Bigger Tires impact fuel milage?

[Jungda99]

Hello,

I have a 06 2500HD silverado Diesel which will need tires soon.

It currently has the stock 245s on it which look cheezy to me. I have had BFG AT 265s on my half tons and was thinking of putting the same thing on this truck or even 285s.

How much will the bigger tires affect my fuel mileage? I think 285s would look the best of the three but I don't want to loose 2 mpg.

I have heard rumours of the 6 liter losing 2 MPG with 285s on.

I know I will have to reprogramm the computer to match the tires size so the shift points and MPH are corret.

Thanks much

[Tyler23]

but if your a truck is a duramax i wouldnt think 285s would do very much to your milage. my boss had a 07 duramax he went from the stock tires to 265 and he claims he got better milage.

[Jungda99]

yeah I had another guy say his ford powerstroke got better mileage with the bigger tires.

anyone else have any insight?

[deadeye]

It depends. Its all up to physics.

If they are a lot wider, it increases friction with the road and could hurt mileage.

If they are just taller but the same width, it will keep the rpms lower for a certain speed, thereby in theory getting you better mileage.

You also have to factor in that with more tire you also have more rotating mass. (a bigger heavier tire)

So a lot of the time its a wash. Most of the time when people go a size up in tires, its wider and taller. You gain the lower rpm's but increase the friction with the road and more rotating mass.

[Wildabeast]

Wider, taller tires will decrease mileage (the added weight can be a killer). A lot of DMax owners report a 4mpg drop with 285's. You may also need to modify the front a bit for 285's to clear (depends on rims and your truck), and you may be tempted to crank the keys a bit.

If you want a taller tire (same height as a typical 265) then look into a set of 235/85 16's. They'll mount on your rims, have roughly the same rolling resistance as the 245's and the height of the 265's. You'll also find that they probably fit without rubbing.

[LMITOUT]

255's would work too, but they don't have BF A/T's in that size.

285's are technically too wide for the 8-lug wheels and some tire shops won't even mount them, but they will fit. (The half-ton's have a 1/2" wider wheel). The bad thing is the sidewalls bulge too much due to the narrow wheel and you have to watch out for the center of the tire wearing prematurely if they are overinflated.

[Wildabeast]

235/85's are properly sized for the stock 16x6.5 8 lug rims and have the same height as a 265.

[311Hemi]

It depends. Its all up to physics.

If they are a lot wider, it increases friction with the road and could hurt mileage.

If they are just taller but the same width, it will keep the rpms lower for a certain speed, thereby in theory getting you better mileage.

You also have to factor in that with more tire you also have more rotating mass. (a bigger heavier tire)

So a lot of the time its a wash. Most of the time when people go a size up in tires, its wider and taller. You gain the lower rpm's but increase the friction with the road and more rotating mass.

I agree with this.

I would expect less of a difference with a diesel, but some of that might depend on your gear ratio and running RPM's. I run 35" BFG AT's (315/70/17) which are 12" wide on my factory Dodge rims (8" wide) and I get about the same mileage that I would expect with smaller/thinner tires, maybe even better especially on the freeway. I can run at highway speeds (70 mph) below 2000 RPM's which helps a lot. If I had smaller tires I would have to run higher RPM's thus more gas. I got about 75k out of the first set which I just replaced even though they were mounted on a narrower rim than recomended.

My current truck (2003 Dodge Cummins) has enough torque to handle the large tires, but on nearly the same truck (2003 1500) with a Hemi and the 35" tires it killed my mileage. I was not able to run 75mph very easily, and the truck was always wanting to downshift to keep the speed up. Part of this was probably due to an unfavorable gear setup for the tires I was running, but non the less the truck had a lot more trouble keeping up speed than the diesel......and my mileage was horrible because of it.

[superbee]

I have a 2003 Dodge 2500 diesel. I had 265's on the truck and went to 285'5. I actually loose about 2 mpg under most driving circumstances. The tires are the same width, I contribute it to the difference in weight of the tire. It will definately depend on the gear ratio in your truck and the driving you are doing.

[311Hemi]

I have a 2003 Dodge 2500 diesel. I had 265's on the truck and went to 285'5. I actually loose about 2 mpg under most driving circumstances. The tires are the same width, I contribute it to the difference in weight of the tire. It will definately depend on the gear ratio in your truck and the driving you are doing.

What was your before and after mileage?

