New Tire Size found 235's to 245's

[Tractorman]

56 minutes ago, Dieselfuture said:

So what about using steel wheels vs aluminum, they are smaller but much heavier. Will it be counterproductive, meaning I'll see same mpg with smaller heavier tire vs my lighter aluminum rims with bigger tires.  

 

I don't think that the increased tire weight will be an issue.  It's more about operating the engine in an RPM range that gives you performance and fuel economy. 

 

It is too bad that car and truck manufacturers don't provide engine performance charts that show engine horse power, engine torque, and fuel consumption in lbs/hp per hour at every RPM in its operating range.  Years ago when I purchased a Cummins 4BTA 3.9 and installed it in a 1991 Ford F150, I was provided with this performance chart from Cummins.  On that engine the peak torque was at 1700 RPM, but the lowest fuel consumption per hour was at 2100 RPM, so it gave me a good idea of what the final drive ratio should be.

 

The Cummins engines in our 2nd generation trucks probably closely fit this performance chart.  This is why the smaller tires will give a significant gain in fuel economy as well as performance

 

Attached is a performance chart for a mid '80's 4BT3.9 (no aftercooler).  Note the fuel consumption information (the horizontal line at the bottom of the graph)  If you save the picture, you can zoom in on the graph for better detail.

 

- John

Edited September 24, 2018 by Tractorman
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[Tittle Diesel Performance]

16 hours ago, Dieselfuture said:

So what about using steel wheels vs aluminum, they are smaller but much heavier. Will it be counterproductive, meaning I'll see same mpg with smaller heavier tire vs my lighter aluminum rims with bigger tires.  

A little unsprung weight won’t hurt it, that’s usually effective more in stop and go or sudden throttle situations. Look more at diameter rather then the weight. Unsprung weight will lose some effectiveness of the shocks but that’s really all you should see. If it was a geo metro that made 60hp and 47lbs ft at the tire I’d be concerned but not on something like this. Personally on 4wd versions I won’t do smaller then a 265/75/16 or equivalent 31 to 32” tire. It looks like crap and takes away from the clearance of the solid axles. I wouldn’t go more then 33” with 3.54 gears. 4.10 likes any where between a 31 and 35, it doesn’t seem to care as much since your rpm is higher and your in the turbo power band all the time rather then being boarder line. If your over 35” tires though a 4.56 is your best bet but honestly at that point economy isn’t your strong suit pushing that much air and weight around.... it can be done but it’s more difficult.

 

If your looking for more economy the 62/67/12 doesn’t really help that. It’s not a bad turbo but it takes a little more rpm to stay in th power band so you use more fuel to offset that creating more heat. You’d be better with like a HX35W with a 16cm housing or even th 18cm. It’ll give you exhaust breathing and low EGT but keep the shaft RPM up and boost would be more responsive. The stock exhaust manifold is ok on 24v trucks but an aftermarket 3 piece DAP unit would help too if you intend to keep that HX35 variant on there. they smooth the pulse and flow into the exhaust housing which ends up running a little cooler and smoother in the long run but boost stays pretty well the same. Just my .02

15 hours ago, Tractorman said:

 

I don't think that the increased tire weight will be an issue.  It's more about operating the engine in an RPM range that gives you performance and fuel economy. 

 

It is too bad that car and truck manufacturers don't provide engine performance charts that show engine horse power, engine torque, and fuel consumption in lbs/hp per hour at every RPM in its operating range.  Years ago when I purchased a Cummins 4BTA 3.9 and installed it in a 1991 Ford F150, I was provided with this performance chart from Cummins.  On that engine the peak torque was at 1700 RPM, but the lowest fuel consumption per hour was at 2100 RPM, so it gave me a good idea of what the final drive ratio should be.

 

The Cummins engines in our 2nd generation trucks probably closely fit this performance chart.  This is why the smaller tires will give a significant gain in fuel economy as well as performance

 

Attached is a performance chart for a mid '80's 4BT3.9 (no aftercooler).  Note the fuel consumption information (the horizontal line at the bottom of the graph)  If you save the picture, you can zoom in on the graph for better detail.

 

- John

 

 

You can can find those models online if you look hard enough. What I find hilarious is they say that 105hp and 260lbs ft is ok for up to 16k GVW but in the modern world where everyone does 70+ and the world isn’t dead flat it just doesn’t work. I’d take that kind of power in maybe a ford ranger but too many Mcdonalds wrappers in the back may get me rear ended trying to merge.... not to say it wouldn’t move 16k but it seems in the real world of factory 320hp turbo 4 cyl cars and impatient people you’d be likely to get hit just going A to B. Of course those power numbers are due mainly to emissions standard as well as reliability/longetivity reasons.

