Odometer Accuracy

[thomas wright]

Hi all,
My odometer appears to be recording less KM that I am travelling and I don't know why. The car was laying up for a number of years before I had it recommissioned with a new fuel tank and sender unit along with ancillary parts for brakes, exhaust system.
I had read on the forum that the correct tyres are needed as the circumstances of the tyre is important.  I measured my spare   unused tyre which is the same as on the car, Dunlop SP Elite 185R 14 90S Radial Tubeless.
Diameter:  25 1/2"  ( 645 mm)
Circumstances:  80  5/16"   (2040 mm)
Would you know if these tyre should record accurately the distances I travel or could you point me in the right direction for other cause of this problem.

Many thanks




280SL W113 1969 auto, 050 paint,  lhd.

[WRe]

Hi,
your tire size is correct, see att. file.
I would check it with a GPS system, e.g. mobile or navigation system. Otherwise your odometer needs some inspection.
...WRe

[doitwright]

Unless you are the original owner, the speedo could possibly have been recalibrated to work with a different size tire. Just a possibility. It used to be and may still be true, before you sent your speedo in for service, they would ask you to count the revolutions of the speedo cable over a specified distance. There is a gear on the transmission that drives the speedo that could also be worn.

[Pawel66]

Is the difference vs. reality substantial? Do you also have similar differences with speed reading? Is it a couple of % points? Or more like between miles and kilometers?

Usually due to tires sizes you may be a couple of percentage points off as small differences in radius give small differences in wheel circumference.

[thomas wright]

Okay,
this is the full story.
After the car was commissioned and then new fuel tank fitted the odometer read 02494km on 3.4. 2020. I topped up with fuel on 4 occasions, last being 30.6.2020 to a total of 99.50lt ( 21.887 imperial gallons).
I used the car for short but regular journeys,  and through winter months until the fuel gauge showed it required a top up which I did on 21.2.2021. With the intention of establishing what my fuel consumption was I took an odometer reading,  it was 02661km which ment I had travelled a mear 167km  ( 103 miles) since 3.4.0202. It was at this point I thought 21gallons vs 103 mile equates to very high fuel consumption or the odometer reading is incorrect. That's the dilemma. 

[Pawel66]

Check the odo with GPS, I would say, or vs. road poles before blaming it.

Then check the miles per gallon.

Then you may need to go through the linkage tour.

This is what i would do.

[thomas wright]

Thanks to all who helped ,you have given me a few options to follow and cure this problem.

[cfm65@me.com]

Hi Thomas,
I live in a ‘metric’ country and my E Type has a mph speedo with a km face. The speed indication in kilos is very close to correct, but the distance travelled is in miles.
I guess this might be the case with you Pagoda.
Regards
Chris
Cape Town

[thomas wright]

Thanks Chris,
The thing is I am pretty sure my odometer dial is original as the car was ordered for the French market according to the Data Card, and I have owned it since 1976 and I believe I was the third owner as the car came out of the factory  on 29 July 1969. Nevertheless your point of view is worth considering as I investigate further.

[mdsalemi]

FWIW, my speedo (which of course, is tied to the odometer) has consistently read between 3-7 MPH higher than actual. Actual checked with GPS speed app. I've been told that many cars do that: read a bit higher. My car has original sized tires, 185HR14.

[MikeSimon]

In Germany, there has always been a law that speedometers - and consequently odometers - are no allowed to read "low". This is to prevent arguments in court that your indicated speed was lower than what you were "clocked" for.
As a result, all vehicles produced in Germany over the decades have speed indication instruments that will show a higher than actual speed. I have a Porsche, a Mercedes 2 Audis and 4 German motorcycles and their speedos all show significantly "higher" than actual.
Some vehicles in Germany have to have officially "calibrated" speedometers/odometers (e.g.: Taxis) and the tolerance on calibrated instruments is around +5%, never under.