I've been reading these posts for awhile now, I just haven't gotten around to registering. Now that I have, I'd like to ask about everyone's experience with changing the tire size on their Expy. I bought new tires late last week and went from the standard 255/70/16 to 265/75/16. Since these are basically the same size as the tires on the vehicles equipped with the 17" wheels, I didn't expect to see any significant change in performance or gas milage. Well, I was right about performance. Can't feel any difference. But on our out of town trip this weekend, our highway milage dropped from 16.5 to just under 14 mpg! No more wind than normal, no extra weight, 75 mph. I can't imagine the tires would have made that much of a difference. Has anyone else had a similar experience? We won't be going anywhere for several weeks, so I'll be watching the milage in town to see what it does.

 

Are you calculating mileage or reading if off of the trip computer. If the overall tire diameter has changed, you may need the dealer recalibrate the ECM for the new tire size. This will also correct the reading on your odometer. The larger diameter tire will travel less revolutions and your odometer will show fewer miles covered for the same amount of gas.

255-70-16 30 in 694 rpm
265-75-16 31.7 in 657 rpm

 

Yeah, I accounted for the change in the speedometer and odometer. Those numbers were based on driving a true 75 and actual milage.

It's not a big concern if the milage does drop because I'll just deal with it. But I'll always be wondering why it changed so much. Maybe I can use that to convince my wife we need to start making some mods to get more power.

 

That's a pretty big difference in tire height. I would bet that's what it is. Only way to fix it is to buy a Superchip and have it programmed to correct the tire size. Mod number 1

 

I second that notion!