Gentle Warning about speedo innacuracies on R8s with ZR rims

Post by **MarkCoupe** » Fri Dec 14, 2007 10:23 am

Hey folks

Just a quick heads up to let you know that if you're running an R8(or 220 at least) on the ZR hairpins with the stock ZR tyres(i think 195/50 or 205/50) your speedo will underread, which makes it a bit more dangerous when it comes to speed cameras etc, as most speedos overread.

At 70mph on the speedo, the car is doing 73. At 90 it was doing 94, so oddly, it undereads more at higher speeds.

It may be of interest to those running 800 boxes that on my friends car, the speedo was 15% out at 90(gps said 78, when speedo read 60 the gps said 51/52) and the higher the speed the more it overread.

All tests done on a private airstrip of course.

Mark

Post by **agw_01** » Fri Dec 14, 2007 11:46 am

Nice info there Mark.

I've replaced my 205/50's with 205/45 profile tyres. According to an online tyre checker, the speed is only out by about .3/mph.

Will have to check that with GPS of course.

mg-richard · Post by **mg-richard** » Fri Dec 14, 2007 4:31 pm

(waits by postbox

)

Bob SLi · Post by **Bob SLi** » Fri Dec 14, 2007 4:45 pm

agw_01 wrote:The Big Green Grandad Machine

WTF is mine then??? lol

Post by **agw_01** » Fri Dec 14, 2007 5:04 pm

The even bigger green grandad machine?

rover220 · Post by **rover220** » Fri Dec 14, 2007 5:07 pm

the margin gets higher as teh speed does as its not an amount of "mph" out but its a percentage out.

if it was 10% out at 30, it would be 3mph but at 90 it would be 9mph

618ireland · Post by **618ireland** » Fri Dec 14, 2007 5:59 pm

Bob SLi wrote:
agw_01 wrote:The Big Green Grandad Machine
WTF is mine then??? lol

The Big Blue Looking Grandad Machine

200coupe · Post by **200coupe** » Sat Dec 15, 2007 10:01 am

618ireland wrote:
Bob SLi wrote:
agw_01 wrote:The Big Green Grandad Machine
WTF is mine then??? lol
The Big Blue Looking Grandad Machine

lol

BlueRover · Post by **BlueRover** » Sat Dec 15, 2007 2:59 pm

rover220 wrote:the margin gets higher as teh speed does as its not an amount of "mph" out but its a percentage out.

if it was 10% out at 30, it would be 3mph but at 90 it would be 9mph

That's ok then as there is an allowance made by speed cameras of 10% to allow for speedo inaccuracies but if your speedo is 10% out then it could be that you're doing 36.3 in a 30 and that could be marginally the difference between camera activation or not.
According to the online tyre size calculator a 195/55/15 is 0.1% out compared to 205/40/17.

rover220 · Post by **rover220** » Sat Dec 15, 2007 3:07 pm

BlueRover wrote:
rover220 wrote:the margin gets higher as teh speed does as its not an amount of "mph" out but its a percentage out.

if it was 10% out at 30, it would be 3mph but at 90 it would be 9mph
That's ok then as there is an allowance made by speed cameras of 10% to allow for speedo inaccuracies but if your speedo is 10% out then it could be that you're doing 36.3 in a 30 and that could be marginally the difference between camera activation or not.
According to the online tyre size calculator a 195/55/15 is 0.1% out compared to 205/40/17.

so what are you saying

all speedos overread anyway by law iirc

Post by **Punx0r** » Sat Dec 15, 2007 7:12 pm

They're allowed to be +10/-0%

t666top · Post by **t666top** » Sat Dec 15, 2007 9:00 pm

10% either way and most speedos are that much out speedos neva work proper, plus heres a fact no speedo needed on mot! which is good as mine is dodgy always ave a sat nav on me dont really matter

victor3 · Post by **victor3** » Sun Dec 16, 2007 11:32 am

t666top wrote:10% either way and most speedos are that much out speedos neva work proper, plus heres a fact no speedo needed on mot! which is good as mine is dodgy always ave a sat nav on me dont really matter

If in doubt, just borrow a plug-in GPS speedo for about 10 minutes to check accuracy on your. Anyone near me is welcome to borrow my Road Angel for this.

Post by **Punx0r** » Sun Dec 16, 2007 7:58 pm

They're not allowed to under-read by anything, hence why they're designed to over-read a little to ensure compliance.

It is illegal to have a non-functioning speedo.