Speedo accuracy

wes1001 · Post by **wes1001** » Sat Jan 06, 2007 3:01 pm

Does anyone know or have experience of how much changing from 15" to 17" rims will throw the accuracy of the speedo out by ? , and is it a plus gain , i.e. indicated 30 is real 33 or is it vice versa ?. Cheers, Wes.

jameslxdt · Post by **jameslxdt** » Sat Jan 06, 2007 3:04 pm

in theory fitting larger wheels will mean, the gearbox will do less rotations at the same speed as it did with the smaller wheels
so the speedo will say your going slower than you actually are, but it will be so minimal i wouldnt worry about it
406 d8 speedo's are so inacurate too, my old diesel would say 70mph when it was actually 80mph

Post by **DaiRees** » Sat Jan 06, 2007 5:44 pm

Here you go Wes, have a look at this.

Tyre Size Calculator

Basically it's the outer circumference of the tyre that's important. i.e the distance the tyre will roll for one complete revolution. If you're going to fit larger rims, you'll no doubt fit lower profile tyres, so keep the outer diameter (and therfore the cicumference) about the same and you'll have no problems.

wes1001 · Post by **wes1001** » Sat Jan 06, 2007 6:24 pm

Cheers Dairees thats a great little bit of kit

Post by **DaiRees** » Sat Jan 06, 2007 9:20 pm

You're welcome mate

I found that site when I was looking to change the tyres on my 4X4, but the goal there was to make them as big as possible without completely knackering up the gearing

(I've gone up over 11%

)

Blue406 · Post by **Blue406** » Sat Jan 06, 2007 10:26 pm

jameslxdt wrote:in theory fitting larger wheels will mean, the gearbox will do less rotations at the same speed as it did with the smaller wheels
so the speedo will say your going slower than you actually are, but it will be so minimal i wouldnt worry about it
406 d8 speedo's are so inacurate too, my old diesel would say 70mph when it was actually 80mph

Thats not good!
My speedo always reads within 3mph of my true speed.

speediepug406 · Post by **speediepug406** » Sun Jan 07, 2007 10:49 am

[font=Comic Sans MS] [/font] Hi there in answer to your question i believe that 45 profile tyres are the correct size for a 17". I am using 225/45/17 on my 6 without any probs with rubbing(lowered 40mm) or the speedo being out.

Post by **DaiRees** » Sun Jan 07, 2007 12:21 pm

speediepug406 wrote:[font=Comic Sans MS] [/font] Hi there in answer to your question i believe that 45 profile tyres are the correct size for a 17". I am using 225/45/17 on my 6 without any probs with rubbing(lowered 40mm) or the speedo being out.

Yup they are exacltly the same outer diameter at the standard 195/65/15, but as the profile is a percentage of the width, the profile you'll need depends on the width you choose. e.g. if you went for a 205/17 instead of a 225/17 you'd need a 50 profile instead of 45 etc...