Speedo Accuracy

[Wilson]

I've always heard speedo's as standard setup tend to give a higher reading than true speed... how true is this and on average how much are they out by?

If I've now got 205/40/17's should this even it out a bit?, I suppose 40/17's are not much different to 55/15s though... I can never work these out !!!

[smithy318i]

I don't know exactly how to work it out, but.
The larger your tyre size, the more wrong your speedo gets.
(because one revolution of the wheels = more distance)

[Brianmoooore]

Nothing to do with wheel size - It's the outer diameter of the tyre that matters, not it's inner diameter.
On the correct rubber, expect the speedo to over read by about 5% or so.
This is not a design fault - it's a legal requirement that a speedo should never indicate less than your true speed, so a bit of inbuilt safety margin is required.
OBC tend to be fairly accurate ( assuming correct diameter rubber).

[Wilson]

Thanks, I'm running 40/17 tyres, standard are 55/15 tyres, would this make the overall size the same?

[Jos]

Need the width of each tyre too, the profile is a percentage of the width of the tyre, ie 40 profile is 40% of the width of the tyre so 205/40/17 would be 40% of 205mm which is 82mm.

You can compare tyre diameters here:

http://www.unixnerd.demon.co.uk/bmw_wheels.html

[Turbo-Brown]

'Ere we go

205/55R15 tyres are 607mm diameter
205/40R17 are 597mm diameter

so with those, you've got even more margin of safety as each revolution equals less distance travelled

[Jos]

oops, should have read up the thread a bit lol, the sizes are there..

[Turbo-Brown]

If ya go to Toyo.co.uk, there's a tyre convertor thing. Quite handy really

[Wilson]

Well I didn't know that, profile is percentage of depth!!!!! Learn something new everyday

So I am now traveling 10mm, less each rev of wheel ... the 15's I had were the wrong size anyhow that were 195/50/15's, not 55!!!!!


Thanks Guys, just wondered really!!!!

[Turbo-Brown]

Eh? NO!

205/55R15 you travel 1906.9mm / rev

with the 205/40R17 you go 1875.5mm / rev

so 28.4mm difference

[Wilson]

Eh? NO!

205/55R15 you travel 1906.9mm / rev

with the 205/40R17 you go 1875.5mm / rev

so 28.4mm difference

Sorry read your last post wrong!!!!

[e30-320i]

its not your fault that your blond

[Wilson]

Your just jealous

[e30-320i]