Speedo Recalibration - Tyre Size ?

[Andy JB]

Having recently changed the rears tyres on my Chim 500 from O/E 235/50 to 245/45 as fitted to later cars on stnd 16" Estorils.

Having just returned from a 1,500 mile trip to Spa Francorchamp I was alarmed to note the speedomoter was overeading by 9-10mph compared to my Sat Nav speed, which previously had been almost identical.

The question here is can the speedo be recalibrated or changed and if so what sort of costs are invloved?

Presumably my mileage will also increase disproportionately all from a small change in tyre depth profile from 50-45. I can't be teh first to notice this?

[7 TVR]

You can get it re-calibrated at Speedy cables or just drive of the Sat Nav, i have ended up leaving mine reading high as it makes me feel like i'm going faster

[dhf]

There is a recalibration screw on the side of the speedo

[ajp70]

I just did a quick google for tyre calculations which suggests that your diameter has changed by just minus 2.18% or minus 14mm. This means that at 60mph the speedo should read about 1.4mph faster so, 100mph should read approx 2.5mph faster.

How fast were you going?!!!!

Edited by ajp70 on Thursday 21st June 16:36

[Andy JB]

ajp70 said:

I This means that at 60mph the speedo should read about 1.4mph faster so, 100mph should read approx 2.5mph faster.

How fast were you going?!!!!

Well I did notice no matter what speed above 50 mph I was travelling, there was no change to the speed differential. Even at a speedo reading of 120mph it was still 9mph over the actual speed or at 50 mph - seems extreme to me and somewhat illogical all things considered!