stellar_bc

i heard that when you change stock rims to a bigger one (17", 18") that the speedometer will not be as accurate...has there been a solution to this?

sage11x

What are you going to? You can often calculate this yourself... or go to tire rack and they'll tell you.
example: I replaced my stock 195/50 16's with 205/45 16's. This tire has a smaller rolling radius than the 195/50 (by 1.9%). So when my speedo says 60mph I'm really going 61 mph... big whupadee doo.
The point of plus sizing is to stay as close to the original rolling radius as possible (within 3% actually). Plus 1 size for our car would be 205/40 17". Bigger wheel, smaller tire, same overall size--or rolling radius. That should keep your speedometer accurate.

Hope this helps.

AzianDragonz

You can change sizes without having to recalibrate your speedometer by getting low profile tires. This will keep your overall rolling radius the same. Just remember though that with the low profile tires you will feel evey bump in the road. My opinion, get some nice looking 16's and drop the car, it'll look sweet.

rustychops41

Sage11x, do you mean 61 indicated is 60 actual? You said new tire is smaller diameter. And a BTW for everyone, once you get past 17" wheels (maybe 18" at the very most) there is no tire low-profile enough to keep you anywhere close to accurate on the speedo. With 35/30 series tires you are asking for trouble unless you only drive car on and off the trailer at car shows.

03ESPro

If his 205/45 is a smaller diameter, then his SPEEDO WILL READ FASTER than he is actually going.

rustychops41

FYI the "cheesy tire diameter formula"..... section width x aspect ratio x 2, divide by 25.4 + wheel diameter. This'll get you close enough for Government Work

goldstar

Just go to: www.miata.net/garage/tirecalc.html

rustychops41

Goldstar: how's that Red-Line gear oil working out? I've thought about it, but it ain't cheap. Many auto-xers swear by it. I'm due for a gear-oil change anyway. Yay or Nay?!