I discovered recently that my MB ML320 shows 5% over the real speed (105kph instead of GPS measured 100kph) with stock wheels/tire (correct diameter).

I've noticed differences in the past with other vehicles but never that big.

My 997 is in the garage for the winter, i can't test it.
Someone did the test with a 997 ?

Needless to say that all the benefits of doing this "speed optimism" goes to dealer and manufacturers...

Unless, of course, the speedometer is not what the odometer takes to put the recorded milleage, which i doubt.

 

Whenever I encounter the "Your speed is..." speed limit signs with the attached radar, it always tells me I am going 2 mph slower than my speedo says. I don't mind much though.

 

at any speed ?

Usually it's a % difference

 

I'm sure it would be a greater difference at higher speeds, but the radar signs I encounter all seem to be for 45mph, so I'm usually going similar speeds.

 

Ok thanks for the input

Looks like you have about the same difference as my MB: 4-5%

Doesn't look much, uh ? Well, on a lease when they charge 0.25$ for each Km extra on the contract, it can make 1000$+ difference at the end.
Not to mention the services that comes faster than it should be, warranty that ends faster, gas milleage better than really is, etc..

I believe manufacturers are doing this absolutely on purpose. I see no reason to accept this.

 

Mine reads about 2 mph slow. Funny that my 2000 Dodge Dakota is right on the money!

 

Mh I am not sure, which is right, but my Garmin GPS says in my ML320 about 3-4 mph less than the odometer. In my 911 it is more like 5 mph

 

Just tested an old Chevy Astro, and it's right on..

Is that mean Americans are better in math than Germans ?

 

I wonder if manufactures do this to help avoid speeding?? I mean why else would they throw the speedo off a couple MPH's. I would be happy to find out I was hit with radar a couple MPH's lower than I had thought I was going. That could be the difference between a reckless and a speeding ticket!

 

seems to be quite typical in the Porsche realm, I have heard many accounts of speedometers showing higher Mph than actual. I know mine always seems to be +4 mph.

 

I think that Porsches read 2% faster than reality - meaning when going 98MPH it reads 100MPH.

 

I'm sure they're doing it for their own benefits..

Think about it, ALL results of doing that is good for them. On every levels.

Sport motorcyle market is even worst..

Anyway, i asked the dealer to make the speedo correction. We'll see..

 

My speedo is also +4MPH.
???

 

Looks like a big difference, same as my Ml320.

It's a shame

 

My speedo reads 2-3 mph higher than the traffic radar signs are showing. This holds true @ 20 mph or 75 mph or anything in between.