i just bought 17x8.5 ssr gt7's for the front and rear with a 35 offset. the tires i have are 245/40/17 in front and 255/40/17 in the rear. i know i will have major wheel gap, but I also know it is another way to change the final drive gear ratio without swapping it out. can anyone tell me what the main difference would be. i havent put them on my car yet. but i do have advan a048 because i am planning to do both drag and wheel to wheel racing. insights please.. oh yea, whatis the weight of the oem 17s with tires.. please let me know. the gt-7 rims weigh 18lbs. and the tire is 27lbs a total of 45lbs. please let me know. thanks..

also on my 18's that weigh 57lbs. i ran 14.1 stock form all day long. with a 2.1 60'

 

The transmission ratio times the drive axle ratio equals the final drive ratio. You cannot change the ratio with tires: it is what it is. That said...

Tire diameter will have an effect on a vehicle's final drive ratio. As tire diameter changes, so will engine rpm at a given speed. If you reduce the tire diameter, the engine speed increases . By installing shorter tires, the vehicle will accelerate as though it has a higher (numerically) gear without the expense of gear swapping. So shorter tires increase engine rpm and are sometimes an economical way to improve acceleration.

 

thanks davidv, thats what i needed to know.... but how much of a change? if maybe someone can help me with that.

 

Easy, determine the height of the stock tire, and determine the height of the new tires. If the new tires are shorter, then you're getting the effect of shorter gears. Divide the difference by the stock height to get the actual percentage.

For example, the stock tires on my car(g35 sedan) are 235/45/18's; that's a tire height of 26.3 inches(235 millimeters * 0.45 / 25.4 millimeters per inch give you the sidewall height; then multiply the sidewall height by 2 and add the inner diameter of the tire). The tires I'm currently running are 245/40/18's, with a tire height of 25.7 inches; that means my current tires are 6 tenths of an inch shorter. That means my new tires are roughly 2.2% shorter.

Hope that helps

 

there are many tire size calculators online, look around, it will tell you all you will need...

 

google tire size calc, the miata one comes up...i use that thing all of the time

 

thanks

 

my new tires are roughly 3.1 inches shorter than my stock size...

 

i think im doing my math wrong someone please double check.... i with the tire mod i got a 3.66?

info from tire rack
(829revsx3.5fd)/(792revs)= 3.66?
revs per mile on yokohama 829 255-40-17
revs per mile on bridgestone 792 235-50-17

 

3.66FD is slightly more agressive than your current 3.5FD, it will help slightly but dont expect huge gains. In simpler terms you will finish the 1/4 mile a few hundred rpm's higher in 4th than stock.

 

hopefully it can put me into 13's with no mods.... except a better tire...

 

FWIW I ran 13.80 with a 2.18 60fty, so given decent conditions and track you should already be 13's...

 

infineon has a slight uphill at the end... my 60s are 2.176 and 2.178 and usually not higher than a 2.210 ... my best 1/4 is 14.107@99.10mph i have a 6 speed in enthusiast trim. what may i be doing wrong? people with dot slicks and mods have run 13.8s consistent at infineon the day i went. so i dont think im off by too much.