Set my dash up at sva today. The instructions said to input 1197mm in the field for 15 inch tyres. The speed was miles out. SVA inspector done a few calculations and came up with the figure of 1816mm. This was fine. 70 mph, actual speed 67mph, was within the limits allowed.

Cheers Nige

My quick maths says the instructions are correct, but you need to take them literally.

15" tyres, not 15" wheels with tyres on them.

((1197/ 25.4)/22)7 = 14.994"

[Edited on 14/8/06 by the_fbi]

To get the exact measurment for the dash.

Use chalk to mark the floor and the tyre the sensor is attached to so the 2 marks line up. roll the car until the chalk mark on the tyre completes one revolution mark the floor again. Then measure between the chalk marks on the floor. This give you you rolling radius. Works with push bike speedo's too! Better than a calculated measurment as tyres seem to differ in size depending on manufacturer and if they are brand new or worn and also tyre pressure.
Run a bike speedo on the sylva and its spot on wth the GPS upto 110 using the above method.

allan