Speedo reading and 19 inch rages being out.

[MoOriginal]

Ok i had some doubts about the speedo readings so took the time to use 2 different external sat nav's to get the readings of actual speeds.

TomTom one
&
Naviman

Both showed that on average there is a 4-5 mph difference upwards between speedo and actual reading.

Regardless of speed it stays the same - I was a bit surprised about this - expected it to be less at lower speeds and getting more the faster you got..

What did I test it between - constant 30 - 40 - 70 - 80 - 100 - 110.

Has anyone had experience of this ?

On my elise its about 7-8 mph but at the upper end of the speedo when your doing over 110 or so...

Was a little surprised that it was so consistent further down the speed range with the CTR

[Crackers]

Originally Posted by MoOriginal

Both showed that on average there is a 4-5 mph difference upwards between speedo and actual reading.

Motor manufacturers are obliged by law to make speedos read a few percent faster than the actual speed of the vehicle. One obvious implication is that drivers cannot use their car's speedo reading in court as an excuse for breaking the speed limit.

[czechplastik]

Well discussed here previously, see the following link

speedo accuracy (merged)

[Mark996]

Making the wheels bigger nearly always makes the speedo even more wrong than on standard wheels......in your case changing 18's-19's.

Yes, you can have the speedo re-calibrated, but as in your case, very few (read: No-one) ever has that done.

Albeit only by one inch, you've still changed the 'Rolling Radius' of each wheel.

[MoOriginal]

yes Id realised that actually it was good to know but not really to take any action

[Cheltenhamshire]

Surely when upgrading your wheels you do a plus 1, 2 or 3 fitment to ensure that the rolling radius is practically the same as the standard wheels to avoid these problems and also more serious ones like rubbing suspension parts / wheel arch trim etc.

[Kremmen]

Mine is exactly the same, the speedo is not inaccurate via a percentage method but it seems to be between 5 and 7mph adrift across a wide spectrum.

[Harps]

Hello, this thread has got me thinking ...may be a silly question ...but is the 0-60 time calculated with the cars speedo or an independent speedo (ie GPS)
Thanks

[MoOriginal]

0-60 ha !

those are quoted figures and to get an accurate reading youd really need a vmax - or data logger...

Im sure someone will do that soon enough...

[Honda100]

Mine is about 4 miles per hour out from 30 up to 90 compared to my GPS. In traffic on the motorway I stayed parallel with a Citroen ** and that speedo was showing the same as my GPS. If Citroen can get the speedo accurate Honda ought to as well.
However I take the 4 mls an hour into account when going through cameras or along motorway restrictions so not a real problem.

[Pottsy]

The 18" tyres are 225/40/18 and the 19" are 225/35/19. In theory, your speedo will read 0.5% slower with the 19" wheels on.

[MC Hammer]

Interesting points here, but the result is not only related to the wheel size but the tyre fitted to it. Different makes have different dimensions so the figures vary make to make.
Standard figures are available for ETRTO (but not for free), however a comparison is available at:
http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC%20calculator.htm

[CivPilot]

Originally Posted by Pottsy

The 18" tyres are 225/40/18 and the 19" are 225/35/19. In theory, your speedo will read 0.5% slower with the 19" wheels on.

I'm with Pottsy on this. Although the wheels are bigger, the sidewalls are smaller and the rolling radius change should actually be very similar. The built in "overread" will be lower on the bigger rims.

Funny thing is, my speedo is extremely accurate compared to stand alone GPS based sat nav systems I have tried.