06 2500 CTD Quad Cab 4X4 AT. Noticed a 2 - 3 MPG loss of mileage after putting on Toyo 285 X 75R17 AT tires. Overhead and speedometer are off because of larger tires. Mileage hand calculated using GPS mileage. Any idea of why the loss of mileage? Truck serviced regularly and all flash updates have been performed.

Thanks
Mike B

 

A some of possibilities come to mind. The bigger tires are a little heavier, the taller tires will increase your truck profile giving more wind resistance, The rolling resistance on the tires may be more than other tires and your GPS may not be as accurate as you think it is. The GPS may be measuring points every 100' ( for example) and missing the finer points giving a shorter distance. Or it could be all the above. I know tires take some time to get the internal bands to begin performing as designed. Just some ideas .