I just got a pair of used 245/75 R16 tires from a friend. So I mounted them up on my old aluminum mags and mount the 245's on rear axle of the truck. 

 

 

They are a bit shorter in height. The gains are great!

 

Engine load is reduced by 5% to 10% roughly. 

EGT's are about 50*F lower.

Acceleration is way better and faster. 

Cruising RPM's are right about 1,960 RPM at 65 MPH.

 

I've got to reset my cruise timing limit on the Quadzilla I need to drop from my current 25% to 20% it was holding cruise timing climbing about 2-3% grade. 

 

I'm going to document MPG usage as well in the coming days. 

Edited July 2, 20186 yr by Mopar1973Man

@Mopar1973Man all this talk of rpm's, egt's, and engine load got me to thinking...I'm still fairly new to the diesel world but I understand that more fuel means more heat in a diesel, opposite a gasoline engine. So would it be safe to say that, as a snapshot of engine efficiency, egt's could be used much the same way a vacuum gage is used in a gasoline engine to determine fuel efficiency? What is your egt cruising down the freeway at 65 with your new setup? 

Friendly reminder.  Before you tow anything heavy... check max load of your new tires, stamped on sidewall and actual load.  Leaving at least 10 percent bewteen those numbers for safety.  Dont think you mentioned the load range of new tires? 

 

8 minutes ago, 015point9 said:

Friendly reminder.  Before you tow anything heavy... check max load of your new tires, stamped on sidewall and actual load.  Leaving at least 10 percent bewteen those numbers for safety.  Dont think you mentioned the load range of new tires? 

4

 

Already done that too. 245's I've got right now are rated 3,042 pounds at 80 PSI Load Range E's

 

38 minutes ago, Scottfunk said:

@Mopar1973Man all this talk of rpm's, egt's, and engine load got me to thinking...I'm still fairly new to the diesel world but I understand that more fuel means more heat in a diesel, opposite a gasoline engine. So would it be safe to say that, as a snapshot of engine efficiency, egt's could be used much the same way a vacuum gage is used in a gasoline engine to determine fuel efficiency? What is your egt cruising down the freeway at 65 with your new setup? 

 

You want to use Engine Load. Engine load is the amount of fuel being injected. EGT's are not a good way being just a small hill and EGT's could climb to 800*F but the engine load could be 20%. Then flatten back out and have EGT's now down to 550*F and engine load of 17%. I'm going to have to capture a few fresh screenshots.

 

 

On 7/3/2018 at 10:34 AM, Mopar1973Man said:

Talk about a huge difference 1 inch of tire makes.

 

EGTs are lower.

Engine load is lower.

Transmission temp is lower.

Exhaust brake performance is better.

Brake performance is way better.

Can hold 5th gear wider speed range.

Acceleration is quicker.

Less stress on the clutch during launch.

 

Just in the 120 miles I'm loving 245's over my 235's. Very impressed with the changes. Now I'm having hard time understanding people with 35 or 37 inch tires with 3.55 gears. I'm going to start fuel logs on my next tank.

 

 

It's great for improving the overall gearing, and it will tow very nice as well. You'll top out in 4th sooner, but towing in 5th is easier. 

 

I went from 285/70R17 to 265/70R17 and lost hill pulling speed, but gained ease on the interstate at 65, then I went to 245/70R19.5 and lost some ability at 60-65 in 6th, but can pull hills at ~2500 and 65 all day long in 5th. 

 

It's all a numbers game finding that optimal rpm for the speed.

 

Downshift to to 4th at 60 and you'll see the same results with EGT, load, trans temp, exhaust brake performance, etc... but you'll actually be burning more fuel. 

 

Regardless of what gear you are in it takes "x" hp to run down the road at a given speed. If you increase the drag in the motor by changing rpms you have increased the hp and fuel requirements and it's a loss. So it really is a sweet spot, but 1950 is a great cruise rpm. I suspect that for your driving routing to Ontario and back all the time you'll notice a slight increase in economy. If you spent more time at 75-85 on the interstate you would likely notice a decrease in fuel economy. 

3 hours ago, Mopar1973Man said:

 

You want to use Engine Load. 

 

Correct, but don't forget that 20% load at 1800 rpms is less total fuel than at 2200 rpms, At 1800 rpms you're burning 81.8% of the fuel as 2200 rpms even for the same load. 

