rear end ratio

Slidd said:

On the speedo, starting in 2015 the readings come from data supied by the wheel speed sensors so the car can adjust to a gear change and no tune for speedo reading is needed like before.

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Does it go off the rears or fronts? One wheel? Average? I have a reason for asking....

Slidd said:

People say the fronts but I think it's an average because every now and then you will still get a hero run that's too quick to be real.

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I ask because last year I did Land Speed Trials and my tickets pretty much exactly matched my speedo. This year I had same size but different brand tires on the rear and the speedo read about 1.5mph higher than the ticket.