 8 hours and is equalized with ambient temps then load them up and run for 20 miles and then check and see what your pressures are again, on average I see between 5-10 psi increase when the tires are hot.

My only caveat, because I have seen many people do it, it look at the difference for record but don't adjust the pressure down when the tires are warm. If your seeing a 10-15% increase in pressure from ambient to pulling off the highway at or near rated load then your fine, if your seeing 20%+ you are overloaded for the pressure.

John,

I don't disagree with you at all. I'm kinda just doing a SWAG here based upon tire posts in the far past and now the near past based on forum threads here!!!

The OEM Michelins on my '04.5, I changed out at 81,000...............not because I had to..........because I had a flat, and I couldn't see patching a tire with that many miles on it........................So I bought the Cooper Discoverer ATR's and got 75,000+ miles on them..................................and I only replaced them because Cooper had a Sale/Rebate going on at the time..................................I now have over 38,000 miles on my Cooper Discoverer AT3's and expect to get another 38,000 miles out of them at least.

Heck; on my '99 with the OEM Michelins..............I got 125,000 on them!! And I was "harder" on those tires than any others I've driven on since!!! I say that because of the roads I was driving on at the time.......................mostly highway...................but brutal expansion joints...................................spaced so that the truck+trailer bounced like a "***** in your dreams"!!!!!

I had a female "apprentice" at the time driving my '99............she used to tell me the it hurt her "boobs" going up and down those roads, due to the bouncing.

Joe

Joe,

First off I have to ask, why are her boobs in quotations? Was there something about them that needed them?? Okay back on topic...

Your life out of the OEM's is more normal than mine; however, I highly suspect your usage was more normal as well. Just look at our fuel mileage differences, I know my truck would meet or exceed your trucks mileage with the same usage/weight. The difference in mileage alone indicates I have a higher average power production, which manifest where the rubber meets the road (couldn't resist).

My dad, who drives even easier than I do, only got 16K miles out of his first OEM sized Toyo AT's before they were not winter safe (still street legal, but not elk hunting smart), and the second set only got him 23K miles. My point is towing on the roads around Idaho is murder on tires, and the extra Smarty torque isn't helping any :-). I wish it were only a small price to pay, but hey at least all roads lead to whiskey and fun.

Tire wear all comes down to use, some get great mileage and some don't.

Tires are one of my biggest frustrations, mainly because they don't last. It's the main reason I am looking at 19.5" LRG tires for my next set. They are built to handle higher weight and torque.

As you can see they are wearing even, and have less than 1/2 of a 32" difference from outside to inside on the front from increased camber while turning.

[dorkweed]

John,So for arguments sakes here, you're saying that towing lightly most everyday of the week is less hard on tires than towing very heavy...............................what, 2 times a month......................every weekend.......................1 time a month?????Just asking...........as usual!!!

[AH64ID]

John, So for arguments sakes here, you're saying that towing lightly most everyday of the week is less hard on tires than towing very heavy...............................what, 2 times a month......................every weekend.......................1 time a month????? Just asking...........as usual!!!

If your talking you vs me then yes I do believe so. 54% of the miles on those tires are towing, very few of them are under 16K GCW. What you have to remember in this instance is I rarely drive my truck as a DD, it's delegated to camping most of the time in the summer. Therefor I have very few easy miles. A vast majority of my empty miles in the summer are on dirt since I tow to the woods and disconnect and drive around. One more way to look at it is average fuel economy. What is your fuel economy average on your tires? My average on my summer tires (I have only had 1 tank since 12 March without any towing, all my towing this year has been 16.5K-18.5K GCW) this summer has been 11.6 mpg. I had a low of 9.26 and a high of 17.84. I have put 5,816 miles on the tires this season, and 3,649 were towing (63%), Even thou it is mostly weekend towing on my truck that's pretty much all it does, so it's no different than towing at the same weight 63% of the time.