 

 

 

 

Edited July 5, 20186 yr by AH64ID

1 hour ago, AH64ID said:

Downshift to to 4th at 60 and you'll see the same results with EGT, load, trans temp, exhaust brake performance, etc... but you'll actually be burning more fuel.

No longer have too... The grade from Council, ID back to New Meadows some nights required 4th because of road conditions and traffic now I can pull in 5th and never downshift. That is one thing this tire change has really help with allowing the truck to stay in 5th longer. 

 

1 hour ago, AH64ID said:

Regardless of what gear you are in it takes "x" hp to run down the road at a given speed. If you increase the drag in the motor by changing rpms you have increased the hp and fuel requirements and it's a loss. So it really is a sweet spot, but 1950 is a great cruise rpm. I suspect that for your driving routing to Ontario and back all the time you'll notice a slight increase in economy.

That's what I'm looking for in the next few tanks of fuel...

 

1 hour ago, AH64ID said:

If you spent more time at 75-85 on the interstate you would likely notice a decrease in fuel economy. 

I don't travel interstates any longer. Just back and forth New Meadows, ID to Ontario, OR. No longer have the Boise run to deal with. That is also something I factored in was the average road speeds I run. Even though the speed limit is 55 MPH between Council to New Meadows I typically ran that road at 45 to 50 MPH get to watch for deer and animal bouncing up on the highway also like I said some days get stuck behind a trucker running slow with nowhere to pass for long stretch.

 

Speed limits typically are 45, 55 and 65 MPH. Small towns: 25 MPH typically. 

 

The change on the lower end of the RPM seems very small. (Speed is off by -1 MPH so its actually 65 MPH as reported by my GPS)

Edited July 6, 20186 yr by Mopar1973Man

9 minutes ago, Mopar1973Man said:

No longer have too... The grade from Council, ID back to New Meadows some nights required 4th because of road conditions and traffic now I can pull in 5th and never downshift. That is one thing this tire change has really help with allowing the truck to stay in 5th longer. 

 

My point was that by dropping to 4th you'd see the same thing, reduced load, reduced EGT's, etc... but fuel economy would drop from the increased drag of the engine. 

 

Yeah smaller tires do sure help out the gearing. I think you're going to like that size for your specific use. 

 From reading all this it boils down to what you are doing with your truck. For me I am mostly empty these days and cruising on the 265's at 75 netting me near 20 mpg. When I tow I try to stay  near 65 and the 245's would most likely make that easier and might generate a touch more on the mpg. But since I do more no towing miles I think the 265's are better for my purpose. And as mentioned above you have to go with what suits what you are doing with your truck. There is not  just one size fits all.

Just now, AH64ID said:

My point was that by dropping to 4th you'd see the same thing, reduced load, reduced EGT's, etc... but fuel economy would drop from the increased drag of the engine.

 

That is kinda flip flop topic there are times where the can be gain from the dropping a gear but there is most times it just plain wasteful running. Hard to explain. I understand what you saying excessively hold 4th for no good reason tend to be wasteful. Most of the gain s would be running 45 to 50 MPH on the old tire up Fort Hall. Above that you need to be in 5th again bounces back to traffic and road conditions. 

 

3 minutes ago, AH64ID said:

I think you're going to like that size for your specific use.

 

So far yes. Acceleration is way better. I can reach 4th as early as 30 MPH now no problem. Gear span is different with a bit better overlap compared to 235's / 265''s. Launching on 2nd gear is way easier on the clutch and just about leaps out there on its own.

2 minutes ago, dripley said:

I think the 265's are better for my purpose.

 

Even the 235's would do you fine. Same revs/mile. 

 

Like 235/85 (9 inches wide tire) R16 and 265/75 R16 (10-inch wide tire) are basically the same.

 

Then 215/85 R16 (8 inch wide tire) is the same as 245/75 R16 (9 inch wide tire)

 

My whole purpose is not for the cool factor but for any small gain I can get. Being the final ratio is so forgotten with all the big tire crazy and people don't pay attention to axle gearing then toss on 37" tire because it looks cool and wonders why they having issues. 

 

More I continue to drive this 250-mile lap course the more I understand about the road conditions I must travel. Being the entire trip from New Meadows to Ontario is all downhill that means the whole trip back is all uphill. Being you follow the Weiser River from New Meadows area all the way to Weiser ID. Since water doesn't run uphill that means the whole trip is downhill following the river. So understand that basic idea you need to have gearing that is suited for pulling that trip uphill and optimize the power for the engine. 

