Thanks for your input so far guys it's really useful. I should have added my speedometer is a SMITHS clock which pretends to be an old mechanical chronometer by jumping up and down by a couple of mph at a time as you accelerate and decelerate.
I'm a bit nervous about using Spanner Juggler's calibration code in case it's incorrect for my device and throws the readings out completely. Can anyone confirm it's the correct code for a 2013 Plus 4?
