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MG TD TF 1500 - ? RPM = MPH

Speedometer again not working, will wait till winter to address. My question is there a conversion of RPM to MPH with standard rear end and tires, just for reference
Jon Levine

There is a chart out there for this, however make sure you figure in your tyre size if going that route.
(and how "accurate" is your tach?)

IMHO: Best way is do some comparison driving with a GPS or smart phone and note your tach readings.

Just about any smart phone out there can load an app for "speed" these days for free if you have no access to a GPS.

I have an old Garman GPS I keep in the TF. Those are all over craigslist and/or fee-bay really under $20. (I did a quick search on CL just now and got over 500 hits...4 on the 1st page were $20.)
David Sheward 55 TF1500 # 7427


Whilst there are charts in various publications, I think Dave's suggestion is your best bet.

Speed is relative when compared to charts. It is dependent on your final drive ratio, effective (road) wheel diameter, and slip of the belt on the three pulleys on the engine.

Try explaining that to a constable when you get pulled over!

Gord Clark
Rockburn, Qué.
Gordon A Clark

Jon, the OEM figure is 14.4 mph/krpm in 4th gear. Bud
Bud Krueger

The only way to get close for your car is as David says, use a Garmin or other GPS and Check it that way.
I have replaced the speedometer, cable, and drive in my TR4A and it still does not work. I used a Garmin, so I know at a comfortable cruising speed I can pretty accurately gauge my mph by the tach. Requires an old man to think, but better than getting a ticket.
Larry Brown


Here are four ways to determine your speed -

My speedometer is broken. I use a free app for my iPhone. The app like all GPS apps uses data, but I have an unlimited data plan, so no worries. Except also like all GPS apps, it uses a lot of battery power, which is a worry because I don't have a power plug in my car.

This is the app that I use. Big easy-to-see digital readout and has a trip odometer, too. Sits in the dash opening in front of me in my TF.
"Speedometer Free Speed Box"
By Hans Schneider

As Gordon noted, these next three ways depend on your tach showing the correct RPM -

A 1950s advertisment for mgtf stated 15.25 mph at 1000RPM. (4.875:1 ratio diff; 5.50-15 bias ply tires)

Fill in three boxes, click "compute ..." by the fourth box.

Since the tire is "flat" on the bottom, do-it-yourself computation using tire circumference may be a little more accurate than using diameter on the web site. Make a mark at the 6:00 position on one of your tires and on the pavement. Roll the car until the mark is again at 6:00 and make another mark on the pavement. Distance between marks = circumference of tire.

1 [divided] by diff ratio [times] inches circumference of tire [divided] by 12" per foot [divided] by 5280' per mile [times] 60 min per hour = speed at 1 RPM

Multiple the number by RPM to compute speed at that RPM Or multiply it by 1000 as an easy reference.

The calulation for my TF =
1 [divided] by 4.875 [times] 81" [divided] by 12 [divided] by 5280 [times] 60 [times] 1000 [equals] 15.7 MPH @ 1000 RPM.

LM Cook

This thread was discussed on 27/06/2014

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