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MG TD TF 1500 - 4.3 ;1 CW & Pinnion revisited

Gentlemen
After fitting a 4.3:1 crown wheel & pinnion, how is the speedo recalibrated?
John James

John, I'm not sure about your side of the Pond but over here we have a few places that specialize in such a recalibration. Typically, they'll ask to to measure the number of turns of your speedometer cable in some specific number of rotations of your front wheel and to provide them with the numbers. They also ask for measurements of your wheel. They then take your speedometer and replace its inner gears with a set that will match those proportions. That's for the odometer. They'll also recalibrate your speedometer for road speed indication. You can expect to pay about $125-150 for this with a general servicing of the speedometer.
Bud Krueger

Thanks Bud, it would be interesting to know how many people bother to recalibrate the speedo/odometer after changing the rear axle ratios. I'll do some research on instrument specialists over here before changing the CW & P. No doubt the cost wil be pounds for dollars.
John
John James

Most of our Club members have not recalibrated the speedometer. We have converted about 60 cars now to 4.3 or 4.1.
We just recalculte the RPM to MPH (based on rear end ratio and tire size) and use the Tach, since that's what you usually do in a T-car.
In the US it only matters in urban areas where the speed limit is 25 to 35 MPH and at these speeds the 15% speedometer error isn't that big in MPH. Where it counts is at the Expressway speeds (70 MPH) and our readings are 15% too low ... so keep up with the traffic and don't worry about the speedometer reading.
Correct odometer reading with a calculator for milage, miles per gallon, etc.
Don Harmer

Don
Is 17.14 mph per 1000 rpm in top correct for a 4.3:1 axle TF?
John James

John, that's quite close. It is also a function of the circumference of your tires. With my Dunlop SP20 165R15 tires I get 16.8 mph/1000 rpm in top with the 4.3 gears.
Bud Krueger

The speed needle can be reset to the correct mph/kph, by resetting the needle. 1st find out the amount of error, then pull/twist the needle off & reset, while holding the drum (on the magnetic speedo) stationary.I multiply my odometer by 22 or so % the speedo & tach are about 10/15% off from new so it's all relative!many of us run the tach. to the middle of the clock , just to find that it is only about 5,000, or mabey 5,500 rpm. 'till the roads wear out> on to the 'Glen! Len
Len Fanelli

I am using a 5 speed transmission in a TD which caused the same speedometer error problem. I use a small gearbox mounted on a short cable connected to the transmission, which corrects the turns per mile for the TD speedometer. Cost was about $75 including the adapter cable. Len, see you Wed at the Glen.

Jan
Janson Hurd

Janson
Where can you purchase the small gearbox?
{8^{
Jay
Jay Dyck

I have decided to get a local mg specialist to fit the 4.3:1 CW&P during the winter. He sends his speedos back to Smiths Instruments for recalibration. They charge £65 plus postage which I think is quite reasonable these days. Wonder if they can fix the clock while they are at it!
John James

John,

After the 4.3 to 1 conversion, with 165 x 15 radials (smaller diameter than original 5.50 x 15 bias ply) you should get 17.61 Mph per 1000 RPM vrs 15.74 MPH orig
(Calculated using tire radius and rear end ratio)

At 3500 RPM this means 62 MPH instead of 55

( I also get 17.14 using a different radius tire)
Don Harmer


Thanks Don, sounds good. I have the same size 165 X 15 radials fitted made by Firestone giving a rolling diameter of 24.5". I am just about to sit down and work out the mph/1000rpm figure mathematically. Thank god for these new fangled calculators!
{No wonder modern kids are hopeless at maths).
John James

Jay -

My speedometer gear box came from:
A.C.I. Engineering
6728 Lovers Lane
Portage, MI 49002
Tel. 616-327-1991
Jan
Janson Hurd

Don
Having accurately measured the rolling diameter of the wheel at 25.5", I have calculated the 4.3 mph/1000rpm as 17.649, which is pretty damn close to your figure and well worth the cost of the conversion.
John
John James

This thread was discussed between 04/09/2004 and 07/09/2004

MG TD TF 1500 index

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