No way should that change make that kind of difference, those tires are not all the different and the weight can't even be noticeable to a diesel. If anything I would expect your mileage to possibly go up a bit because of the lower RPMs your running on the highway. Did you have your speedo recalibrate for the bigger tires? Bigger tire means your speedo is reading slower....and your logging less miles than you are actually making. If you didn't recalibrate...run a GPS to figure out how much your speedo is off and add that to your calculated mileage. That tire change would probably decrease your speedo by about 5-7% under actual.

[deadeye]

I have a 2003 Dodge 2500 diesel. I had 265's on the truck and went to 285'5. I actually loose about 2 mpg under most driving circumstances. The tires are the same width, I contribute it to the difference in weight of the tire. It will definately depend on the gear ratio in your truck and the driving you are doing.

285 is 20cm wider than a 265. So it is a wider tire. Thats why you lost some mileage.

The aspect ratio is what makes it taller.

I.E. for a 265/75-17, taller would be 265/85-17.

The middle number is what makes it taller. Its the ratio of sidewall to the tread width. So a 265/75-17 would have a sidewall thats 75% the size of the tread width. The higher the aspect number the more sidewall you got.

[LMITOUT]

The difference in the total miles driven between the two sizes of tires (265 and 285 in this case) is negligible and won't have a great effect on the mileage if the speedo isn't calibrated. On a 200-mile tank it's maybe 5 tenths of a mpg. The more miles you get out of a tank the greater the error, but it won't be 2mpg worth unless you can go 1000 miles before filling up.

A large contributor to the loss of mileage is the added weight of the larger tires. It doesn't matter if it's a 500HP diesel or a 250HP gas, it's more unsprung weight that the engine has to rotate. Sure, the diesel will handle the larger tires better because it has more power, but it's still more weight than it had been turning before and therefore requires more energy (fuel) to do so.

Another factor causing the decrease of mileage is rolling resistance which is obviously greater with the wider tire.

[311Hemi]

In comparing these sizes of tires (using two of the same tires) it is about a 3% difference between the 265 and 285 based on revolutions/mile. So, roughly 3 miles difference over 100.......still less than I originally estimated if I am doing my math right.

So you think if I went from 18-19 MPG with my 35" BFG AT's (315/70/17) that I am currently getting to a 265/75/17 I wound increase my mileage by more than 2 mpg because of the weight decrease? On the freeway as it is I see 18-19 mpg.....maybe I could get 20-21 fairly easily by running smaller/lighter tires.

Just comparing a few tires, in same brand AT's so it might not be what this poster has seen:

265/70/17: 31.8" diameter 10.7" wide

285/70/17: 32.76" diameter 11.5" wide

315/70/17: 34.5" diameter 12" wide

I just don't see how you could lose 2mpg with such a small change in tire size....but maybe you can. I would be curious to see if he would lose 2-3 mpg more if he ran the tires I am running which are 2" taller. Maybe the 315's are big enough to lower the RPM's enough to make it a wash over the weight of the tires.

Still curious to know what he is actually seeing before and after the tire change. If he was getting 20+ on avg then maybe that could be the case, but I don't think he would drop of 18mpg to 16mpg hwy in the truck unless something else is going on.

[BobT]

One thing that can fool you too. With larger diameter tires, your drive train will be turning at a lower rpm to go the same distance. Your odometer is driven by your running gear and so will record less distance traveled than reality. Speedometer will also show slower than real speed. Something to keep in mind when you get that speeding ticket for some reason.

Anyway, since your odometer is recording fewer miles per gallon it doesn't necessarily mean you are getting fewer miles per gallon. Engine rpm will be lower at the same speed, which may actually improve your mileage and offset the mass factor mentioned earlier. There are ways to calculate an estimated change but I'm certainly not qualified to do it.

[Redlantern]

I admit I don't know the first thing about mechanics but I find this thread interesting. How would the aerodynamics be affected by having taller tires? I had a truck that seemed to get better mileage with a heavier load in the bed. My neighbor (a diesal mechanic) suggested it might be due to the bed sitting lower than when it was empty.

[rcatcherman]

When I run Michelins I get the best mileage. Same size different make loss of mileage. Thats where I would check. I get between 25 and 30 thousand more miles with my michelins also. Plus the ultimate traction on snow and ice plus heavy rains. And the cost is usually only 10 to 15 dollars difference.