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MG Midget and Sprite Technical - Odometer reading 10% high!

Folks,

I have an issue with the odometer in my 71 midget and would appreciate some help.

The odometer is over-reading by 10%. It is the original, so is the correct one for the car (serial number 6142/06A). The wheels are the original mkI Rostyles sitting on Firestone 145/13 tyres.

I had the speedo rebuilt by Richfield Speedograph, but the problem remains.

HELP! Any ideas as to what the problem is, and how to fix it?

James
James30

I think 10% is within the average for most cars, even my modern over-reads by 10%.

AndyB
a borris

Thanks for the reply Andy.

With every BMC/BL car I have ever owned, the speedo has always over-read, but the odometer has been pretty much spot on

But the Midget is the only one where the odometer is ALSO wrong.

Since the wheels are standard, the speedo head has been rebuilt and the diff is standard, I am very puzzled!

James
James30

what profile are your tyres?
David Smith

I too find that strange James, like you most odometers I have ever had have been about spot on. Have you a different gearbox or diff?
Robert (Bob) Midget Turbo

Thanks again for the replies!

Bob - no, diff and gearbox are the originals.

David - if I'm honest, I don't know. As I mention in my original post, they're 145/13s made by Firestone which I thought was standard. Not being an expert, I don't know if 145/13s can vary enough to change the odometer!
James30

Tire sizes have 3 primary components such as:145 r 60 - 13. The last number is the nominal wheel diameter, the first number is the tread width, the number in the middle is the profile heighth as a percentage of the tread width. A 60 would be shorter than a 70. This will have an effect on the outside diameter of the tire & will have a direct effect on the speedometer & odometer accuracy. Change any of these 3 & you can be sure of an inaccurate reading.

Check out these sites for more info.

http://www.miata.net/garage/tirecalc.html

http://clubs.hemmings.com/clubsites/chevylist/tech/tire_size_conversion.htm

http://www.dsm.org/tools/tiresize.htm
SK Kowznofski

PROBLEM SOLVED!

This has been bugging me, so with the help of Gary Frogeye (thanks Gary!) we jacked the car up on one side, and observed how many times the propshaft rotated for one turn of the wheel.

Result - I have a 4.22 diff which ties in with the 10% over-read. Slightly embarrassing, but it must have been a 4.22 for at least 35 years (or been ordered that way).

Anyway, does anyone want to buy a almost brand new rebuilt diff, with only about 275 miles on it....? :-)

James
James30

Yep ;-)
Robert (Bob) Midget Turbo

James car is absolutely stunning. it looks about as original as it's possible to get with almost all the panel original to the car. And it felt great to be driven around in it.
With James permission, I'll post a couple of images up of it.
Garyfrog

Yeah, no problem Gary - go ahead.
James30

I'll put some up tonight and email them to you too. I apologise in advance about the quality.
Garyfrog

Just a small contribution. I had the original Frog speedo recalibrated last year by Speedy Cables to suit Type 9 and 3.9 diff. It came back totally rebuilt but both speed and odometer over recorded by 10%. You do get used to it. I did have to return it this year when the odo recorded my return trip to Birmingham Tunnels from Kent at a little over 5000 miles! Speedy Cables appear to have done a fantastic job on it all under warranty it seems and both read probably within a couple of percent of true.
John
J R Clark

This thread was discussed between 26/04/2009 and 30/04/2009

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