[dripley]

I average 25k a year. About 4500 towing the 15k 5th wheel, maybe another 4k with material for the job, and the remainder with the truck around 7400#. When working on the east coast I see very little in the way of steep grades of any length. For the past 2.5 years I have been back on the west side of the blue ridge mountains and see a lot of grades to climb. Almost no dirt roads, mostly interstate miles. The first five years in stock configuration, the past 7 with the Banks set up. I have always been jealous of Dorkweed's mileage on tires, but after reading AH64Id's mileage I feel pretty lucky. I am about midway between ya'll. Michelins get me around 60k, Pirelli Scorpians and a Cooper highway tread got me about 40k each. I am on a set of Cooper ATP's and I might get 50k out of them. For the price they will probably be the best i have bought on a cost per mile basis. I know a lot of things effect tire wear and I do hot rod around some. I am also guilty of leaving 80 psi in the tires after a trip with the 5th wheel. I am doing a better job on the air pressure with my current set, but wont see what Mr. weed is getting with his. So I guess I am still jealous of Joe, but not envious of John.

[Mopar1973Man]

I have to agree with AH64ID about te roads here in Idaho and the impact they have on tire rubber. I typically see about 30-35k miles from tires. My biggest eater of tires is haulling firewood. The miles of hauing heavy up and down the mountain chews the rubber up fast. Even my exhaust brake eats the rubber quickly. But as for the formula its a good base line number to start from and tweak. I've been through 2 sets of tires now and never had a blow out or serious tire damage from poor inflation numbers.

[ISX]

My truck came with michelin LTX A/S on it when I bought it and I put 80k on them. Who knows how many they had on it before. They sure didn't look new when I got it. I aired them up to 80psi in the front year round.. These hankook dynapros were a flop, wore very quickly but they were $150 each so you get what you pay for I guess. Those A/S tires are more like $220. When I put them on the back I only go to 60psi. I never can tell a difference between air pressures so I just run a lot so I never have to mess with it.

[Wild and Free]

Most of my driving is about 50% gravel, drive 35 miles of it daily to and from work and it does wear tires a bit faster but it comes out to about the same wear as if loaded with trailer most of the time on hot pavement from my experience with all my rigs.Ryan the Hankook Dynapro ATM are going good on my cummins and I have talked to many others who are running them and all in my area are having great luck with them. I have 30K miles on the set now and they are still look almost new.Best tires life I got was on a set of BF AT.s 285-75-16 I had them siped when new and put 85K on them sold them to a friend who put another 30K on them. They were D rated with a lot of gravel miles on them as well.I got 85K out of the factory BF rugged trail AT's on the 05 and they still had some life left when I pulled them off. I would say overall I get 60-70K from tires.My wife got 110K out of a set of Toyos on her neon a few years back.Like Mike said the biggest tire eater is pulling trailers on gravel and especially wet gravel.Wet tires are extremely more vulnerable to damage than a dry tire, I saw a demonstration while at Michelin school where a guy took a large heavy blade knife and laid the full length of the blade on a lug of a dry tire, it was a large loader tire, and he hit it with a large mallet to try and get the blade to penetrate the rubber and all it did was bounce off, he then took a spray bottle and gave the same spot 2-3 squirts of water and did the same thing and one single hit sunk the blade all the way into the lug just showing how good of a lubricator water can be and how a 10 inch blade could be a rock cutting a tire as easily when wet, gravel roads are like driving on razors when it is wet.At work at the mine we run Toyo MT's and the roads are wet most of the time as we need to control dust with water trucks and we average 8-10K on a set of E rated tires on our pickups that never see pavement in their lives.My problem is finding a tire that gives awesome traction year round PLUS gives long life. So far the decent traction tires are 60K and the sucky treads are the longer life tires, so far the BF AT"S have won in my book but like I said they were siped when new, but the Hankooks are doing good so far will be several more years before I find out total life as I only put 10K a year on the cummins.