Peeps,

I think my Speedo is under reading. There are two reasons for this.

Firstly, I could (given the correct environment) get 150+ mph on the clock with the standard Jap box in my '94 WRX. Since changing to a UK spec box and rear diff, I struggle to get up to or beyond that figure.

Secondly, I was unfortunate enough to pick up a NIP at 41 mph in a 30 zone. I'm sure when I spotted the mobile camera van my speedo was reading 37/38 mph.

I'd like to understand if and why its under reading and get some advice on the cost of recalibration and where it can be done? The car is currently on 17s but may be on 18s soon.

Any advice would be apprecieated, thanks

Kevin

 

Kev, not sure about gearbox ratios but wheel sizes will affect your speedo, have a look here

 

Andy,

Yeah I know there would be some effect in that respect. I suspect that my overall rolling radius is smaller than the standard size (205/50/16?) and therefore 17/18" wheels need to revolve more times to register the same speedo reading - hence you travel faster, but your speedo says not (i think that's the right way round!?).

But the car has always had 17s on and it has been more noticeable (i think) since the gearbox and rear diff were changed. I suspect this may be to do with final drive ratios etc.. but am no expert.

Kevin