TridentTested
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Don't forget you have to account for old, classic cars as well.
For example the width of the needle on my 74 Stag, and the thickness of the paint lines on the speedo, plus a little bit of needle flutter, you have no chance of any accuracy between 30 and 33 mph!
On my GTV6 the speedo's needle stayed dormant until the car reached 100 mph at which point it spun up to the mark (don't ask me how I know this). These were the days before GPS so I had to learn what 30 mph and 60 mph were in revs in various gears. Doubly annoying as mine was the earlier so-called 'split dash' with the rev counter over in the middle of the dash. It was a known problem with worn speedo senders on the gearbox, I meant to get it fixed and never did.
The QP's speedo seems very accurate when compared to my TomTom (which one assumes is totally accurate). The QP seems to over read by 1% or 2%. It helps that it has a digital readout so you are not trying to guess where the needle might be pointing.