Speedometer accuracy.

[splitpin]

While out on a run at weekend with sat nav on I noticed that at an indicated speed of 60mph on the speedometer, the sat nav was showing around 51mph. The tyres are the standard sizes. Is the speedometer likely to be so wrong or are sat nav speeds not very accurate.
Thanks.

[shaun.m]

sat navs are usually good when my speed is 80 on the clock the sat nav reads 74 mph and i also have road angel which reads the same

[Bongoplod]

Most modern Satnavs work from readings from 8 or more satellites,so I would think they are more accurate than the speedo .

Brian

[Matt&JoyBongling]

It is likely to be your speedo - ours is about 10% out compared to our GPS & Sat Nav.

[M 80NGD]

Have you got a "stick on face" fitted to your bongo??! I have and it's 6 mph out! When I'm doing 30 mph,my clock reads 36 mph!! It was fitted in Derby,by Hillside motors. Suppose I won't be done by the speed cameras if i stick to MY speed!!

[mikeonb4c]

While out on a run at weekend with sat nav on I noticed that at an indicated speed of 60mph on the speedometer, the sat nav was showing around 51mph. The tyres are the standard sizes. Is the speedometer likely to be so wrong or are sat nav speeds not very accurate.
Thanks.

Interesting one that. When I had my std 15" wheels on I had a similar over reading situation (maybe not quite as sever as yours but more or less like that). I put 17" wheils on with 55 depth tyres and now my speedo is virtually 100% accurate according to the SatNav. I seem to overtake other cars now in 50mph motorway restrictions. I wonder if a lot of cars have over read built in for reasons like this. So I get a 'competitive edge' but no safety margin if I exceed the speed limit, if you get me

[madmile]

It is true that most cars have a safety limit built in, and I think it can be as high as 15% legally, but it cannot underead at all.
Every bongo will have had its speedo messed around with to some extent so if you are being fussy (I usually am ) then you drive up the road with it half assembled with the sat nav and get it as close as possible. On that point, the sat nav will be correct, particularly at steady speeds- if you can maintain a speed for a few hundred yards, the sat nav will be correct.
If you want an equally accurate check and are running standard 15" wheels with mazda recommended tyres then 3000rpm will be exactly 75mph, and assuming you are locked in final drive then 2000rpm will be 50mph.

Mike is right as I run 215/55/17's and at 3000rpm I am now doing 78mph.

[You've Been Bongod]

Have you got a "stick on face" fitted to your bongo??! I have and it's 6 mph out! When I'm doing 30 mph,my clock reads 36 mph!! It was fitted in Derby,by Hillside motors. Suppose I won't be done by the speed cameras if i stick to MY speed!!

no but you will be done for kerb crawling

[M 80NGD]

Have you got a "stick on face" fitted to your bongo??! I have and it's 6 mph out! When I'm doing 30 mph,my clock reads 36 mph!! It was fitted in Derby,by Hillside motors. Suppose I won't be done by the speed cameras if i stick to MY speed!!

no but you will be done for kerb crawling

[welsh winger]

When I changed the face on my clock from KPH to MPH, the speed was miles out (little joke there) 10mph + so drove round with the plastic front off the clocks and kept moving the needle on the speedo till It got close, I say close as I have left it a little on the slow side, it reads 27/28 when the sat nav says 30. worked for me hope this may help.

Nick
Welsh Winger

[splitpin]

Thanks for all replys, no stick on face on clock M 80NGD. madmile, I have the standard wheels and tyres, so do you mean that when gearbox is locked and revs are 2000 true speed will be 50mph so my clock will show almost 60mph. welsh winger, did you drive at a steady speed and move speedometer needle to sat nav speed, is this correct way to calibrate clock?
Thanks.

[bigdaddycain]

Hi splitpin, vehicle manufacturers have speedo's over-read by default... Just in case a customer tries to sue ford/mazda for that speeding ticket! 10- 15% is the normal amount of over-reading.

[Bob]

Manufacturers also have to allow for the difference in tread depth from brand new to well worn tyres. Makes quite a difference to the circumference, and hence gearing.

[welsh winger]

Thanks for all replys, no stick on face on clock M 80NGD. madmile, I have the standard wheels and tyres, so do you mean that when gearbox is locked and revs are 2000 true speed will be 50mph so my clock will show almost 60mph. welsh winger, did you drive at a steady speed and move speedometer needle to sat nav speed, is this correct way to calibrate clock?
Thanks.

Hi, yes and no, the answer nothing so technical, yes I drove at a steady speed for untill the Gps was reading a steady speed, then if it was a quarter of an inch out one way or the other I moved it quarter of an inch, did this until I was happy with the result.

Nick
Welsh Winger

[splitpin]

Thanks for all advice, very helpful.