Technical question

[RabButler]

Hi! My first Toyota and already I have discovered that I need a new wheel socket, 21mm. It's actually the only size I don't own in the range up to 32mm although I do have a 13/16" plug spanner that might fit and since the car doesn't come with a jack, it's all a bit academic. Anyway, as I have done on my last 3 cars (all Honda Civics) I tested the speedo against a TomTom GPS (an excellent way of legally overtaking everyone in motorway roadworks). The Honda digital speedometers all read 31, 42, 53, 64, 74 at the respective speed limits, but my Corolla is +4 mph across the board. Is that a typical reading? I thought that the speedometer had to be within 10% high and must not under read at all.  I assume there must be some tolerance built in to cope with the slight variations in operating diameter at the different wheel sizes and my 16" wheels will be at one end of the acceptable tolerance.

[FROSTYBALLS]

The EC directive covering speedometers requires that:

• The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
• The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

However, UK legislation requires that the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from the EC in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph. For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

Although most manufacturers build in a tolerance which ensures their vehicles meet the EU directive, for the UK market, they also need to ensure they meet the UK requirements. 

[RabButler]

Many thanks for the detailed response. So when drivers are doing an indicated 60 mph they are actually doing 50.  That may go some way to explaining the queues behind Auris vehicles!

[Clockwork]

I've checked mine against a TomTom but only at 70mph and I have an error of 5mph.  My last car, a Mazda, was a little bit worse at about 6.5mph error.

[nielshm]

My car is very accurate. Doing 50 mph only has 1,5 mile error, doing 75 has 2,7 mile error. The car is running on winter tyres. 

My old Auris was terrible. A full 10-12% error from 50 to 60 mph. My tachometer how ever, showed 2-3% less than the actual distance covered. 

The new Corolla shows 1% less than the actual distance, so fuel economy is slightly better than calculated at the pump. 

[PeteB]

If you access the car's internal computers with something like a Techstream, it knows the exact correct speed.  For the UK/EU it fudges the figure for the speedo.

I've seen people from countries like the US and Japan state that their speedos were spot on too.