Brian Lacey

The local Subaru specialist told me that the tyre for my Forester T/tb new 17"wheels was 225/45/17 but these have a rolling circumference 8cm less than the original 215/60/16. I thought the overall diameter and circumference had to be the same in order to maintain a correct speedo reading? Has the specialist given me the wrong size? Anyone know?
Thanks
Brian

kart_man

Check out http://www.tyresave.co.uk/tyresize.html

They recommend no more than 2.5% differance and this page lets you put in both sizes to compare.

Jolly Green Monster 2

In a rush but I think perhaps you have worked out the rolling circumferance wrong..

215/60/16 = 16inches for the wheel + 60% of 215mm = 129mm

roughly what 30mm to an inch.. so 16 * 30 + 129 = 609mm

225/45/17 = 17inches + 45% or 225mm = 101.25

17 * 30 + 101.25 = 611.25

so 16inch wheels have a rolling circumference of 609mm and the 17 inch wheels 611.25mm

I have used roughly 30mm to an inch tough so could get more accurate if you wanted..

The key and I still find it amusing is the profile is a percentage of the width..

I dunno percentage, millimetres and inches all to measure one tyre!!!

JGM

kart_man

Hmmmm. Just checked that tyre calculator and it seems to be way off !! Unless there's something we're missing !
There are actually 25.4 mm in an inch which just makes that calcualtor even worse !! I'd assumed that if someone went to the trouble of writting something like that, that it would at least work !!
Getting the calcualtor out is the best route I guess !

Brian Lacey

Hope I've used the correct terminology here and not confused anyone!
Fact- the new tyre has a smaller circumference than the old. The old tyre is larger when placed against the new. When the circumference is measured around with a tape measure the difference is about 8 cm. By circumference I mean the measurement around the circular tyre i.e. one rotation.
Thanks
Brian

Jolly Green Monster 2

So using Kartmans figure of 25.4mm to an inch we have:

215/60/16 = 16 * 25.4 + 129 = 535.4mm

225/45/17 = 17 * 25.4 + 101.25 = 533.05mm

so only a difference of 2.1mms now worth worrying about..
that diametre so half that for the radious and only 1.05mm difference.. so your speedo might read 30 and you are doing like 29.98mph now..

JGM

kart_man

What is the total circumference ?? Around 1990mm ?? (for the smaller one).

Brian Lacey

Old tyre has a circumference of 2070mm
New tyre is 1990mm

Brian

Jolly Green Monster 2

old tyre being higher profile and therefore squashes a bit the rolling circumference on the car is probably similar as the diameter is..

JGM

kart_man

OK, I've got it. The tyresave calcualtor is correct. You need to multiply the height of the tyre by 2 for each side of the wheel (top and bottom !!).

ie
(16x25.4) + (2 x (0.6x215) = 406.4 + 258 = 664.4mm diameter

(17x25.4) + (2 x (0.45x225) = 431.8 + 202.5 = 634.3mm diameter.


Circumference= pi x d (I'm really stretching my memory now !!).

= 664 x 3.14 = 2086mm
or
634.3 x 3.14 = 1992mm

So rolling circumference diff = 94mm = 9.4cm = around 5%. I guess in the grand scheme of things it only equates to 5mph per hundred, you'll get better acceleration, but your speedo will clock up quicker !!

You'll probably be ok with that, but personally I'd try and get closer. 225/50/17 would be closer (if they're available).

HTH !

Dave

Brian Lacey

Thanks for all the advice and information, it was really useful. Outcome is that the tyre dealer is changing the tyre for me, with full credit.
Yes, they are a rare size the 225/50/17 and that means expensive!!!
Cheers

Brian

Jolly Green Monster 2

Oops... very true I only included one side of the tyre

DOH!!

Glad you got a good result at the dealer though

JGM