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MG TD TF 1500 - Rear axle ratio vs tire size

I recently had the real axle gear ratio changed from a 4.875 to 4.3 on my mgtf. I was expecting to get around 17 mph/1000rpm but am getting only 16 mph/1000 rpm. I think Bud /Krueger is correct when he says tire size has to be taken into account. I currently am riding on Dunlop 165SR15 radial tires. As near as I can figure the tire diameter is a little over 25". Am I correct in thinking the old bias tires of 15 x 5.50 would have measured 26" in diameter? If so this might account for the diffeerence between expected 17 ,ph/1000rpm and actual 16 mph/1000rpm.

Bill
Bill Schroeder

Radial tyres generally have a lower profile and the walls squash more under load, so measuring the free diameter off the car is not the best indicator. What you need is the rolling radius on the vehicle, which will certainly be less.
Dave H
Dave Hill

Some time ago someone posted an Excel spreadsheet which calculated rear axle ratios taking into account your own measurement of tyre diameter or circumference, and gave resulting mph per 1000 revs etc.
I think they gave their own speedo indicated speed v. gps speed as well.
Anyway I saved it, so here it is again -

https://www.dropbox.com/s/9dulce4zp4hwkax/TF_rear_axle_ratio_calcs.xlsx?dl=0

David
David Wardell

http://www.dropbox.com/s/9dulce4zp4hwkax/TF_rear_axle_ratio_calcs.xlsx?dl=0

edit out the "S".

....
MAndrus

After Dad and I changed the rear end on the TF we took a GPS with us to see what we had done to our speed. To our amusement, the change in the ratio compensated for the reduced radial tire diameter. The GPS's digital statement of speed was right in the middle of the much-oscillating speedometer range.
Safety? Fast?
Scott Ashworth - '54 TF
S. R. Ashworth

Put a piece of tape on the sidewall straight down, and another on the pavement to mark starting point, then roll the car/tire exactly one revolution, and measure the distance traveled. That should give an actual working circumference. Or go 2 revs and divide by two to be a bit more accurate. (Not my idea- that is how directions say to calibrate bicycle odometers!)
George Butz

I downloaded & tried to use the calculator above as it seemed like a valuable tool but I'm not sure I entered in the correct values.

I was able to enter my ratio of 4.300 on line 11 which shows a calculated speed of 95.143 mph @ 5,500 rpm which would be really nice but just doesn't seem right. Anyone able to advise me on exactly how to use it? Cheers
Peter TD 5801
P Hehir

Peter, not sure I can help that much as I didn't invent it, and bits of it puzzled me as well.

As for line 11, I think you enter values for both turns of wheel AND propshaft turns - i.e. you have to count both. Then it gives you your rear axle ratio. After that you enter the result into B17 and add your tyre diameter into B15 and then the table below gives you speed v. revs.

I think ...

David
David Wardell

Peter,

I can't help you on using the spreadsheet mentioned above, but I can send you one which I wrote when I changed from crossply tyres to radials. If you are inerested send me your email address and I will forward it to you. john dot scragg at wanadoo dot fr

John

52 TD


J Scragg

Peter,

The calculation of these values is very easy, and follows the equation (for 4th gear):

Car Speed (in mph) = 0.0009525 x Wheel Circumference (in inches) / Diff Ratio x Engine Speed (in rpm)

In case of a differential ratio of 4.3, the formula is:

Car Speed (in mph) = 0.0002215 x Wheel Circumference (in inches) x Engine Speed (in rpm)

The main unknown is the circumference length of the wheel, which for accuracy, needs to be measured at the right tyre pressure, following the procedure explained by George.
An approximation can be always computed from the tyre size code, but different manufacturers, and even different wheel designs of the same make give different diameters.

Jesus
J Benajes

Thanks all for your comments and suggestions. You have confirmed my suspicion the circumference of a radial tire for our cars is less than the circumference of the old bias tires. I must say I am pleased with the 4.3 ratio but think a 4.1 would be even better if radials are used.

Bill
Bill Schroeder

John my address is pjbm at bigpond dot com. Jesus I'll measure my wheel circumference & apply the formula that you've posted. Be interesting to compare both results. Much appreciated. Cheers
Peter TD 5801
P Hehir

On my website, in the Technical Index section, is a paper I wrote on calculating the Turns Per Mile of your speedometer. From that ratio it is pretty simple to calculate the new speed per 1000 RPM.

Warmly,
dave
Dave Braun

One should be careful with using the value o TPM, since this is basically a relationship between the speedometer cable revving speed and the car speed indicated by the speedometer. As such is just a characteristic of the gage, and not of the car. Of course the correct case is when the speedometer TPM value is well matched to the car (gearbox + wheel) characteristics.

In our TDs, TPM is very often 1600, which means that 1600 turns of the cable will advance the odometer one mile. The speed indicator needle will show the same relationship (provided that magnet and spring are in good condition): 1600 cable turns per minute should move the needle to indicate one mile per minute or 60 miles per hour.

When the speedometer is matched to the car, the following formula should be followed:

TPM = 62992 x DiffRatio / PinionRatio / WheelCircumference (in inches)

In the common TD case of DiffRatio = 5.125 and PinionRatio = 2.5, the correct match corresponds to a wheel circumference of 80.7 inches (or a wheel diameter of 25.7 inches).

It is also worth noting that TPM links wheel speed with speedometer indication (and if correctly matched also with car speed), but the engine revving speed is not included in the concept or calculation of TPM.

Jesus
J Benajes

This thread was discussed between 03/09/2015 and 04/09/2015

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