Possibly a silly question, but, at around 70 mph my phone satnav reports my speed about 62mph. Which to believe? I'm trending towards the satnav (not just because I can go faster 🙂 )

Hi Phillip,

An old conundrum I've struggled with for ages (even with an early TomTom)!

I'd love to believe the SatNav, because I would guess that the speedo overestimates by a bit (two reasons: makes the car look faster, might also save you points).

Just had to do a speed awareness waste of time (AKA Speed Awareness Course, of course!) for being flashed at 45 in a 40 limit in the middle of Wales, returning from an Irish ferry. It was 02:00am. How P'd O was I? Enormously!!!

They're out to get ya, just play safe!!!

J

Hi Philip,

Speedos are designed to "fail safe" by which I mean that when a speedo indicates 70mph, it is either accurate or is over indicating, ie the true speed might be 68mph. This is why claiming your speedo was inaccurate doesn't cut much ice as an excuse for speeding.

However, yours does seem to be more inaccurate than I would expect. Do you know if the original size tyres are fitted? If not and if not re-calibrated with SDD your speedo / mileage will be off kilter because speed / distance is calculated assuming a specific rolling circumference for the wheels.

Having seen your post, I checked mine this morning and found that the speedo was less than 2mph different to my GPS speed.

Thanks for the replies. I've no idea what size tyres were fitted originally, so either I have to get my speedo recalibrated or, possibly the route I'll take, fit the larger tyres. I'll check my tyre sizes tonight. 

Typically you car speedo is 10% higher that what you are actually doing, car manufactures do this so they dont get sued. If you dont have standard tyres even optional upgrades will reduce the tolerance. 

Gps will be more accurate probably