Speedo Error

[dempsey]

My speedo (2002 4.5 with later digital mileometer) reads well out.
A minimum of 5 mph overstated at low speed rising to showing 160 mph
when the GPS only shows 148 mph.

This hacks me off even when I know what the true speed is. I guess it
could be mis-calibrated with the wheels/tyres (18" spiders, 255/35 ZR18 rears).

Anyone know where I can send the unit for adjustment to read accurately (and how would they do it anyway)?

[Tyre Smoke]

They will always over read, never under read. The spiders and tyres might have some effect, but a 10% margin is 'normal'. Even if you have the speedo calibrated, it will always over read to allow for tyre wear marginally reducing rolling diameter.

[jbvipercerb]

I don't know the make of your speedo, but most after market electric speedo's have a set of dill switches in the back of them, sometimes under a rubber cover. If it has these switches can be reset to adjust the speed shown. It will take a bit of trial and error but you should be able to get it spot on.

[kojak]

Tyre Smoke said:

They will always over read, never under read. The spiders and tyres might have some effect, but a 10% margin is 'normal'. Even if you have the speedo calibrated, it will always over read to allow for tyre wear marginally reducing rolling diameter.

This is quite true. Imagine it being the other way round. You think you are doing 30/32..and you'd be knicked.

[kieronsri]

Tyre Smoke said:

They will always over read, never under read. The spiders and tyres might have some effect, but a 10% margin is 'normal'. Even if you have the speedo calibrated, it will always over read to allow for tyre wear marginally reducing rolling diameter.

Manufactures are only allowed to have an 8% over read on speedo's so ten percent is a bit high