 

Now there is always that chance that this doesn't work out. Fine... I can just stick with 235's and continue. That why I'm running a pair of used tires on my old aluminum mags for the testing purpose. Nothing lost in the tires or anything. I'm hoping this gets to other that are considering larger tires to consider the gearing impacts of the current axle ratio and what the tires will produce for final ratio.

It's been a while since I took the '97 with on the interstate and even longer since I checked fuel mileage so I took it to Albuquerque yesterday. Total trip was 226 miles, about 165 miles of that was interstate. When I got back home I topped off the tank which took 9.8 gallons of fuel. 23 miles a gallon and was doing close to 80 mph on I-40 most of the time.

 

If the '01 still looked good with smaller tires I would change it over to 245/75R16 just for the drastic improvement in drivability and fuel mileage.

 

 

39 minutes ago, 04Mach1 said:

It's been a while since I took the '97 with on the interstate and even longer since I checked fuel mileage so I took it to Albuquerque yesterday. Total trip was 226 miles, about 165 miles of that was interstate. When I got back home I topped off the tank which took 9.8 gallons of fuel. 23 miles a gallon and was doing close to 80 mph on I-40 most of the time.

 

If the '01 still looked good with smaller tires I would change it over to 245/75R16 just for the drastic improvement in drivability and fuel mileage.

 

 

It would be good to see if the 01 got the same mileage as the 97 on the same run.

1 hour ago, Mopar1973Man said:

Even the 235's would do you fine. Same revs/mile

Wont disagree on this, just not sure to buy the different size since the 265's have done so good every where i have used them. Might try a used set one day or check into the price of the narrower tires. 

 

44 minutes ago, dripley said:

It would be good to see if the 01 got the same mileage as the 97 on the same run.

 

 

The '01 in the same scenario would have seen 18-19 mpg at best. I have driven from Grants, NM to Westminster, CO which was about 550 miles on 18 gallons of fuel in the '97. Of course the '97 being 2wd and closer to the ground definitely helps its aerodynamics. I wonder what would happen if I got a free flowing exhaust on the 12 valve. It's still got the stock exhaust with the catalyst.

 

Best I've seen from the '01 to date is about 24 mpg and I was babying it at 65 mph. I need to see if the recent exhaust upgrade helped the '01 fuel mileage.

7 minutes ago, 04Mach1 said:

It's still got the stock exhaust with the catalyst

Catalytic converter? On a 97?

2 minutes ago, dripley said:

Catalytic converter? On a 97?

Yep... Catalytic converter. The only reason I can think of Chrysler would put one on a diesel back then would be to detune it since they couldn't rape the ECM. Reading through Cummins Forum and similar it seems the catalytic converter was common on 94-97 Dodge Cummins.

3 minutes ago, 04Mach1 said:

Yep... Catalytic converter. The only reason I can think of Chrysler would put one on a diesel back then would be to detune it since they couldn't rape the ECM. Reading through Cummins Forum and similar it seems the catalytic converter was common on 94-97 Dodge Cummins.

Would not have thought that since my 02 did not have one.

Must be a Cali motor. IIRC they also had some form of smog pump or EGR 

From what I remember the CAT was a last ditch effort of sorts to make emmissions requirements of the time. It worked but barely. After that we got twice as many valves and wires under the hood .

37 minutes ago, AH64ID said:

Must be a Cali motor. IIRC they also had some form of smog pump or EGR 

Might be... I don't see anything on it about being California emissions. I bought it from the original owner who ordered it from a dealer in Belen, NM. Truck spent the first 7 years of it's life in Truth or Consequences, NM. Looks like a normal P7100 12 valve 6BT,  just has a catalytic converter right behind the down pipe. Definitely doesn't have EGR or any other emission control devices on it.

 

 

4 hours ago, 04Mach1 said:

If the '01 still looked good with smaller tires I would change it over to 245/75R16 just for the drastic improvement in drivability and fuel mileage

 My 4X2 truck on the left with 245/75R16 on factory rims.  @JAG1 4X4 truck on the right.   

 with the weight of that camper My 16's are the best thing. Those are Alcoa wheels BTW.... the strongest 16 inch wheel available.

 

IBMobile, I've tried to post some of our pics at camp with this new computer. Need to get the wife to help with that.

Edited July 6, 20186 yr by JAG1

10 hours ago, AH64ID said:

Must be a Cali motor. IIRC they also had some form of smog pump or EGR 

 

California models had EGR valve and Catalytic converter.

 

Edited July 6, 20186 yr by Mopar1973Man