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Speedo difference

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What tyres are fitted for a start.

Since the circumference of a tyre is pi x diameter and the rolling circumference is what affects the speedo, you can work it out as the ratio of the old and new sizes anyway so if you have 32" tyres fitted and wish to go to 35s, then the speedo will read 32/35 x whatever so instead of 50mph it will read about 45.7, and so on.

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Thanks for the replies but... Bogmonster, i have put on the post that they are 35" simex's after that you have lost me! :huh: .

Landymanluke, good link have seen these before but i dont know original tyre size for a 90 also these calculators dont allow you to enter 35" only 235 etc

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take for example:

90 original 205R16 as base

1. move upto 235/85 or 7.50R16 then out by roughly 10% on 205 speedo pinion

2. move upto 255/85 (33" high) then out by 20% roughly on 205 pinion speedo

3. move upto 35" tyres then 30% out on 205 pinion speedo

Some 90s eg those with alloys and 235/85 or 265/75s from factory will have the 110 speedo pinion fitted therefore

from above

1. LR accurate

2. out by 10% with 255/85 on 235/85 pinion (for 235/85 read 265/75 or 7.50R16 they all use the same pinion)

3. 35" tyres out by 20% on 235/85 pinion.

Note LR accurate means accuracy LR build in (or out which can be +/-10%) mine for example on 255/85s with 235/85 speedo pinion fitted is GPS speed bang on. its then 10% low with 35" simex on. eg reads 54pmh when your doing 60mph.

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with a 205 pinion i reckon on 35s

35 + 30% = 45mph

the % is on the speedo reaing not on the speed. if that makes sense,

i woudl check with GPS, how do you knwo that your not on a 235/7.50 pinion?

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As others have said, borrow a GPS - any type (TomTom through to handheld Etrex type ones) will tell you your speed. I find mine is a bit out, at an indicated 60 it's doing 57 really. Thats on 32" 265/75's, and I'm guessing a 100 speedo drive gear.


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