Reduced tire size and shortened lift caused large drop in gas mileage

[true.believer]

I have a 2018 Silverado 1500 with 5.3L engine. I originally had a 7.5" lift (leveled front and rear) with 35" diameter / 12.5" width tires on it, with a recalibrated speedometer, that was consistently getting 14.6 mpg.

 

About a month ago I reduced the front end lift to 6" (kept the original rear lift at 7.5" so that the truck resembled more of the factory pitch forward) and installed a smaller set of tires that are 32" diameter and 11.5" width (kept the same wheels). After recalibrating the speedometer I am now consistently getting 13.2 mpg under the exact same driving conditions.

 

Of note is that with both sets of lifts/tires, I used external measurement methods to verify that my speedometer and tripometer were reading correctly after the recalibration.

 

I have also forced a throttle body reset to try to get any of the automated computer fuel controls to reset, but it hasn't made any difference.

 

I was expecting my fuel mileage to at least improve a little bit, but cannot think of any reason why I am seeing a near 1.5 mpg decrease in fuel efficiency with the lower lift and smaller/lighter tires. Can anybody help me figure out what I am missing here?

[M1ck3y]

On 3/16/2022 at 4:14 PM, true.believer said:

I have a 2018 Silverado 1500 with 5.3L engine. I originally had a 7.5" lift (leveled front and rear) with 35" diameter / 12.5" width tires on it, with a recalibrated speedometer, that was consistently getting 14.6 mpg.

 

About a month ago I reduced the front end lift to 6" (kept the original rear lift at 7.5" so that the truck resembled more of the factory pitch forward) and installed a smaller set of tires that are 32" diameter and 11.5" width (kept the same wheels). After recalibrating the speedometer I am now consistently getting 13.2 mpg under the exact same driving conditions.

 

Of note is that with both sets of lifts/tires, I used external measurement methods to verify that my speedometer and tripometer were reading correctly after the recalibration.

 

I have also forced a throttle body reset to try to get any of the automated computer fuel controls to reset, but it hasn't made any difference.

 

I was expecting my fuel mileage to at least improve a little bit, but cannot think of any reason why I am seeing a near 1.5 mpg decrease in fuel efficiency with the lower lift and smaller/lighter tires. Can anybody help me figure out what I am missing here?

 

I'm not sure if this directly applies to you, but larger tires will get better mpg on the highway and worse mileage in the city. Obviously tire weight is a factor as well, but I THINK size is the main contributing factor.

 

A larger tire will effectively raise the gear ratio, allow's for a further distance traveled for any given rpm (longer contact patch), less rolling resistance (increased diameter) etc. 

 

It MAY also be possible that because you have big lift with basically stock tires, your wheel well opening's could be creating additional drag/turbulence etc.