[Mopar1973Man]

If you want to know how much loss there is the rotational mass you have here is the web site for mathing that out all the way down to the lug nuts.

 

http://hpwizard.com/rotational-inertia.html

 

15 minutes ago, Tittle Diesel Performance said:

I won’t do smaller then a 265/75/16 or equivalent 31 to 32” tire.

 

Be aware the LT truck had 245/75 R16 tires and the SLT truck had the 265/75 R16 tires. Still in a stock range of tires just better ratio.

[Tittle Diesel Performance]

17 minutes ago, Mopar1973Man said:

If you want to know how much loss there is the rotational mass you have here is the web site for mathing that out all the way down to the lug nuts.

 

http://hpwizard.com/rotational-inertia.html

 

 

Be aware the LT truck had 245/75 R16 tires and the SLT truck had the 265/75 R16 tires. Still in a stock range of tires just better ratio.

 

That was a factory optional tire size regardless of trim level. I’ve had LT with 265 and SLT with 245 each truck was different. 265 was classified as the off-road package.

[dripley]

My 265's were an option. Paid a few extra bucks for them. My door sticker lists 245's. Mine is an SLT.

[Dieselfuture]

So far it looks like my average is 20mpg, engine load is down and egts too, but my speedo is off according to GPS. If I'm doing 70 by GPS truck says 72 or so. PO said he flashed pcm I believe  (or abs module) whichever one is responsible for speedo, for 295/75/16 I believe. 

So if I'm getting 20mog by the truck and qwadzila what am I really getting. I'm guessing it's about 5% off or so. Still better then my 18.5 with 285s

[Mopar1973Man]

Bigger looks cool but small is better for the economy.

[trreed]

Heh. I'm making a mental joke right now.

Edited September 24, 2018 by trreed

[Mopar1973Man]

42 minutes ago, trreed said:

Heh. I'm making a mental joke right now.

 

Do I want to know? 

[AH64ID]

20mpg based on odo miles with a 5% error is 19mpg actual. 

 

Having ran several different sizes of tires on my truck none have had more than a 2-3% change in economy. That from stock to 285’s to my 125 lb 19.5’s. 

 

Each tire size has its benefits. The OEM tires pulled better in OD at 65, but the current ones do better at 60 in 5th. 

 

Personally I wouldn’t run something as small as a 245 if you ever see back roads and/or snow. The pumpkin drags enough as it is on my truck with 255/80R17’s coming it at 33”. It’s not as bad in the summer, but even with the stock 265’s I would find more sticks, rocks, etc bumping the undercarriage than I do with 33’s. 

 

On my dads 17 he has some winter tires that are 265/65R18 vs 265/70R18 for his summer tires. He gets better economy on the bigger tires, even after accounting for the speedo error. The 6.7 is a different beast thou, and prefers lower rpms than the 5.9’s.    

[Dieselfuture]

So I just filled up 22.24 gallons and had traveled 454 miles. Now my GPS says I'm going 70 when dash is telling me I'm almost doing 72, so figured for every 70 miles I travel truck adds 2 miles because speedo is off. Since I'm illiterate and can't do hard math, I took 454ml and divided it by 70 and x 2 which is 13ml so then 454-13 is 441 of actual miles traveled ÷ 22.24 comes to 19.82 mpg. Is this correct? @AH64ID. If so going from 285/70/17 to 265/70/17 gained me about 2mpg and I wasn't even trying, I even had more then one ocasion of getting on it pretty hard getting on ramps and passing. 

So if this is the case ( I'll can't believe I'm saying this, I remember how existed I was getting these tires lol) I'll probably be selling my current 285s and getting a new set of 265, and some day some day maybe I'll consider 245, but for me I don't tow every day so look still mean something. If I had to guess, if I put on 245 now I'd see somewhere in 21-22mpg 

Biggest difference I noticed, is its easier to take off, cruise down the road and turn when not moving much. 

The other thing is before I could cruise at 75 without working it too hard I was in 2000rpm range now mpg seems to drop off around 75 because rpms are over 2000 at that speed, seems like 1900-2000 is a sweet spot for crusing with close to 21 degrees of timing. Idk just some random thoughts. 

Edit 

If I stay around 65mph I can probably gain another 2pmg as qwadzila was telling me on flat ground I'm around 23-24mpg, and I have it dialed in pretty close.  Time will tell I guess. 

