Taller Tires and 6.0L MPG?

Bob Q · September 19, 2005

This past weekend, I took my '04 2500HD CC 4x4 on a little trip that totaled 193 miles. It was 95% freeway and I had the cruise set at about 67 mph. Mileage figured out to be 14.11. I was wondering if a little taller tire (255/85 or 285/75) would help mileage since it would knock off about 200 rpm. The truck is all stock with 11,000 miles and a 6.0L, 4L80E, 4.10 gears, and 245/75 tires. Thanks.

BADGERMAN · September 20, 2005

I Lost about 1-2 mpg when I put the 285's on. It does reduce the rpm, but the 285's tires are a heavier tire and you have to take in account the heavier rotating weight.

I put on the predator programmer and I got back about 1 mpg.

Hope this helps.

builderpaul · September 20, 2005

This past weekend, I took my '04 2500HD CC 4x4 on a little trip that totaled 193 miles. It was 95% freeway and I had the cruise set at about 67 mph. Mileage figured out to be 14.11. I was wondering if a little taller tire (255/85 or 285/75) would help mileage since it would knock off about 200 rpm. The truck is all stock with 11,000 miles and a 6.0L, 4L80E, 4.10 gears, and 245/75 tires. Thanks.

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I would think you would get better milage. It would be hard to calculate without speedometer recalibration for the new tire. With a bigger tire and without recalabration you will be going farther than the odometer says you are.

BADGERMAN · September 20, 2005

I would think you would get better milage. It would be hard to calculate without speedometer recalibration for the new tire. With a bigger tire and without recalabration you will be going farther than the odometer says you are.

I thought the same thing, that it would get better mileage because of the reduction in rpm's. But it didn't.

I have a girlfriend who is an engineer and she calculated it out for me before I was able to fix the speedometer with the predator programmer. Don't ask me how, she is the nerd with the degree.

I have checked the speedometer with a gps unit and along side two different cars, the speedometer is coorect after the programmer was installed.

dewfpo · September 20, 2005

With the taller tire, theoretically you could gain mpg IF all your driving was highway and you didn't tow much (it would have the same effect as going from say a 4.10 to a 3.50). AND you drove the same speed as before, AND IF you were NOT starting and stopping.

Bigger tires take more throttle application to accellerate at the same rate as with the smaller tires. And, provided you are not moving your cruising RPM out of your engine's sweet spot.

Where larger diameter tires eat the mpg is in accellerating from a stop.

Also braking is reduced since your rotor spins fewer revolutions in the same distance covered.

DEWFPO

71GMC_3/4T · September 20, 2005

I've wondered if significantly narrower tires would help MPGs. You'd have less contact with the ground, therefore less friction.

Also, for long runs, how would zero tow-in affect economy? Could you run the front end really "loose" and gain enough to overcome the fear of having it run off the road by itself? I'll wait around for someone else to try it first though...

Freyguy · September 20, 2005

When I went from 245's to 285's, I figured out my speed via GPS and took the average of a couple rations....like when I was doing 65, I was really doing 61 (just a guess), and 35/33 or something. It worked out that if I multiplied my mileage by 1.067, I would have the correct mileage. Now I got my computer redone which made life easier!

I too lost about 1-2 mpg. I was getting 11.5 city/hwy with 285's. Now with the programmer on 93 octane, I'm getting 13 city/hwy.

Bob Q · September 20, 2005

Thanks for the replies. I suppose I shouldn't complain with 14 mpg, but I was hoping I'd be able to squeak out 15 or 16 mpg. Some older posts that I've read on the forum from guys with 8.1Ls are claiming 13-14 mpg on the highway. Are there any other improvements that might boost mileage? I'd be happy getting an honest 15 mpg on the highway, but maybe I'm dreaming. Heck I know a couple of guys with older Suburbans with TBI 350s getting 16 mpg. Why doesn't the 6.0L get better mileage? A CC 4x4 can't be much, if any, heavier than a Sub.

BADGERMAN · September 20, 2005

Heck I know a couple of guys with older Suburbans with TBI 350s getting 16 mpg. Why doesn't the 6.0L get better mileage? A CC 4x4 can't be much, if any, heavier than a Sub.

Bob Q

It might depend on what gear they are running. If they are running a 3:73 gear, then that might be the difference.

My CC 4x4 weighs in a 6250# with me (160#) and a full tank of gas.

You might be able to get close to 15-16 if put on a programmer and stay with the stock tires.

I did get 14 mpg once, but it was all highway and it was about 2 days after I bought it, so I was taking it a little easy.

I have averaged about 11.5 mpg since I bought the truck. I get about 9-10 mpg pulling a 6500# travel trailer.

silverado3500 · September 21, 2005

I would be tempted to try a 235/85R16, (same height as a 265/75R16, but narrower) or a 255/85R16 (slightly taller than a 285/75R16, but again narrower.) I have no doublt that the 235/85 would provide a slight improvement, but the 255/85 may be getting a bit too tall.

I would also consider that it will take a long time to pay off the "investment" of different sized tires from improved fuel economy, and that if improving your fuel economy is your only reason for wanting to change the tires, I don't think its worth your trouble until your current tires wear out.

Thats my 2cents, hope that helps.

mudbug660 · September 21, 2005

On my truck I went from 235/75r16 to 285/60r17 and I am losing milage. even after calculating the speedo difference I'm still losing milage and the 285's are 1/2" bigger but alot wider.My rims weigh 25 lbs a piece which I think is around what the stock ones are. I think the contact patch of the tire has a great amount to do with mpg differences. It's like asking what would get better gas milage: a truck with the tires correctly inflated or one with the tires underinflated, ie. causing a larger contact patch. I'm not saying that the contact patch is the only thing but like silverado3500 said a narrow tire might just give you better milage. when my tires wear out I'm defintly going down to 275/60r17 which are the same height of my last tires. So i hope that might help my milage a little. But since I'm still staying in the 20 range I'm not going to complain.

JRKRACE · September 22, 2005

For the life of me. I cannot break 11.5 cityhighway 50/50. Got 285s and a 93 octane tune from PCMS for Less. With the GPS dead on, I can't get more than about 230 miles to a 20 gallon fillup.....

silverado3500 · September 22, 2005

IMO the increased width of the tire is what hurts the mileage (more rolling resistance) more than anything else.

BADGERMAN · September 22, 2005

IMO the increased width of the tire is what hurts the mileage (more rolling resistance) more than anything else.

I agree, but I also think that the additional weight of the tire plays an effect also.

I would be curious to see what the weight of the stock rim and tire combo weighs, I remember right the tire that I put on my truck weighed about 20# more than the stock tire.

Gary8.1 · September 22, 2005

IMO the increased width of the tire is what hurts the mileage (more rolling resistance) more than anything else.

I agree, but I also think that the additional weight of the tire plays an effect also.

I would be curious to see what the weight of the stock rim and tire combo weighs, I remember right the tire that I put on my truck weighed about 20# more than the stock tire.

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I also agree that width and weight of the tire play a large role. I also think your starting gear ration plays alot in it. I replaced my donut 245/s with 255/85/16's because I needed more clearance. I was playing road grader with the bottom of my truck out hunting :-)

I have been very happy with them, but I have 4.10's in my truck and by going to the larger tire size it changed them to about 3.78's. I think if you have 3.73's to start with and you put larger tires on you will change your gear ration to 3.43's or close by putting on a 33" tire. This I think would really kill your milage especially in town driving with all the stop and go.

I am currently getting between 9.5 and 10 in town driving and around 14 highway with my 8.1/allison long bed.