MB odometer accuracy

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MB odometer accuracy

Post by drcdb15 » Wed Apr 03, 2019 7:46 pm

Has anyone ever checked the accuracy of their Middlebridge odometer? On a recent journey of over 250 miles, accompanied by a modern car, Google Maps showed the distance covered as 263 miles. The modern car showed 268 miles which was in part due to extra distance around services areas. However, the MB indicated (unless I've made an error somewhere) 299 - an overreading of 13.7%. I've never had occasion to check this before, so I plan to do a controlled test when I can, but in the meantime wondered if anyone else had observed such a high discrepancy?

I also plan to test the speedometer accuracy with a GPS speedo. The speedo accuracy has obvious implications for legal compliance, but the total mileage is of concern too when one has mileage-capped insurance cover. Furthermore, the age (and hence value) of the car is impacted - a car that seems to have done 100K miles may only in fact have covered less than 90K.


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MB odometer accuracy

Post by philhoward » Wed Apr 03, 2019 8:09 pm

Have you checked the speedometer against a GPS reading?


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MB odometer accuracy

Post by scimmy_man » Wed Apr 03, 2019 8:11 pm

what size tyres are you using? that is usually the problem.


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MB odometer accuracy

Post by Andrewsait » Wed Apr 03, 2019 8:28 pm

I'd be a little bit wary of relying on a GPS speed reading. I have a TomTom satnav which shows the speed and no matter what car I am driving, whether old or new, the GPS speed has always been 3-4mph slower than the speedometer. Bear in mind a GPS only measures your linear speed, it cannot make an allowance for the fact you might be going up or down hill and therefore it will under-read your speed.

I have had my Middlebridge only a few weeks so I have not yet sussed out all of its foibles but the speedometer seems to be reading around 5-6 mph below what speed I think I am travelling at, as measured by the TomTom and making an allowance for the fact it under-reads. Also, at a slow speed the needle jumps around quite a bit, trouble is I don't know what is normal in these cars.

I hadn't thought to also check the odometer reading but I will do so over the next few weeks.

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MB odometer accuracy

Post by scimjim » Wed Apr 03, 2019 8:45 pm

welcome to the forum Andrew :D

I’d trust GPS over a speedo every day - your car speedo is designed to over-read (around 5% on a modern). Modern GPS accounts for gradient by a form of triangulation (as long as it has enough satellites).

My modern reads 41 at a GPS 40 and 82 at 80.


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MB odometer accuracy

Post by gtcse8 » Wed Apr 03, 2019 8:49 pm

Andrew welcome to the loony home/club.

Needle jumping about is usually due to cable faults, either kinked or damaged/lack of lubrication.
You are correct about the pratnag not being too accurate over height changes.

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Post by Roger Pennington » Wed Apr 03, 2019 8:56 pm

I've used a GPS speedo for years, and would trust it's accuracy over the normal speedo (unless I was to spend my life driving through tunnels! :lol: ).

And welcome to the forum Andrew :D


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Post by gtcse8 » Wed Apr 03, 2019 9:16 pm

Roger Pennington wrote:
Wed Apr 03, 2019 8:56 pm
I've used a GPS speedo for years, and would trust it's accuracy over the normal speedo
Don`t forget the "Normal " speedo" is allowed to be 10 per cent out, I have never heard about the "Absolute" accuracy on pratnag units, they will have trouble on hilly roads such as in my area. :shock:

I used to travel ALL OVER when as a service engineer and I did find a slight lag or deficiency between the differing pratnags that I used. :roll:

The MB speedo is a different type to the 15,000+ Smiths unit that Reliant used, and who knows what MB did to make it work :pc


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MB odometer accuracy

Post by Will Anderson » Wed Apr 03, 2019 9:26 pm

Over reading of MB speedometers is common. I assume the odometer is similarly out. MB14 overreads by about 10% against a GPS and the GPS is accurate according to the speed trigger signs locally. MB42 reads about right, within 1-2% of GPS but IIRC (Mark?) is on 14" wheels. Which always made me wonder if the speedo drive in the boxes was wrong but then they would have had the cars tested against timing equipment when developing.

Lossiebob on here (MB48) was going to get his recalibrated he thought it was so inaccurate. I am sure Mick, MB33, uses a GPS speedo and I tend to rely on the sat nav speed display and have not been caught speeding yet by following it!

Welcome Andrew! :wink:


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Post by drcdb15 » Thu Apr 04, 2019 12:51 am

Thanks for all your comments folks. I shall explore further and report back in due course.


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Post by gtcse8 » Thu Apr 04, 2019 8:12 am

Yes Will MB42 is on 14" Wolfrace.
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Mark


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Post by peter freeman » Thu Apr 04, 2019 8:22 am

My MB seems to be very accurate when on MB wheels ( going by sat nav and speed radars ) but during the summer I fit Wolfs and then it over reads - not big problem as it keeps me at the speed limits.



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Post by gtcse8 » Thu Apr 04, 2019 8:37 am

15" MB`s are supposed to be same/very nearly rolling radius as 14" full fat Wolfrace tyres??.
I suppose what makes the difference is the abundance of different profile/height/width 15" tyres available.
IF you fit the same profile tyre that Middlebridge originally used then you should be within the Legal 10%


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Post by peter freeman » Thu Apr 04, 2019 9:14 am

I have 195x65 tyres on the Wolfs mark



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Post by Old and Slow » Thu Apr 04, 2019 11:33 am

I have two Satnavs, so I think I might run a test with them both in parallel, along the flat bit of the M4 and see what the comparison shows. The Ford S-Max certainly over-reads by 10% at 30 mph according to the Satnav and the roadside radar displays that are installed on the country lanes round here.


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