I bought the 2007 GT with the stock 17" bullitt wheels and tires.

Then I put some 20" Ronaele wheels on (8.5" wide in front, 10" wide in back)
with low profile Toyo tires.

since the wheels are bigger, do the speedometer and odometer need to be adjusted?
If I get meatier tires with a larger sidewall do the speedo and odo need to be adjusted again?

If I do need a computer adjustment, where do I go, and what do I ask for?

 

Any change to the stock tire's revs per mile will require a change to the stock setting. It's an easy change with a handheld tuner. My SCT xcal2 allows changes in 10's (i.e. 470, 480, etc).

 

http://www.miata.net/garage/tirecalcold.html
http://www.1010tires.com/TireSizeCal...?action=submit
Rim size does not effect revs per mile or the tire.
Use the above OD calculator to find the revs per mile for both the
stock tires and your new ronaele (eleanor spelled backwards) and
compare the two numbers.
20" rims are bigger but the 'low profile' tires you have
on there should compensate for the bigger rim and give you
the same revs/mile. You dont state tire size so I cant figure out what
you are asking. Not enough info.

 

+1 to the above comments. you need to check the revs/mile of the outer tire diameter and go from there. a tuner will do the adjustments.

 

Wow, I never caught that!

 

Yeh those two sites are complety differnt do a 274/40/18 and its TWO differnt numbers on each site one says 780 rev the other 756 ...big deal cause at 60 ill be really doing 70.. SOOOOOO which site do i trust ..??

 

Go to the manufactures site and get the size rev's per mile. Or give me the model and size and I can see if I can find it for you

 

Here you go
Sumitomo High Performance HTR Z II Tire (275/40-18)

 

783 rev's per mile.

773 stock pirelli's

759 stock BFG's

Don't forget to adjust for those gears.

 

can you post that link were you found so in the future I can grab it again when I do tuning adjustments ! Thanks sooo much ! my tuner has it set at 726 right now ! LOL