Edited September 27, 2018 by Dieselfuture

[AH64ID]

70/72=.972

454x.972=441 

 

That's how I do it, but it's the same answer. 

 

The most accurate way would be to check the difference is to use the GPS odometer and compare it to the truck odometer. My odo is within 1%, but my speedo is a little more off. 

 

[dripley]

I wonder what difference you will see in tire wear the smaller you go. Seems a little less rubber would wear quicker but i dont really know that. Just pondering the question.

[Dieselfuture]

33 minutes ago, dripley said:

I wonder what difference you will see in tire wear the smaller you go. Seems a little less rubber would wear quicker but i dont really know that. Just pondering the question.

Yes I agree a lot of variables, and does it pay in the long run to run smaller tires. All the research and driving that @Mopar1973Man done says smaller tires are way worth it, in mpg and wear and tear. Plus sounds like he gets good amount of miles out of his tires too, forgot what brand of tires he runs 

And you had 265 entire time with good results and MPG, so I think 265 is the ticket for me.

 

Edit 

Oh yeah I forgot to mention, my truck always seem to pull to the right a little bit and not just cuz of crown in the road, but now it seems to be pulling left a little. So it has to do something  with the way tires wear

Edited September 27, 2018 by Dieselfuture

[Mopar1973Man]

9 minutes ago, Dieselfuture said:

Plus sounds like he gets good amount of miles out of his tires too, forgot what brand of tires he runs 

 

Hankook ATm 245/75 R16 now.

Hankook ATm 235/85 R16 previous set.

 

10 minutes ago, Dieselfuture said:

Yes I agree a lot of variables, and does it pay in the long run to run smaller tires. All the research and driving that @Mopar1973Man done says smaller tires are way worth it, in mpg and wear and tear.

 

235's have done excellent in the past and extended the front suspension lifespan considerable. Now see how the 245's hold up to the test of time. Like I'm heading for 200k on these ball joints. 245's should extend the brake pad life. I can already tell the brake performance is better with the 245's over the 235's. 

[dripley]

 I have the 265 s on mine, always have. Just thinking the tire wear might balance with the fuels savings. But I get 19.5 mpg at 75 on pure interstae runs at about 2200 rpm. Thinking of the other parts you could see savings might just justify it for sure.

 I would like the 245 s for towing with the boost in rpm, be much for 6th gear. I guess any gear for that matter. But most all of my towing is interstate too. Just call me a pavement chicken.

[Dieselfuture]

8 minutes ago, dripley said:

 I have the 265 s on mine, always have. Just thinking the tire wear might balance with the fuels savings

I figured with current prices I'll save 20c per gallon with smaller tires, so it would take a lot of driving to make a big difference, but just how easy i just got 20mpg is nice, wasn't even trying. I really think if I can do all hwy at 55-65 I should be able to get 22 to 23 

 

[dripley]

1 hour ago, Dieselfuture said:

I figured with current prices I'll save 20c per gallon with smaller tires, so it would take a lot of driving to make a big difference, but just how easy i just got 20mpg is nice, wasn't even trying. I really think if I can do all hwy at 55-65 I should be able to get 22 to 23 

 

I can see that. Most of the interstates I run will get run over at 55-65. I remember a trip I made north to the next job and fair amount was on secondary roads. I was towing and kept it down around 60 or less for 2 or 300 miles. I got 12.5 mpg on that trip which was good considering I usually get 11 on the interstate running 65. But it was a slow trip.

 

plus you just stepped down the size I have always had.

Edited September 27, 2018 by dripley

[Tittle Diesel Performance]

ANYONE WHO NEEDS TO RECALIBRATE SPEEDO:

Any shop with a snap on Solus can enter the ABS module and correct for tire size. If the size isn’t on the list there’s is an option based on revolutions per mile. S03, super chips and any other plug in flash type programmer will usually have ABS correction factors. I just corrected one yesterday. Someone put the tire size in as 215/70/15 and it was running 33’s so I was doing 55mph indicated and pulled out the GPS cause I was blowing by traffic and turned out I was doing 81mph. Now it’s within 1mph GPS and radar based. He always said he got like 13mpg empty, guess that didn’t help much huh haha truck gets about 20 now with some tuning and 90hp injectors

[Mopar1973Man]

1 minute ago, Tittle Diesel Performance said:

ANYONE WHO NEEDS TO RECALIBRATE SPEEDO:

 

If @Chris O. finishes his CCD Network tool we will have another tool that can calibrate the ABS speed.