Tire size?

[ahunt01]

Hi, I currently have 185/65R 14 tires on my car, how bad would me speedometer be thrown off if I put 185/60R 14 tires on the same rims?
Also, currently my wheel gap is gone because I lowered my car, if I put the 60R tire on, would that ugly wheel gap come back, and would my car be lower to the ground?
Should I not go down to a 60R tire, or stick with the 65?
Thanks

[bretfraz]

Going to 185/60 will throw off your speedo by over 3%. And yes the gap will show again since you are using a lower profile tire.

Go with a 195/60 tire to maintain speedo and size.

[HellrotCi]

3% speedo error really isn't that much is it? 100 mph on the speedometer would only be off by +/- 3 mph. Under normal driving conditions you'd only be off by 1 mph. I guess the real problem would be the change in in your odometer reading. Wouldn't that register more miles than actually driven?

[ahunt01]

How would 195/60 maintain the speedo and size? I thought the 185 &195 were just tire width, and not the diameter like 65R or 60R. The diameter is what I thought would affect everything.
I've heard from some people that dropping from 65 to 60 wouldn't really be noticable in the speedo, but I did hear from just as much people that it would. I'm equally concerned about the ride height, I like the height and wheel gap now. If I had a bigger wheel gap and then in turn had to lower the car more to make up for it, I would not be able to make it over the speed bumps at my college, so that's my biggest concern.

[TortoiseAWD]

The "60" in a tire measurement of 195/60 isn't diameter, it's "aspect ratio". If you take the width of the tire (195mm) times 60%, you'll end up with the sidewall height in mm. So, to figure the overall circumference of a tire, it's

circumference = (diameter of wheel + 2 * sidewall height) * pi

The diameter of the tire is the diameter of the wheel plus 2 times the sidewall height. In your case, you have a 14" wheel. Let's convert that to 355.6 mm so that all of the untis are the same. Your sidewall height is 185mm * .65, or 120.25mm. So, the overall circumference of your stock tire size (185/65) is

c = ( 355.6mm + ( 2 * 120.25mm ) ) * pi = 1872.70mm

To calculate the overall circumference for the 195/60 tire:

Sidewall height = 195mm * .60 = 117mm
c = ( 355.6mm + ( 2 * 117mm ) ) * pi = 1852.28mm

Because the sidewall is measured as a fraction of the tread width, a wider tire (195mm > 185mm) with a smaller aspect ratio (60% < 65%) can have almost the same overall circumference. In the case of the two sizes we're talking about here, it's about a 1% difference in the speedo reading.

If you change from a 65 to a 60 aspect ratio while leaving the tire width the same, you reduce the sidewall height, which increases your fender gap. It's not much of a difference in this case, though; about a 1/4" difference in the diameter of the tire.

Check out http://www.miata.net/garage/tirecalc.html for a nifty size calculator that will let you compare two tire sizes to see what the difference in diamater and speedo error will be.

Tort

[bretfraz]

Daaannnnggg!!!

Mad propz to Tort for that post. I was too lazy to post that same Miata.net which I used to calculate tire sizes prior to my first post.

[4DSC]

Yeah, big :bigup to Tort for posting all that!

I've found the miata.net tirecalculator to be indispensible, and it's now also hosted on another site..... which I don't have bookmarked for some bizarre reason....

ahunt01, I'm not sure what your motives for wanting a 185/60 tire are, but if it's because a certain tire model isn't available in the correct 185/65, then I'm with Bret and would look at 195/60, hopefully the same tire would be available in that size. Most cars (with a few dumb exceptions) should be able to fit one size wider without issue.

[ahunt01]

4DSC, yeah I was looking at 185/60 tires because there is a little more performance tires in that size compared to 185/65. I never knew I could use 195/60 tires, that definately should open more options for tires.

TortoiseAWD, thanks a bunch for that thread, it helped tremendously

[TortoiseAWD]

No problem . . . glad to have helped. Just sharing some of the useless trivia that clutters up my grey matter these days . . .

Tort