Speedo v Actual speed

[dibbly_dobbler]

Hi All. As a matter of interest (to me anyway !) how fast do you reckon I would be really be going (in my 1 series) if the speedo was reading 85 ? I though an overread of 10% was about normal but not according to my speeding ticket (for 81 when I thought I was doing <85). Cheers, Mike.

[SpeedIsProgress]

When I had a mates sat nav in my E46 for a couple of weeks touring Europe, my speedo was only ever 1 or 2 mph out even when I was going 130+ but I guess it depends on how much tread you have on the tyres, the angle you are looking at the speedo from etc. But I was shocked at how accurate it was.

[PJR]

In my experience BMW's don't over read as much as some other cars do, but still, at 85mph you can be pretty sure its reading 5mph over. Thats about what I find with my 120DSE and comparing the speed readout on my Tomtom Go 700 sat nav.

P,

[dibbly_dobbler]

Thanks guys - I will revise my assumptions accordingly (ie keep it under 80 on the speedo !)

[eliot]

my 540 over-reads by about 4mph at 85.

And it would appear delibrate, because the car knows how fast its going, Try this:
On a flat stretch of road, say at a steady 70mph - hold the speed precisley (or use cruise control) - then reset the average speed indicator (if fitted) - you will find that it gives a slower average speed, which is identical to your GPS speed!

[Mroad]

The Speedo cannot read under the true speed.
It can however run over the true speed by a tolerance of (true speed/10) + 6.25mph. In other words at a true speed of 70mph the speedo is allowed to indicate up to 83.25mph. This is the SVA standard, manufacturers type approval standards may be tighter.
From various cars I have driven the speedos are lot tighter than the maximum allowable SVA limit. At a GPS 70mph they have varied between 5mph and 1mph over.
In other words believing your speedo is a good idea if you don't want a ticket.

[srebbe64]

I've got a Beemer and a satnav and can confirm that the when the speedo says 80 mph the true speed is 74.

Funnily enough, the most accurate speedo on any of my cars is, bizarrely, my TVR. It's accurate to within 1%. Beats the hell out of me.

[blackburnbmw]

My 330ci auto speedo reads EXACTLY the same as the speedo on my Indago sat-nav up to 120mph. After that I tend to concentrate on the road ahead, ahem. However my wifes'03 Focus speedo always reads 3-4 mph faster than the Indago (any speed). A traffic cop friend suggested that he would trust the sat-nav for accuracy more than a car speedo as a general rule...

[dcb]

Mroad said:

At a GPS 70mph they have varied between 5mph and 1mph over.

On 98 RON petrol, my car has a documented top speed of 152 mph.

I've had it to 154 mph on 100 or 101 RON near Frankfurt.

For BMWs, I think the speedo at 70 mph isn't very far
out, maybe only 1-2 mph.

This of course assumes the tyre pressures are correct
and you've got standard wheels and tyres on it.

Legally, speedos aren't allowed to underread.

[dibbly_dobbler]

Thanks again all - that's useful info

[soprano]

i have compared my speedo with my satnav

at 80 indicated on the satnav, my speedo reads 81 - so pretty accurate

1999 328 coupe if that helps

[Shropshiremike]

According to the V-box that some mags use for tests, the one on my 328 is pretty accurate - read 73 at true 70. The one on my Primera GT was much less accurate - overread considerably more

[plowy]

In france during the summer an indicated speedo of 160 gave a GPS on the sat nav of 157..... I find mine is reasonably accurate, an indicated 70 is 68...

[podman]

Get well into 3 figures thou (like 170+)and your speedo will be miles out and you sat nav will be out by quite a few MPH as well..

If you want to really know what its doing, pop along to a starightliners day and run it thru a trap...be ready to weep thou at the actual figure after you have seen the indicated!

[plowy]

podman said:

Get well into 3 figures thou (like 170+)and your speedo will be miles out and you sat nav will be out by quite a few MPH as well..!

GPS readings do not vary like analogue, a sat nav reading 10mph is as accurate as the same sat nav reading 200mph....

[podman]

plowy said:

podman said:

Get well into 3 figures thou (like 170+)and your speedo will be miles out and you sat nav will be out by quite a few MPH as well..!

GPS readings do not vary like analogue, a sat nav reading 10mph is as accurate as the same sat nav reading 200mph....

No its not..GPS is not 100% accurate, especially at 200MPH!

GPS works by measuring the difference in position from satellite signals and time delays the signal back to the unit which then computes the speed.

Timing traps are mere inches apart in the setting up and give an electronic signal back to the computer which measures the time between signals to compute the speed.

It would also seem logical that a satellite thousands of miles up in the sky isnt as accurate as a bespoke timing trap. I dont think there are any speed records officially logged via GPS...

As I say, pop along to a speed testing day and see for yourself.

[RobM77]

dibbly_dobbler said:

Hi All. As a matter of interest (to me anyway !) how fast do you reckon I would be really be going (in my 1 series) if the speedo was reading 85 ? I though an overread of 10% was about normal but not according to my speeding ticket (for 81 when I thought I was doing <85). Cheers, Mike.

You got a speeding ticket for doing 81mph?! Was that in a 70mph limit? Seems a bit harsh!

[dibbly_dobbler]

RobM77 said:

dibbly_dobbler said:

Hi All. As a matter of interest (to me anyway !) how fast do you reckon I would be really be going (in my 1 series) if the speedo was reading 85 ? I though an overread of 10% was about normal but not according to my speeding ticket (for 81 when I thought I was doing <85). Cheers, Mike.

You got a speeding ticket for doing 81mph?! Was that in a 70mph limit? Seems a bit harsh!

Yes - speedo had about 83/84ish on it so I assumed (wrongly ! See other comments above) that I was doing 75/76. My own fault of course and a limit is a limit but I was a teensy bit put out !

[RobM77]

dibbly_dobbler said:

RobM77 said:

dibbly_dobbler said:

Hi All. As a matter of interest (to me anyway !) how fast do you reckon I would be really be going (in my 1 series) if the speedo was reading 85 ? I though an overread of 10% was about normal but not according to my speeding ticket (for 81 when I thought I was doing <85). Cheers, Mike.

You got a speeding ticket for doing 81mph?! Was that in a 70mph limit? Seems a bit harsh!

Yes - speedo had about 83/84ish on it so I assumed (wrongly ! See other comments above) that I was doing 75/76. My own fault of course and a limit is a limit but I was a teensy bit put out !

A limit is a limit, yes. Was this a human being that pulled you over, or one of those silly cameras? I find it hard to believe how a modern car can be safe at 70mph but so unsafe at 81mph to warrant a conviction and a fine. If that was an actual human policeman I doubt his intelligence to be doing his job to be honest - he should be stacking shelves somewhere.