Speed measurement

[Ian Simmonds]

Driving up the M1, M6 today, (in the usual boring way) I was playing around with the trip computer and speedo and noticed a difference. If you do an instant 'speed trip' with the speedo showing 80mph the speed trip settles to a 74mph reading. We all know speedo's are optimistic. But where does the Speed Trip 'reading' get it's digital speed probe output from to contradict the speedo?

[Ross]

[Evil Thought]Wonder who's had the highest speed on the trip meter?[/Evil Thought]

[Greg_S]

yeah ive noticed that before. i guessed the auto check must be more accurate than speedo

[HPmuirt]

Try it against a GPS system and you will find that the instant reading is very close to your 'real' speed, as it does not have the built in over-read that speedos have.

Cheers n

Si

[Fat Audi 80]

Oh! Never tried that. I will press it at 5400rpm in 6th and see what I get :mischeif:

[djavant]

speed measurement

hi i live in germany 6700rpm if you reset it while driving you get 155 mph and the speedo is reading 167 mph the car is an avant with standard power hope this helps stops you getting caught in uk by the old bill

[Ian Simmonds]

Thanks guy's, probably won't try the 6.7k rpm in 6th bit on the M62 though!

But it's interesting to see that through the 40 mph 'Average speed' check roadworks on the M6 my speedometer was reading exactly 40 mph and everyone around me was moving at the same speed (e.g. 36mph!!!) - So they're all out! Drove through a dual carriageway 40 limit today at an instant speed trip 40mph (44/45mph speedometer reading) and it felt decidedly 'Iffy' - everyone else was doing 36mph..... Is the world conditioned to 10% inaccurate speedo's?

[HPmuirt]

Is the world conditioned to 10% inaccurate speedo's?

YUP.....well except me

Cheers n

Si

[Chris5044]

how do u do this speed check?

[richslay]

When driving along at a steady speed, reset the average speed function on the trip computer and it will give you your current 'real speed', in my experience very close to GPS readings.

[Fat Audi 80]

hi i live in germany 6700rpm if you reset it while driving you get 155 mph and the speedo is reading 167 mph the car is an avant with standard power hope this helps stops you getting caught in uk by the old bill

Which is why I have always doubted dubious claims of 165mph mildly tuned S2's....

[4WheelDrifter]

Is the world conditioned to 10% inaccurate speedo's?

YUP.....well except me

And me.

I still look at my tacho the calculate speed, since i was without a working speedo for about 2 years.
So I'm also used to travelling faster than most other people in speed restricted areas.

During a high speed run some time ago I managed to get a GPS verified speed of 163 mph. Then I just divided the speed by the rpm, and got just less than 25mph/1000rpm in 5th (top) gear. So I still use that to judge my cruising speed.

[s2carlo]

noticed that too.
So I mounted my navigation system and did a recheck.

It shows that the speedo is about 7% optimistic.

I think the GPS is rather accurat