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Why match donor tyre circumference on different diameter kit wheels?
jps - 22/10/18 at 12:56 PM

Is there any specific reason for making sure that the overall circumference of the 15" wheels + tyres on my Haynes is as close as possible to the circumference of the 13" wheels + tyres of my Sierra donor? Does it impact on the suspension effectiveness?

nick205 - 22/10/18 at 01:27 PM

If you've already calibrated your speedomater then altering the outer (rolling) circumference of the tyre will make your speedometer read incorrectly.

Angel Acevedo - 22/10/18 at 02:29 PM

If you adjust your suspension to maintain your Ride Height there may be an impact as you will be on a different point of your camber curves.
It may not be noticeable though.

SJ - 22/10/18 at 03:44 PM

Why not just go with 13" wheels? The car will handle, ride and steer better.

steve m - 22/10/18 at 03:45 PM

"Why not just go with 13" wheels? The car will handle, ride and steer better."


And look better, imho

steve

voucht - 22/10/18 at 04:26 PM

quote:
Originally posted by SJ
Why not just go with 13" wheels? The car will handle, ride and steer better.


Because rear lower wishbones on a Haynes Roadster (book specs, Sierra donor) do not fit in a 13'' wheel, it is too wide. I remember trying to fit the the 14'' alloy wheels that where fitted on my Sierra donor, and the head of the bolt was touching the inside of the rim.

quote:
Originally posted by jps
Is there any specific reason for making sure that the overall circumference of the 15" wheels + tyres on my Haynes is as close as possible to the circumference of the 13" wheels + tyres of my Sierra donor? Does it impact on the suspension effectiveness?


It is important only if you kept the original speedometer from your donor (assuming that you kept the same diff. and gearbox as well). I don't see how it can affect the suspension.

ianhurley20 - 23/10/18 at 08:15 AM

On mine I was using an MX5 engine and gearbox driving a ford 3.92 diff and 15" rims so that they would fit the Haynes rear wishbones. I used an MX5 loom and MX5 instruments. (I assumed they were accurate to start with). The MX5 uses a 4.1 diff on 185/60 x 14 tyres. I calculated that for the prop to rotate at the same rpm for each speed then I would need 195/45 x 15 tyres. This worked out to be correct as at IVA the tester congratulated me on having a speedo that was EXACTLY correct.

jps - 23/10/18 at 10:45 AM

Thanks chaps. Ideally I would fit the original steel wheels from my Sierra (13" i think) - but Voucht is correct - minimum of 15" needed for clearance.

My speedo will be an aftermarket, so I expect to be able to program it as required.

I'll look at what common sizes are available in R15 and then go from there.

[Edited on 23/10/18 by jps]