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MG TD TF 1500 - Speedo Input RPM

Just finished a tach rebuild and calibration. It worked out well so now will try to repair the broken speedo. For troubleshooting and calibration I need to know the speedo RPM input per 1000 RPM in top gear to enable bench testing the repair. I have been searching the archives but to no avail.

TIA


Jim Haskins 1953 TD
J M Haskins

Jim, it's in the archives, but I'm not sure where to find it, so I'll do it again. Look at the attached image of my speedometer and spot the number 1600. That's the number of turns of the input shaft per mile. So, 1600 turns/minute would equal 1 mile/minute, i.e., 60 miles/hr. Bud

Bud Krueger

Bud,

Thank you very much. Your information is just what I need. I read dozens of archive entries but never found the right one. The knowledge available here is truly remarkable.


Best regards,


Jim Haskins 1953 TD
J M Haskins

Keep in mind that was accurate for original tires (outside diameter) and rear end ratio. When my speedo was rebuilt some years back, they had me mark the tires, roll a set distance, and count turns of the cable (put a pointer on it). Of course I have no idea of the distance, etc. now. George
George Butz

I actually just found my calibration correspondence- which I don't think will help due to lack of the formula needed. " For the calibration check please jack up one rear wheel and rotate for 25 turns while someone counts the number of cable turns for you, then change places try again and hope you agree! We also require the tyre size and make." This was from John Marks, Vintage restoration. George
George Butz

See Dave Braun's writeup on his website.
http://www.dbraun99.com/technical%20index/Calculating%20TPM%20of%20your%20speedometer%20cable.pdf Sometimes the '%20' for 'space' gets to be a problem. If so, just got to his home page and follow the links. Bud
Bud Krueger

Jim, if you want to send me your email address I'll send you a copy of Dave's directions. Bud
Bud Krueger

Jim,
If you run into trouble you can always fit one of my custom built corrector gearboxes-plug and play.
Here's one I built recently for a Delahaye.
Regards
Declan

Declan Burns

Declan,

Thank you for the kind offer. I would be proud to have an example of your excellent workmanship in my car. It remains to be seen if my patience and frustration can hold up for the duration of the repair. Hopefully I get it right with lots of careful effort.

Best regards,

Jim Haskins 1953 TD
J M Haskins

When I had my TD speedo calibrated by Nissonger they had me roll the car forward in a straight line for 52.8 feet and count the cable revolutions. 52.8 feet being 100th of a mile.
John Quilter

John,
With my gearboxes there is no need for any of that. I ask potential client's to compare the speedo read out at say 50mph with a GPS and fully inflated tyres. I then choose the closest gearing to correct and build the gearbox to match. The original speedos are not linear over the whole range so hence I choose cruising speed. The speedo on my TD at cruising speed is accurate with 1/2 MPH-more accurate than my modern daily driver! There was possibly a bit of luck involved.

Regards
Declan
Declan Burns

For speedo or tacho calibration, remember this usefull thread for triumph indicated by Jesus.
http://www2.mg-cars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2016020720400219215

Laurent.
LC Laurent31

In the TD with the usual gear ratios, the actual TPM of the car can be calculated as

TPM = 3280/Lw (meters)

Being TPM the turns per one mile in the speedo input spindle and Lw the length in meters of one wheel turn.

Or TPM = 129137/Lw - with Lw measured in inches

So, the value of 1600 (as shown in Bud’s photo) is correct for a wheel turn length of 2.05 meters or 80.7 inches (or a tire diameter of 25.7 inches – close to an old 5.90x15 tire)

Even in the case of matching that wheel turn length, the value of 1600 TPM is valid for the odometer, since it is a fixed input/output system, but not in general for the speedo needle, whose input/output response depends primarily on the strength of the magnets.

Jesus
J Benajes

Here's the info from Carls book with a bit of my math. Hope it helps

Dave

Dave Clark Arizona

Gents,
Thank you all for the information and advise. Speedo is complete and road tested. Used a 1600 RPM drill as an input. I did not account for the rather rapid loss of RPM as the battery discharges. Started out a 1596 RPM which was close enough for me. By the time the trial and error was done the drill had lost nearly 500 RPM. Thus the final setting was way off. Oh well, just do it again. The story of MG restoration.


Jim Haskins 1953 TD
J M Haskins

This thread was discussed between 02/03/2016 and 17/03/2016

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