Actual speed
 

Finally found the actual speed readout on my Garmin satnav, and wonder which instrument is correct.

70 on the satnav is pushing 80 on the speedo, but when (on cruise control on a level road) I reset the message centre average speed (obviously it doesn't have actual speed setting), it read out 75.2mph. None of the readings changed for a mile or so, till I had to slow down. Car is CDT with Ron's Tu3 on 7, so I know mpg on the computer is optimistic, but will that also affect the speed reading?

I expected the speedo to be optimistic, but not by that much.

Suppose I ought to reset satnav and both trips for distance travelled, to see what differences there are.

Malcolm

Even the Sat Nav seems to have a Delay, and the Hi Line Speed readout is correct only at that second of computing then just an average, but mine is nothing like that far out on readings even with Remap and Synergy.
At true 70MPH mine is showing 75MPH on Speedometer and 71.0MPH on Digital Readout.

I have noticed this in both my 75s - speedos read fast. My current 1.8 being more inaccurate than my old diesel.

I have a Snooper S6R and according to that an indicated 80 on the speedo is a true 72. The error gets worse the faster you go.

I'm quite glad really as it means I'm less likely to get nicked if I believe the speedo!

All above sounds about average for a 75. My speedo reads 10% fast throughout the range ie True 30 indicated 33. True 70 indicated 77.

I do wonder what the speedo was calibrated for and on which tyres although as far as I know the biggest difference in circumference for any of the available wheel/tyre combinations was about 2% which is not nearly enough to get me speedo anywhere near accurate.

Perhaps thats why 75's have got a pipe and slippers image:lol:

Tim,

Your figures match the ones I get from my sat nav. The 75 speedo does under read by a significant margin. At 90 indicated I have true speed as 81 and at 100mph indicated (on the continent in May) true was 90/91.

Does anyone else find it frustrating when people in front slow too much for cameras because they are reading from their speedo? lol

Hi Malcolm

As every device any speedometer has a range of tolerance. And by law a speedometer must not show a speed slower than the real one. Therefor all speedometers display more or less deviation on top of the actual speed, as the diameter of the approved tyres is varying, too.

The board computer can measure a more accurate speed (for calculating the average) without the risk of violation a law (which refers to the speedometer). Therefor this value should be lower and a little bit more accurate.

A good sat nav should calculate a very accurate value.

My 75 has been on a rolling road to get the speedometer calibrated. Comparing the now known deviation of the speedometer with the readout of the sat nav is an exactly match.

juergen - when you say you had it calibrated, did you actually alter the readings on the car and reprogram it so it reads true?

I was wondering whether this can be done, and whether if it could, it would have any effect on the overreading mpg readings too.

I wonder if the auto - like jdc - is better calibrated? anyone else have experience something more like his?
Mine - manual - is reading 80 at 71 too.

Hi pondweed

I think calibrate isn't the right word, I should have used test instead. :bowdown:

The deviation is less than 10% up to 116 mph in my case.

But the calculation of the average fuel consumption can be calibrated for certain (somewhere in the instrument test mode).

so my idea about reducing the tyre wall size might still be the only way of sorting the problem out!?
Sady, I was of the illusion our car was FAST. Now I realise it was illusion. Me in my own little bubble. (:driving: We must have been looking like 90 year olds when keeping to speed limits...)

Most cars speedos read up to 10% over so our cars are as fast or slow as any other. I believe the percentage rises as speed increases. Possibly 1% for every ten mph.