Spiffyjiff

for all you physics types out there...

like some of you have done, i also switched my stock rears from 295/30/19 to 305/30/19. according to my calcs, i guess i gained 6mm in overall diameter? regarding the physics for size only (i.e. holding all other aspects constant - weight, horsepower, etc) my car should hypothetically:

• have a higher top speed
• be less quick
• get better gas mileage

is this logic correct? and with such a subtle difference - <1% change - is it possible to notice any of these? (i could swear i notice the first 2 on the track but just my imagination??)

Nugget

I made the same switch on my C2S and couldn't tell a bit of difference, on the street or on the track. In theory the slightly wider rears should cause the car to exhibit more understeer, but the differences (if any) were more subtle than I could detect.

A useful tool for this is the tire size calculator that the folks over at miata.net wrote:

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

Mike in CA

Your logic is definitely correct. By going with a taller tire, you've basically installed taller gearing in your car. The thing is you almost certainly can't detect a <1% difference on the butt dyno; if you think you can I'd suggest you either have an incredibly precise BD or it is in your imagination.

OTOH, my street tires are 305/30/19 but my track tires are 295/30/18's (going for wider ones next time but that's another story). The 295/18's result in gearing that's almost 5% shorter than stock. The track wheel/tire combo is also quite a bit lighter esp. in the rear. I'm certain that I can feel better acceleration off the corners with the effectively shorter gearing of the track set up.