Guys, I'M going from the stock 5 spoke GT wheel diameter of 27.3inches to a diameter of 26.5..do i need to Reset my speedo...this is with the 265/40-18 MT drag radial most folks use......

 

Need to?

Only if you want the speedo to be dead on, but then of course it will be off again when you put the stockers back on.

If you have a tune on a SCT tuner it is a simple change, otherwise it isn't an easy change.

 

Thanks, I WILL most likely be swiching back and forth a lot, wonder how far off that 3/4" will make it????

 

About 3.5%. You're looking at 740 revs per mile stock vs. 766 revs per mile on the drag radial.

 

Meaning?? Needs Reset, will only be off by 1-2mph??

 

Here's a Q about the SCT: can an SCT be borrowed and used solely to change the diff gear cal? I ask b/c a friend has one for his Lightning and he'd be willing to loan it to me but doesn't want to lose his stock tune, which presumably is stored within his SCT.

In other words, does the SCT become more or less dedicated to a particular car once its been used to download a modified tune to that car?

 

No, they can only be used for one vehicle at a time.

 

Don't worry about it.

The shorter tire will just make the speedo read faster than normal.

 

Once the Stock tune for the other vehicle is in it it can not be used until the original vehicle is returned to stock, So even putting it on to change gear ratios on yours it cannot be used on the Lightning until your is returned to stock.
So yes it is dedicated to one vehicle.

BTW, Import your tire size into http://www.miata.net/garage/tirecalc.html

 

Your speedometer will read about 3.5% higher than actual speed. For example, the speedometer will read about 72.5 when you're actually going 70. Probably not enough of a difference to worry about.