W123 300D speedo 8mph high at all speeds

[okto]

The speedometer on my 84 300D reads 8mph high at all speeds, i.e., at 70mph indicated, I'm traveling at 62mph, according to GPS on two different phones and stopwatch times between mile markers.

I've checked it at 80 indicated, 60 indicated, and 30 indicated to verify that it wasn't just the wrong speedo gearing, and it's 8mph off at all those speeds to within speedo needle accuracy.

Any ideas?

[BWhitmore]

Has the speedimeter been replaced at anytime? The proper speedometer is mated to the rear axle ratio. The 1985 model used a different speedometer than the 83-84 model and the earlier models used a different speedo also.

[Phillytwotank]

tire size can effect this. what size tires are you running?

[97 SL320]

Speedo / rear axle calibration / tire size will give a percentage of error not a fixed 8 MPH offset.

The only thing that would cause a consistent 8 MPH error is a needle that is offset relative to the speedo clockwork.

Does the needle fully return to the zero stop pin or where all other cars show the needle when stopped?

[optimusprime]

The speedo may have been taken out of the car before. If you work on them you have to lift the pointer over the end stop and mark its resting place , on the dial with a pencil .Then you know the right spot to replace it to., This was not done .so the speed is out ..Take the needle over the end stop ,mark it, so you know . Advance it off the pencil line just a little .It will be trial and error .

[okto]

I don't have service records for this vehicle (the previous owner was, ah, not a Mercedes person), so I don't know if the speedo was replaced.

optimusprime: The speedo may have been taken out. The needle wags a lot coming off the stop pin, so I'm guessing at some point it was removed and replaced incorrectly.

[okto]

Additional data: the odometer seems to read high according to GPS, as well.

[gmog220d]

Originally Posted by okto

Additional data: the odometer seems to read high according to GPS, as well.

That would indicate that your speedo is not matched to the rear end gear ratio, and/or that tire diameter is not spec.

[97 SL320]

Originally Posted by okto

The needle wags a lot coming off the stop pin, so I'm guessing at some point it was removed and replaced incorrectly.

That's usually a sign of a dry or bent speedo cable. ( unless you have the electronic speedo )

[okto]

Originally Posted by Phillytwotank

tire size can effect this. what size tires are you running?

OTTOMH, I wanna say 195/75R14, but I'll check in the morning.

[okto]

Tire size is 195/70R14, I don't know what stock is.

[gmog220d]

Recommended tire size and inflation pressure should be on a plate or sticker in the driver's door jamb.

[97 SL320]

What part of my post 4 " Speedo / rear axle calibration / tire size will give a percentage of error not a fixed 8 MPH offset. " does not everyone understand?

A gear / tire error will produce a percentage offset not a fixed XX MPH offset across a range of indicated road speeds.

Try this using 2 rear gear ratios.

Tire Size, RPM, Speed, and Differential Ratio Calculator

To get this:

GEAR . . . . . .RPM . . . . . MPH. . . .GEAR . . . .MPH. . . MPH DIFFERENCE
3.45 . . . . . . 1000. . . . . 21.1. . . . 2.00 . . . . 36.5 . . . 15.4
3.45. . . . . . .3000 . . . . . 63.4. . . . 2.00. . . . .109.4. . . 46
3.45 . . . . . . 7000. . . . . 147.9. . . . 2.00 . . . .255.2 . . .97.3

[97 SL320]

Another calculator for changing tire diameter and speed offset.

Actual Vehicle Speeed Calculator and MPH Offset (New Tires and/or New Gears)

Type in some different MPH numbers, the % of offset stays the same, but the actual MPH difference changes through the range.

Our guy has a constant 8 MPH offset over a wide range of speeds.

A question for the OP would be: " Is the odometer accurate?"

[gmog220d]

Originally Posted by 97 SL320

A question for the OP would be: " Is the odometer accurate?"

See Post #7.