Do bigger alloys affect the accuracy of the speedometer?

[stu__grant]

Hi Everyone

I have been lurking on here for a while, researching my perfect Z4 which I bought from another forum member yesterday, I'm now the proud owner of a 2004 3.0 SE!

My mate who followed me home after I bought it said I was averaging 90 on the motorway, I'm sure my speado was reading 80.


Is it possible that the wheels & tyres fitted to the car are making it faster than the speado reading?

Can this be calibrated to account for different wheel sizes?


Heres my tyre info


http://www.laddsguns.com/tyre.jpg


I would be really grateful from any help / advice before the speed cameras start assisting with the recalibration processes!

 

[srhutch]

In short yes. Even if you were to fit the alloys of an ///M which are 18" replacing 18" on a standard Z4 the speedo will be out as the tyres on an ///M are bigger.

If you go for bigger wheels you then need smaller tyres to bring the rolling diameter back to where it should be.

 

[corsaire77]

You can do your calculations here:
http://www.willtheyfit.com

Assuming it's your rear tyre, here is the calculation compared to OEM:
http://www.willtheyfit.com/index.php?width=255&aspect=35&diameter=18&wheelwidth=8.5&offset=50&width2=245&aspect2=40&diameter2=18&wheelwidth2=8.5&offset2=50&Submit=Submit

 

[mmm-five]

The answer to your first question is yes, wheel/tyre combo could make a difference to the speedo reading. So if you go up a wheel size, you need to go down in the tyre profile to compensate for the increase in wheel size. A 225/40r18 would upsize to a 225/35r18 IYSWIM.

Is that a front tyre (for which it is about 6.5% too big - should be a 225/40r18), or a rear tyre (for which it is about 2.7% bigger - should be a 255/35r18). Which means that the speedo thinks the tyre is covering 1 mile when it's actually covering 1.065 miles due to the larger circumference.

Neither would account for a (roughly) 11% difference between your's and your mate's speedos - so depending on what car/tyres your mate has, it could be that his is over-reading and your's is under-reading.

 

[Toe-side]

You can do your calculations here:
http://www.willtheyfit.com

Assuming it's your rear tyre, here is the calculation compared to OEM:
http://www.willtheyfit.com/index.php?width=255&aspect=35&diameter=18&wheelwidth=8.5&offset=50&width2=245&aspect2=40&diameter2=18&wheelwidth2=8.5&offset2=50&Submit=Submit

Nice site Corsaire77

Use a GPS /sat nav as an easy or quick way to check /compare speedo accuracy also.

 

[BRC]

Interesting observations. My sat nav/gps shows a consistent 3 mph light reading when compared with the car. Somewhile ago lower profile tyres 40 rather than 45 were fitted and the error increased to 5 mph. BTW this wasn't on my Z but another car I have. Check it out on gps and add or subtract as necessary at the critical speed limits i.e. 30, 40, 50 and 70. Also useful to know how accurate the speedo is so you can safely cruise past plod without wondering about/fearing accuracy!

 

[the cueball]

it's useful to bare in mind that all vehicle speedos over read... :wink:

"ACPO Guidelines on speeding, and the UK law regarding prosecution.
Vehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of -0->+10%. The implications are that it must never under-read - for obvious reasons - but may over-read. As the cost of manufacturing a speedometer with -0% error would be very costly they all over-read by a few percent without exception. Even if speed is measured correctly the display may not be accurate, so a speedometer error is allowed. Because of this, the Association of Chief Police Officers (ACPO) have an official formula for calculating a speeding offence. It allows a leeway of 10% plus 2mph. In reality, most speed traps are triggered at higher speeds than this because if they were set bang-on those guidelines, the sheer amount of paperwork generated would overrun the police speeding departments."

 

[stu__grant]

Very interesting, thanks for the input everyone.

I have Z4M wheels fitted which once you add rubber certainly look bigger than the standard wheels even though the rim is an 18''?! (I don't really understand all the numbers on the tyre wall)

A quick test with the sat nav shows that the speado is pretty much spot on although I haven't had a chance to test it thoroughly... I expect my mates Golf is just being super cautious!