Speedometer conversion from MpH to KmH

[TEJOLX47]

Hi there,
My car is a 280SL originally delivered in 1970 in the USA.
Having bought it in Europe I decided to convert it to Euro specs, mostly for convenience.

At some point in time, I have bought a Km/h speedometer.
I was concerned about differential ratio, but went forward with the change.
After installation, I have noticed two issues: a) the Kilometers were correctly and accurately counted; and b) the speedo showed a speed about 10Km/h less than the real speed (measured with GPS).

So, on the first opportunity, I sent the speedo to recalibrate.
It came back showing a speed 5Km/h ahead of the real speed.
And, for the time, I have done nothing.

Last week, I was talking with an oldtimer MB mechanic, and he told me that to swap the Speedometer, I need to replace the instrument as well as a "part" in the engine (and he pointed to the distributor shaft).

Does anyone know anything about this?
Many thanks
Francisco

[Jonny B]

Interesting comment about the "part in the engine" - the speedo is hooked to the transmission. I don't think there is any connection to the engine.

The tachometer is hooked up at the engine. There is plenty of information on the website about that.

[ja17]

No relationship to the engine. It is driven off a gear in the transmission.

[MikeSimon]

Hi there,

It came back showing a speed 5Km/h ahead of the real speed.
And, for the time, I have done nothing.

Last week, I was talking with an oldtimer MB mechanic, and he told me that to swap the Speedometer, I need to replace the instrument as well as a "part" in the engine (and he pointed to the distributor shaft).

Does anyone know anything about this?
Many thanks
Francisco

At what speed does it show 5km/h ahead? In Germany, by law, the manufacturers MUST NOT install speedometers that show LESS speed than actual. As a result, all km/h speedos in German road vehicles are calibrated to show MORE than actual. 5km/h more at 100km/h is absolutely normal.

[Jordan]

At what speed does it show 5km/h ahead? In Germany, by law, the manufacturers MUST NOT install speedometers that show LESS speed than actual. As a result, all km/h speedos in German road vehicles are calibrated to show MORE than actual. 5km/h more at 100km/h is absolutely normal.

Thanks Mike, good information to know.  Mine was originally a German road vehicle and the speedo has always shown 3km/hr more at 100km/hr.  It's irked me a bit so its nice to know that is normal.  I'm less irked.

[TEJOLX47]

Thank you all for your contributions.
@Mike,
Thank you for that valuable input.
The speedo shows circa 10 Km/h more than real speed until 80km/h.
Then it reduces the gap to 5Km/h more or less.
I will take that into consideration.
Cheers

[MikeSimon]

There are a few other things that play into this:
- Was the km/h speedo originally for an SL
- The speedo is calibrated to match the rear axle ratio
- The wheel diameter also needs to be considered.

All these things will affect the deviation of the indicated-to-actual speed.

[TEJOLX47]

Hi Mike,

Speedo matches the partnumber for Euro specs Speedo in the EPC;
Rear axle ratio was taken into consideration;
Tire measure is standard.

To confirm this, I have measured GPS distance and odometer distance a few times and for a few distances. They match.
I believe the only issue I have is with the real speed, and that - I was told - is set by a magnetic mechanism in the instrument.
Many thanks
Francisco

[stickandrudderman]

Whoever calibrated the speedo for you should be able to answer your questions.
The normal routine is to make a chalk mark on the tyre and the floor, roll the car forward (or back) one wheel revolution and mark the floor again. Measure the distance travelled and give that to the speedo shop.