By law they cannot read under
i’ve heard this a few times but surely if so then it would be an MOT check point.
I had a car with a non-working speedo for at least a couple of MOT’s and never got more than an advisory.
I’m not sure if it’s a legal thing, but most manufactures will air on the side of caution with them through fear of being sued if it’s shown people are speeding because their car under-reads.
I’m told when they’re designing a new car the get the various suppliers in and they might say “okay, you can have this one, it’s accurate +/- 2% and costs £30 per car, or you can have this one that’s accurate +/- 10% and costs £5 per car”.
They’ll usually choose the cheaper one – then they’ll instruct the makers to ensure that they are set to over-read by 10% so at best they’re completely accurate and at worst they over-read by 10%.
There really is no downside for the manufacturer – an over-reading speedo makes their car seem faster than it is, makes it seem more efficient than it is for a given (false) speed and it means services come around sooner (and the mileage is recorded at a faster pace).
They can’t be “got” for it, because they will always claim that it’s only prudent to make them over-read by 10% based on a device that’s accurate +/- 10% to ensure none of their customers are speeding when they don’t think they are.
80’s Fords were renowned for their ability to travel at fantastic speeds, far in excess of what they were claimed to be able to do – claims of 140mph XR2s weren’t uncommon 😉