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Reduced fuel mileage

Since buying new tires a few months ago, my gas mileage has dropped by 20-25% . I went from OE size of Cooper? P245/70-17 to Mastercraft 265/70-17 AT2 for better mud/snow traction since I live on a farm. It is an 06 GMC with 4.3 V6 auto trans, 2 wheel drive. I have not changed my normal driving habits, or carry excess weight. I have about 64K miles on it and I regularly maintain it. Are the new tires heavier and may be cause. Thanks

did you recalibrate your speedometer? The new tires are about 1 inch taller than the previous size. You also effectively changed your rear end gear ratio as well. Since you have the small engine it has to work harder to move the truck since there is now a bit less mechanical advantage.

It sounds like the new tires have a very aggressive tread pattern. If they are making a roaring noise going down the pavement, the extra fuel consumed is going into moving those tires. After a long drive you might feel or use a remote sensing thermometer to measure the temperature of the tread to get an idea of how hot it is getting. You can then reference that temperature to a more economically rolling tire – like on a Prius.

I have always wondered the gas mileage penalty of the big tired, 4WD raised pickups, roaring around city streets versus a highway tired, 2WD, similar stock pickup. Hope this helps.

For Fodaddy: I think It may have changed the rear end gear ratio, but would I recalibrate my PCM to compensate for tire size. I think I heard that it changes how the trans shifts and change engine timing
For Researcher: I do hear tire noise more so than before when I had the previous set. I will try to hand feel the tires after a long trip. Thanks to both of you for your help. And I am still open for any and all advice.

This is probably normal. Changing to a larger tire also adds weight and there is more rolling road resistance for the truck to overcome on the highway. This is in addition to the other previously stated reasons.

Thanks, Missleman, that does give me more ideas that can help me solve this problem. After all, gas aint cheap.

Are you sure that your mileage is down that much? You are now traveling further (and faster) than your odometer says you are, so unless you are using to GPS to determine how far you have traveled between fillups, your mileage calculations will be off. This also applies to any mileage readout your truck might have.

Thanks tardia: I am comparing my speed and distance with a Garmin 205 GPS. I noticed that both the gps and speedometer match almost perfectly, as before the new tires, the speedometer indicated 2-3 mph less than gps, After about 140-150 miles the gps reads 4 miles more that odometer. I can actually see a very noticeable difference even on a short trip on my fuel gage (less than 50 miles.) I used almost 1/2 tank in a 150 mile trip. my truck holds 26 gal. Any help from any one is deeply appreciated. THANKS

There’s no “may have” about it. It did change the effective gear ratio. Not dramatically, but noticably. This could cost 2-3 MPG depending on the type of driving, the new tires are a bit heavier and if they have more aggressive tread that could cost another MPG or so, and when you take into account that your odometer speedometer are now reading about 4% too slow and undereporting your distance traveled by the same amount that could account for the rest. You’re probably not actually losing as much mileage as you think you are, but you are losing some.

I don’t see how they can be 1 inch taller, they went with a wider tire, not a taller tire. Aspect ratio and rim size stayed the same, the width was what changed by 20mm, 2cm, or 0.79 inches

I don’t see the tire size making that big a difference in mileage; assuming these new tires are not underinflated.

Put aside the GPS and fuel gauge for a while and check the mileage with the tripmeter/gallons used method and see what happens; unless that is what you mean by the 150 miles trip and half of a 26 gallon tank comment.

Other than that, it might be time to scan the truck for any codes. If the mileage is really suffering that much there should be a code or two present. AutoZone, Checkers, etc. will do this for you free.

Aspect ratio 70 means tire height is 70% of tire width.
So 70% of 20mm width increase is 14mm, increase of tire diameter is double this: 28mm.

I think there is a tire size recalibration field in either the PCM or BCM that can be used to regain spedo accuracy for this degree of change.

I think you have three factors combining to hurt your mileage. Two have already been mentioned.

  1. you’ve gone to a more aggressive tread, increasing rolling resistance.
  2. you’ve increased your romming circumference, slightly changing your final ratio for a bit less efficiency getting the truck up to speed
  3. the larger wheels and tires have more weight, well away from the rotating axis. It takes more energy to get more weight spinning up to speed.

Hold a “roadie” bike wheel and tire up and spin the wheel with your finger. Now do the same with a “mountainbike” wheel and tire. You notice it takes much more force to get the mountainbike wheel spinning. You can easily feel the difference. The same physics are at work in your truck. Accelerating a heavier wheel and tire simply takes more force.

You didn’t reduce the mileage as much as you totally screwed the data you are using to figure the mileage as now both your speedometer ans odometer are both off.