Thanks for all the help! This means I got 21mpg coming back from OK to south TX last week not bad!

 

So wait a minute - are you guys saying that the dealer can adjust for tires LARGER than 285's on trucks earlier than 06? I thought that's what I just read....

Oh - here's a cool little calculator for determining tire sizes / rotations, etc.

http://www.miata.net/garage/tirecalc.html

 

Yep! My 04.5 has 315/70-17 (essentially a 35" tire) & the speedo/odometer is calibrated for them.

Actually thought about not doing it so I could get a 109K mile warranty.

 

Spend a few $$ and stick a GPS on your dash... It will show actual mileage, compared to the calculated mileage shown on your overhead/odometer. This will give you a true reading of MPH and MPG, regardless of tire size.

 

Day to day, sometimes I don't turn the GPS on, or have it tucked away, so it misses mileage. There are times when the signal is too low to pick up, so the unit just guesstimates. The truck odometer is always working I think.

I don't have a whole lot of confidence in consumer GPS model history records, especially when I notice a record that I was traveling over 175 MPH sometime in my recent past... although there was that flying saucer incident.