Will MPG go down that much moving up from 265/75/16 to 285/75/16?

 

You might lose 1mpg. Mine didn't lose anyting measureable. You'll have to take into account that your speedomenter is a little bit slow now, so your actually putting more miles on your odometer reads.

 

It depends alot on what kind of driving you do also. If you are doing alot of stop start driving it will have more impact. I lost like 2mpg when I just drove in town but saw little or no diff on the highway and maybe 1 if I did a combination.

 

It is complicated.

If the new larger diameter tire
is built to have lower rolling resistance
(more and more tire designs are being improved)
you might even pick up a MPG or two.

http://www.us.pirelli.com/en_US/brow...resistance.jpg

The larger diameter tire also affects the gearing so that you turn less
rpms per mph with the engine
which will slightly improve MPG on flat and downhill highways
but cause the transmission to downshift more often on hills
...a common complaint on forums from annoyed pickup owners.

However, working against you are these 'bad' changes:

1. the new tire probably has a deeper tread that will increase rolling resistance until it wears then with time and mileage.
2. the taller tire slightly lifts the truck which lets more air at highway speeds
hit the rough underside which increases aerodynamic drag.
3. if the new tire is wider this also increases aerodynamic drag...especially if it goes outside the fender opening.

 

[quote]ORIGINAL: Cereal Killer

You might lose 1mpg. Mine didn't lose anyting measureable. You'll have to take into account that your speedomenter is a little bit slow now, so your actually putting more miles on your odometer reads.


So you are saying a vehicle won't accurately measure miles on the vehicle when tires are oversized??

 

Not unless your speedometer has been corrected to compensate. Otherwise your speedometer is reading slower than you are really going, and your odometer doesn't have has many miles on it as your truck truley has.

 

Yikes. If I went to the dealer, they would be able to see if my odometer was correct? And if it wasn't, could they fix that? I would hate to know my truck wasn't getting accurate miles

 

Your might already be fixed since you bought it with those tires on it. Test it against one of your friends. If it's not I'd leave it alone and then fix it right before you go to sell the truck. Hopefully you'll get some extra money for the "low mileage"

 

The speedo can be correctly set by a properly trained technician at the dealership if he follows this
(not well known) set of instructions:
------
originally posted by grfraz
Augusta GA

Here is the recipe.

1) Have the tech plug his hand held into the OBD port and turn it on. A menu
screen comes up with a bunch of info. Select YES to go to next screen.

2) On this screen select menu item 1, "DRB III Standalone" to go to next screen.

3) On this screen select menu item 1, "1998-2005" to go to next screen.

4) On this screen select menu item 1, "All" to go to next screen.

5) On this screen select menu item 5, "Antilock Brakes" to go to next screen.

6) On this screen select menu item 9, "Miscellaneous" to go to next screen.

7) On this screen select menu item 2, "Set Pinion Factor" to go to next screen.

8) On this screen there are a several tire sizes and Other. Select menu item 8,
"Other" to go to next screen.

9) On this screen there are two items,
an Axle Size, (9.25" on the 1500's and 10.5" on the 25/3500's)
and Other.
Select menu item 1, "9.25" Axle" (or 10.5 if your case)
to go to next screen.

10) You have now arrived at the point in this process where you will enter the
revolutions per mile for your tire size.
Select the menu box for RPM.
It will indicate 400 revs per mile initially. Here is where the tech will change the revs per mile with either
the up or down arrow. It will take a while to go from 400 to 604 (for the BFG
315's) so be patient. As you are waiting on him to finish, take this time to let
him know that he shouldn't charge you more than say $20.00, especially since you
trained him on this procedure.

11) The last step is to select "OK" and the changes will be excepted, then the
hand held will display "Reprogramming Successful".

Good luck!

======
There is an online Java calculator to compute approximate
revolutions per mile at:

http://pw1.netcom.com/~sgalaba/tiresize.htm