Reduction In Mileage Tire Change 245 To 265

Stevens03 · November 22, 2011

I love it when people post an urgent question and never come back

Wolverine350R · February 22, 2012

I'm going to drag this back up to the top of the list.

I have a 99 Sierra: standard tire size 245/75R16, optional 265/75R16 (I think those are correct). Do you guys think that the factory accounted for the two different sizes when the trucks were made or just put an average in the computer to get the speed and miles? I did go from the 245s to the 265s and never recalled anything, but the spare is a 265 so I assumed that is what the truck originally had.

newdude · February 25, 2012

I'm going to drag this back up to the top of the list.

I have a 99 Sierra: standard tire size 245/75R16, optional 265/75R16 (I think those are correct). Do you guys think that the factory accounted for the two different sizes when the trucks were made or just put an average in the computer to get the speed and miles? I did go from the 245s to the 265s and never recalled anything, but the spare is a 265 so I assumed that is what the truck originally had.

RPO codes in the glovebox show the OEM tire RPO code. You could go to a dealer and have parts or service run it and they should be able to give you the OEM size no problem.

rat454 · March 8, 2012

I started this thread to see if anyone else changed tire sizes (245/70/17 to 265/70/17) and saw a significant reduction in mileage. I fully understand how larger tires can reduce mileage due to increased weight, rolling resistance, speedo effects etc. I expected some drop but not the 2-3 mpg I see. Before changing tire size the speedo was actually a little fast (2-3mph at 30mph). This was confirmed by GPS and speed traps in my area. So when putting the larger tires on my speedo was more correct. I recently got a BB tune and gave Justin the new tire size. I just took my first road trip this past weekend and the truck got slightly better (with the new tune + 0.3 mpg) mileage but still down compared to when I had the stock size tire. The speedo is now within 0.2 mph compared to GPS. Could it be that the new tire I have just has a very high rolling resistance compared to the OEM tire and that is the major factor in reduction?