Speedometer Acuracy - E320CDi

[Bowruss]

I've had my E Class for about a month now and I'm very impressed.

My previous car was a Volkswagen Passatt TDI and the handbook had some useful tech data which included final drive ratios - even for the 5 speed Tiptronic box that mine had.

From this, I was able to calculate that in 5th, the car was doing 70 mph at 2100 rpm with the speedo showing 76 mph. This was presumably the usual under-reading that manufacturers build in.

Does anyone know the final drive figures for the E320 CDi with the 5 speedo auto box? Or alternatively, does anyone know what speed is actually being done when the speedo reads (say) 75 mph?

The other difference between the cars was that the VW cruise control simply set the speed you had reached whereas the E Class cruise control and the speed limiter show selected mph figures.

I suppose the question reall is - if you set the cruise control for 70 mph, are you actually travelling at 70 mph - or 66 mph?

Regards to all, Russ.

 

[124coupe]

I have checked my E280CDI (not your model!) via GPS and also the distance markers on the motorway - at 70 indicated its doing 68.

Will change a bit as the tyres wear (!) but near enough for me.

 

[jberks]

As you suspect, you set the digital cruise to 70 and your GPS will read 66-68.
Get up to 85 on the clock and you're more like 79 on the GPS.
Explains why when you inadvertantly fly past a police car at 90, they ignore you. You're probably really only doing early to mid 80's - but as everyone elses speedo is out too, you still get the effect!.

 

[124coupe]

I set the cruise to what my GPS says is 84.

OK so far.....

 

[Blobcat]

I set the cruise at 76, which is 73.5 according to my CPS and 2250rpm. AS the speed goes up the speedo is even further out. But on average 2-3mph out

 

[Uncle Benz]

I am very disappointed with my C32!! When it shows 160MPH my sat nav says its only doing 146MPH. The error seems to get bigger when the speed is higher...

 

[Blobcat]

I am very disappointed with my C32!! When it shows 160MPH my sat nav says its only doing 146MPH. The error seems to get bigger when the speed is higher...

They do that to make you feel better and so it looks good on Top Gear etc. Had my E320cdi up to 145mph on the GPS in Germany with still some left so well happy with the car. Wasn't looking at the speedo at the time so I cannot comment on how far out it is.
Above around 100mph the speedo is 'for indication only'

 

[television]

I am very disappointed with my C32!! When it shows 160MPH my sat nav says its only doing 146MPH. The error seems to get bigger when the speed is higher...

the car is limited to around 155 take the limiter off and try it at 225

Malcolm

 

[Bowruss]

Thanks everyone. Looks like my suspicions were right about the overreading.

What now interests me (you see, the forum gets you going!) is the references to GPS showing your speed. As you know, I'm very new to all this and certainly never had satnav before.

How is this done? What do I look for?

Thanks again, Russ.

 

[Blobcat]

A Road Angel (or equivalent) will display your instantaneous road speed. My Garmin GPS will store the Max speed as well as show the instantaneous. IMHO the Garmin is better than the Road Angel and both are much better than the speedo.

 

[Bowruss]

Thanks Blobcat

Do you know if Comand has this facility deep in its bowels?

Russ

 

[David Pemberton]

Many speedometers do over-read, but it isn't always a safe assumption. I just read a magazine road-test about a race to the alps between an SL and an ML. The journalist assumed that the SL would over-read, whizzed down to Dover and was nicked on the way for doing exactly what it said on the speedo...

A GPS is a good way to get an accurate idea, also very helpful at allowing you to set the cruise to do exactly 50mph through a SPECS regulated roadworks section.