Legal tyre/rim diameter size

by **teflon** » Wed Apr 13, 2011 10:57 pm

Was wondering if I could get some advice.

I'm changing from my stock 17s - 215/45/17 tyres to the Spec B 18s. I wanted to get 215/45/18 tyres to maintain the same tyre width for comfort. However, in qld (and I also believe nationally) from what I understand of the laws, you can only go up by 15mm max in diameter legally.

http://www.infrastructure.gov.au/roads/ ... 1%20v3.pdf

Is this what others understand? Has anyone here run 215/45/18s when they've changed from their stock 215/45/17 tyre and rim combo?

EDIT: The spec b from the same year got 215/45/18s, so I assume it would be legal!

by **smythie** » Thu Apr 14, 2011 10:22 am

I would suggest going to 225/40R18 rather than 215/45R18. You only increase diameter by a nominal 12mm and you get into a MUCH cheaper size range with MUCH greater option of tyres. Only issue for me is the compromise in comfort inherent with dropping profile (from the already comfort compromised 45 series IMO) but for most that is not an issue.

by **tangcla** » Thu Apr 14, 2011 11:47 am

www.miata.net/garage/tirecalc.html

by **teflon** » Thu Apr 14, 2011 12:11 pm

Thanks guys... saw a few online calculators tangcla... my diameter will increase by 4%... my speedo according to my gps is out by about 5% so overall the speedo should be ok.

The 45s will fill the guards a bit better especially once the billies are installed. Have decided to go for the 45 profile for now (got a good deal), but when new rubber is needed will probably opt back for the 40 profile as suggested to increase my options.

by **Gambit** » Fri May 20, 2011 5:13 pm

guys a question along similar lines, will it be an issue to fit 245/45/18 onto 18x8.5 +43 offset with standard bilsteins and springs? Or is that going to be too wide a tyre and cause scrubbing?

by **Adrian2627** » Fri May 20, 2011 5:22 pm

I'm running 235/45-18 on and Outback and only just have enough clearance so the 245 on a Liberty will most likely scrub inside and out especially as you near full lock.

by **Gambit** » Fri May 20, 2011 5:37 pm

Yeah thought so..bugger

by **coyote** » Fri May 20, 2011 6:58 pm

Gambit wrote:guys a question along similar lines, will it be an issue to fit 245/45/18 onto 18x8.5 +43 offset with standard bilsteins and springs? Or is that going to be too wide a tyre and cause scrubbing?

I think you'll be fine on the inside (tyre won't touch the strut body), but it will most definitely be well outside the guard.

I doubt either would matter as it would probably not even get under the spring perch.