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HW Offline
#1 Posted : Tuesday, 24 October 2023 7:54:22 AM(UTC)

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Hi everyone,
Just registering XC after recent rego lapse. My XC has had ROH 15" wheels from a Ford Falcon GLI since 2003. Tyres are Mastercraft LSR 215/65R15's. Wheels were no problem with usual rego since 2003, but this time around I need blue slip for the first time ever. Perusing the regulations, they state that with a wheel change from original only 7% extra diameter allowable.

Anybody know what percentage my current 15" wheels above original 14" would calculate to?
HK1837 Offline
#2 Posted : Tuesday, 24 October 2023 8:44:07 AM(UTC)

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Find out what the largest diameter optional tyres were on an XC and either work it out yourself or use an online calculator. Pretty sure those XC coupes with the blue stripes had 15" wheels so they would have had ER70H15, and I guess some of the other XC were optional with these too.

I just had a look and most online charts say that E70H14 (the added R means Radial) is equivalent in diameter to 195/75/15 or 205/70/15. So a 215/65/15 should be similar diameter.

Formula for diameter using the format XXX/YY/ZZ is:

(2 x (XXX) x YY/100) + (ZZ x 25.4). So using 215/65/15 gives you 279.5 + 381 = 660.5mm. The 279.5 is the width x profile x 2 (the x2 is for the tyre at the top and bottom of the wheel). 381mm is simply 15" x 25.4mm.

If you do that for 195/75/16 you get 673.5mm. 205/70/15 is 668mm. Actual tyre sizes may vary from these though and they will vary between bias ply and radial especially with growth at high speed.

If you look at the % difference between 215/65/15 and 195/75/15 you only get about 2%.

If you Google hard enough you'll find published data for some tyres, below is a 205/70/15 which is 26.5" or 673.1mm:


Here is the only E70-15 tyre I could find, remember it is a bias ply, diameter is 26.73" or 678.94mm:


Here is published 215/65/15. It has 26" which is 660.4":


Comparing the published data for above E70-15 and 215/65/15 the difference is the E70-15 is 2.8% larger diameter than 215/65/15.

Edited by user Tuesday, 24 October 2023 8:49:31 AM(UTC)  | Reason: Not specified

If we all had the same (good) taste, who would buy all the Fords?
 1 user thanked HK1837 for this useful post.
HW on 24/10/2023(UTC)
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