gtbguy was pointing this out before he went offline...
i went from a 195/60/15 ls wheel/tire combo to a 205/55/16 gt setup. have i gone too big? he says i should be 205/50/16?

ben :confused:

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

You can check out the the differences here. Personally, I would have gone for the 205-50/16's because it has a lower profile! ;)

the 50/16 is 0.6% too fast, where as the 55/16 is 2.7% too slow. The 55/16 should be alright since it's within a 3% variance.

LaterZ!
Darren!!

am i correct in assuming that 16" sizes were only available with the ej25? what was outback sizing?

ben :)

Benjamin: It's no biggie.. Now you would have a reason to buy the Apexi Rev/Speed meter to correct the speed of your vehicle! :p

Penphoe - you have it a little mixed up, a larger diameter will tend to cause your car to be moving faster than it would be indicated on your speedo.

Ok Here's the Calcs :

195/60/15 :

195mm = 7.6774676in
A/R = 0.60

(7.6774676)(0.60)(2)+15 = 24.212961" (OD)

205/50/16 :

205mm = 8.0711839in
A/R = 0.50

(8.0711839)(0.50)(2)+16 = 24.071183" (OD)

205/55/16 :

205mm = 8.0711839in
A/R = 0.55

(8.0711839)(0.55)(2)+16 = 24.878301" (OD)

195/60/15 -> 205/50/16 = (-)0.58554% vs OEM Legacy L O.D.

195/60/15 -> 205/55/16 = (+)2.7479% vs OEM Legacy L O.D.

Hence, when you're travelling 100km/h (speedo) using the 205/50/16's you'll actually be travelling (reality) 99.41446km/h

For the 205/55/16, you'll be travelling 102.7479km/h (reality) vs. 100km/h (speedo)

GTBGUY -- I just copied what I saw at the miata.net tire calculator. In any case, they're both off by just a tad.

LaterZ!
Darren!!

ps -- thanx for the more thurough mathematical explaination! :D

if i bring a copy of this, which i be able to use it in traffic court?

i'm sorry your honor but my tires created a 2.7479km/h variance....

ben :)

Ben - actually when I had my 1st car, I got a "photoradar" ticket of me going 165km/h + on Deerfoot trail... I had tires which were about 5% larger in overall diameter... they took my car to a dyno and then gave me the print out, verifiying that my speedo was off, displaying only 157km/h. But in anycase, this was in a 100km/h zone, so needless to say my fine was quite hefty! thank god however that it was photoradar and not a real cop!

u guys have an awd dyno there? wow...

ben :)

Ben - my 1st car was FWD not AWD :)