Advice Please - Speedo Calibration

[SMB]

Hi Guys,
OK Ive felt for a long time the speedo overread by about 10% , but I've now done all the checks and used Steve Heath's spreadsheet and I'm certain,

It's a 95 4.0 Chimaera, 225/55ZR16 rears and Borg Warner gearbox.

3000 RPM in 4th ( easiest to measure as 1:1) is 74-75 MPH on the speedo, but according to the the spreadsheet says it should be 69!

Now, the car had a new speedo not after a year of ownership (odometer stuck!), My question is how do you adjust/calibrate these to get to the right readings? Is there some sort of adjustment on the back of the speedo? As I assume the sender unit just creates pulses which i counted by the speedo, is this correct?
thanks

>>> Edited by SMB on Sunday 22 September 17:31

[shpub]

You can't unless you want to spend a lot of money. The speedo may have been accurate ish when the car was built but there have been tyre size changes and this can add a couple of percent.

You can either get paranoid and spend a lot of money getting the unit recalibrated as it might need an extenal pulse changer to be made or you can enjoy the car.

I would enjoy the car as most other manufacturers have speedos that are equally inaccurate.

If you want an accurate speedo, I recommend a SPA unit with the RaceLogic sensor . Costs just under £500 exc fitting, doesn't look like the TVR unit but will fit and can be calibrated to suit any tyre size.

Steve

[rwalker1]

I believe all speedos have to read between correct and plus 10% - but not low (like mine!). At a true 69mph your speedo could read up to 78mph and be within spec. Yours actually sounds more accurate than most!

[shpub]

quote:
I believe all speedos have to read between correct and plus 10% - but not low (like mine!). At a true 69mph your speedo could read up to 78mph and be within spec. Yours actually sounds more accurate than most!

Actually the MOT spec states it has to be within 10% at 30 mph so a straight line on the glass pointing at 30 mph would do.

Steve