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Larger Tires and Loss of Fuel Mileage

RI Chevy Silveradoman

RI Chevy Silveradoman

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A few months back (July 3, 2009) I needed new tires. I thought the stock 245's were a little small, so I decided to go with 265/75/16 General Grabber HTS tires. I had the tires installed, and noticed the ride itself seemed much better, but I could notice a difference in the way the truck drove as far as power. After towing to NH, and ME for a summer vacation, (not a good time to check mileage), I checked my mileage when I got home. I was consistantly getting between 19.5 and 21.5 mpg hand calculated with the stock tires. I was only getting 18-19 mpg hand calculated with the 265's. I used my tuner to adjust for the larger tires. I recently took a trip to Loudon, NH last week, and I only got 19.1 mpg for the round trip (Last year I got 24.1 mpg for the same trip).

I am not totally convinced that I am losing that much fuel mileage by only going up 2 tire sizes to a 265. Is it possible that the mileage on the odometer is off due the the tire size, thus giving me the wrong numbers? The adjustment that I made with the tuner only adjusts for speed if I am correct. For the record, I drive the truck like an old man, and I am very consistant in my driving habits. All of my Preventative Maintenance is done in July (new filters and fluids). Everything was done on July 1, 2009.

Do the RPM's (Rotations per mile) for the tires have anything to do with proper odometer readout? The RPM's for the stock tires are 683 RPM, and the RPM's for the 265's are 657 RPM.

Any help or insight is appreciated. Thanks in advance. :thumbsup:
 
I noticed a similiar drop in my 6.5, and am regretting going with 265's now with the DMAX. I took a big hit in the power department with them although the ride was much improved. I haven't had a chance to check milage, but my EGT's have gone up some around town driving which normally isn't a good sign for milage.
 
Do you happen to know the weight difference between the 245 and 265 series tires? If there is a big enough difference, that could account for the loss.
 
Do you have a tuner to correct the speedo?

It will defiantly be off.
 
8100 Power said:
Do you have a tuner to correct the speedo?

It will defiantly be off.
Click to expand...
:thumbsup: Exactly!

I went to 255/85/16s on my dually and the speedo and odometer are definitely off! My speedo is off by 6mph and a trip back home to W.Va. is usually 525 miles from my door step to my mom's house, with the new tires it is now 470 miles. That is a 55 mile difference about 10%. My new tires are 3 inches taller so I expected things to be off, yours are only about 1.5 inches taller still enough to skew your results when figuring mileage using your odometer reading.
JP
 
I agree it will be off somewhat Jeff. If you have a GPS compare all your number used for calculations and see what is off.
 
What tune are you running Jeff? Your speedo will definitely be off. One thing though with these trucks and well, any vehicle, is that when you change tire or rim sizes, your speedo and odometer will be off.
 
Your odometer readout is now displaying/logging about 4% less than it was with the 245's. I think if you use a GPS now, you will find that your only displaying/logging about 1% less than actual miles traveled with the 265's.
 
Mileage

RI Chevy Silveradoman said:
A few months back (July 3, 2009) I needed new tires. I thought the stock 245's were a little small, so I decided to go with 265/75/16 General Grabber HTS tires. I had the tires installed, and noticed the ride itself seemed much better, but I could notice a difference in the way the truck drove as far as power. After towing to NH, and ME for a summer vacation, (not a good time to check mileage), I checked my mileage when I got home. I was consistantly getting between 19.5 and 21.5 mpg hand calculated with the stock tires. I was only getting 18-19 mpg hand calculated with the 265's. I used my tuner to adjust for the larger tires. I recently took a trip to Loudon, NH last week, and I only got 19.1 mpg for the round trip (Last year I got 24.1 mpg for the same trip).

I am not totally convinced that I am losing that much fuel mileage by only going up 2 tire sizes to a 265. Is it possible that the mileage on the odometer is off due the the tire size, thus giving me the wrong numbers? The adjustment that I made with the tuner only adjusts for speed if I am correct. For the record, I drive the truck like an old man, and I am very consistant in my driving habits. All of my Preventative Maintenance is done in July (new filters and fluids). Everything was done on July 1, 2009.

Do the RPM's (Rotations per mile) for the tires have anything to do with proper odometer readout? The RPM's for the stock tires are 683 RPM, and the RPM's for the 265's are 657 RPM.

Any help or insight is appreciated. Thanks in advance. :thumbsup:
Click to expand...

