Fitting different sized tyres or a non-standard ratio ring and pinion in the diffs could well mean the speed displayed is inaccurate. Now there is a way to fix that problem!
Fitting and programming this small box is easy. Behind the handbrake backplate, on later Defenders, is a green 3 wire transducer which generates a pulse. This is then sent to the speedometer to show your vehicles speed. Once installed it allows you to adjust the output pulse, both up and down, which means your speedo will indicate the correct speed at which you are travelling.