Speedometer Accuracy?
 

Do you think your speedo is accurate? I am not sure if I mentioned this before but when ever I go past one of those mobile detector devices it always shows my speed lower than what the cars gauge reads. What could cause this?

Different rims and tires (rolling radius) plus the usual manufacturer 'error'.

I have 20's on my car now and I have not seen that wheel anywhere yet on any other car. If it came stock with 19's could have caused the speedo error. Please discuss how changing sizes affects the readings. My wheels are a seven spoke chrome 20 have you seen that one before?

Tires.... look up the tire's rotations per mile spec. I can easily see speedo diff between stock P Zero and the Blizzak LM60.

Try a GPS against the digital read out on the dash.

Speedometer Accuracy?
 

Reply to an old post --- I have 20" Mulliner package rims which came with the car as a factory option. I am consistently about 3 mph higher on the speedometer than radar signs indicate. On another car approaching the same sign it is spot on. It's my understanding that rim size should make no difference as tire sidewalls are adjusted to compensate which I guess is why we see the ultra narrow sidewalls on extreme 22,23,24" rims.

Some time ago, maybe 5 to 6 years in a magazine review, i read that the Bentley Continental GT had a speedometre missreading of around 7-8%. I was surprised a car like this had such a big error margin because at 320km/h on a highway its a very big difference:D

That's what I am experiencing --- 42mph on radar display vs 45mph on my speedometer. My speedo always high

I have a 2007 mulliner with 20" speed wheels and my speedometer is at least 5 - 6 mph off when I go though speed zones with radar showing my speed.

If you think about it, its the fastest and cheapest way of getting extra performance without extra cost of fuel or other expensive alternatives, and its also good for the planet. Well thought by Bentley, just tweak the speedometre.

speedo
 

Legally they can be up to 10% off the speed reading as long as they are reading fast, they legally cannot show you going slower than you actually are. All my British cars (Lotus, Bentley, Aston Martin) all have been reading almost 10% fast from the factory.
When I put larger wheels and tires on my Ford it threw off the speedo by 6% so non stock combinations can also affect it.
Paul

hilarious

You THINK you are getting performance.:p

My Harley Davison motorcycles are the same way. It may help keep me from getting to many speeding tickets.

I found this post after a search for an incorrect speedometer reading. So i'm 6 MPH faster. At 80 the GPS speed is actually 74. So that equates to ruffly 1/5 of a mile additional for every mile driven. So if a vehicle has say 50,000 mile on it that would be an additional 2500 mile on the speedometer. If my math is correct. But the big question is this something that can be reprogrammed with VAS maybe? Anyone have or had and corrected this problem? T :cool:

Speedo
 

Your math doesn't appear to be correct. You are over by 6/74 which is around 8.1%, which is within the legal limits. 1/5th would be 20%. Also I don't believe the Odometer is tied to your Speedometer. Tampering with the odometer is very likely illegal. Simple way for you to check is to use the mile markers on the side of the road. It will be obvious if it is off by the 8%. I checked mine, speedometer off by just under 10%, odometer was spot on correct.

I have the 'Torque pro' app on my Android with a Bluetooth adapter that plugs into the OBD socket.
The true speed as shown by the on-board diagnostics is always lower than the speedo.
When I upgraded my wheels to 20" the difference was less as the rolling radius was slightly greater.

As for GPS, it's normally more accurate on a level road so don't try it on hills.

Les.