I am still running the Hypertech on Level 3. I did adjust for the tires with the tuner. I am not sure on what that actually does, speedometer or odometer? The larger tires are only 2 lbs. heavier than the stock tires from what I can see (45 lbs. to 47 lbs.) which should not equate to that much of a change or difference.

I appreciate the responses! That leads me back to my original thought that I am not sure my odometer is giving me the most accurate results. I think I am still getting about the same mileage as I was before, due to my driving styles and habits, and the fact that I was on a highway run, with no traffic and the cruise set to 63mph. Although my calculations say I got 19.1 mpg, I probably got around 23-24 mpg. :thinkerg: :confused5:
 
Jeff, you definately will lose some to rolling resistance of the more aggressive tread. I'm pretty sure the Difference is about 4% (mine is 7%+ with 285s) for the 265s. I'm sure I put a tire calculator in a sticky here.
 
I've found the sizes you enter into the programmer are never right. I had to get a GPS and play with the tuner to get the sizes to read the correct MPH.
 
Hello RI Chevy Silveradoman,

Assuming the vehicle's computer has been properly calibrated...

The General Grabber HTS has a less aggressive tread than the stock tires, so, if anything, there should be less rolling friction (loosely corresponds to a quiter ride, too, because there is less "slapping" of the tread on the road surface).

The 265s are 2.5% larger than the 245s, so your final gear ratio is smaller by that ratio (e.g., previously if diff ratio is 3.42, now 3.33). This would translate to your engine working a little harder to turn the larger diameter tires. It has to do with mechanical advantage, much like a shorter lever requiring more force to turn a wheel fixed to an axle (conversely for this application because the engine is turning the axle to turn the wheel).

Remember, because the engine is already "handicapped" by the larger diameter tires, it has to work harder to perform the same functions it used to do with the smaller diameter tires. For example: stop-n-go driving, or carrying additional weight over the suspension, or towing a trailer, or a combination of the three, etc. In these instances, the larger diameter tires will work against your vehicle getting better fuel mileage.

The tires are just under 1-1/4" taller than the stock, so the increase in aerodynamic drag would be almost insignificant.

I would try increasing the tire pressures to a safe value so that they are "squished" less and generate less rolling friction. When you accelerate from a stop or while already in motion, peg the tach to the engine's maximum torque@rpm value until the desired speed is reached.

If you do a significant amount of the above examples (stop-n-go, etc.), either get a taller gear ratio or get smaller diameter tires.

Regards,
Franko
 
Thanks for the responses. I run 60 PSI in the front and 65 PSI in the rear when I am not towing. I figured I would lose a little fuel mileage, but not that much. That is why I am still not totally convinced that I am getting accurate fuel mileage results. I will try and check mileage on the odometer with a GPS and see if I can find anything out and report back.

The trade off with the larger tires is a better ride though! ;)
 
the biggest differance i found with the tires is the weight of the stockers to the weight of the new ones. i had my old truck had 265's on it, i replaced them with my bfg at same size and they weighed alot more. the junk 245's dont have much to them so anything you replace them with will be heavier. i bet you lost a little but not as much as your saying.
 
Check with your GPS. My speedo (and odo) are low by an average of 2mph at anything 50ish and higher. It's pretty near dead on below 50.

Your mileage isn't as bad as you think... I drove to Atlanta last Thanksgiving and very very very nearly broke the 23 mpg barrier using the mileage off of the gps. The odo calculated out to around 20 even. I'm running Michelin 265/75's with no correction to the speedo.

That said, I'm going back with 245's (and was looking at the HTS also ;)). The Michelins have plenty of tread left, but are starting to get squirrely in the rain, plus I never did like the loss of acceleration and funky ride with the taller tires.
 
saratoga said:
Check with your GPS. My speedo (and odo) are low by an average of 2mph at anything 50ish and higher. It's pretty near dead on below 50.

Your mileage isn't as bad as you think... I drove to Atlanta last Thanksgiving and very very very nearly broke the 23 mpg barrier using the mileage off of the gps. The odo calculated out to around 20 even. I'm running Michelin 265/75's with no correction to the speedo.

That said, I'm going back with 245's (and was looking at the HTS also ;)). The Michelins have plenty of tread left, but are starting to get squirrely in the rain, plus I never did like the loss of acceleration and funky ride with the taller tires.
Click to expand...

I've only got about 125 miles on my 265's, but I'm ordering a set of 245's tommorrow. Took a short trip yesterday and just was not happy with the 265's. My EGT's are up, I had pulling under braking now, and the loss in accel really sucked. Luckily I know somebody who has an 05 3500 SRW and needs a new set of MICHELIN's.
 
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