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MG MGA - Back axle ratio

I have fitted 185/60r15 tyres (dia.602mm) on alloy wheels to my A with the standard 4.3 diff. After reading Barney,s site the original tyres where nearest to 165/80r15(dia.645mm). I have a B 3.9 diff and wondered if this fitted would bring the speedo and performance back to original. Anyone have a formulae for this calculation
Brian Paddon

you need to look at Mick Andersons gearbox Ratios/Rear Axle Ratios Spreadsheet, it is brilliant for this.

You measure your tyres circumference (best to actually measure it with a tape measure) and then work out the tyres diameter using D=3.142/C

You then input the tyre Diameters and your diff ratios onto the spreadsheet and it lets you compare the effects of any changes that you make.

You can also change the individual gear ratios.
I used this to help me work out the best ratios to put into my 5-speed box when I made it into a much closer ratio gearbox.

I think you will probably need to get your speedo re calibrated to correct it for your 3.9 diff.

Hope that helps

Colyn Firth

Tis simple

4.3 devided by original tyre -645
Then multiply by new tyre -602
Answer is diff ratio requireded-------4.013

3.9 would be a lot closer choice than 4.3
William Revit


In case you misread Colyn's intention in his formula.
The diameter is the circumference divided by Pi (3.142).

M F Anderson

Just a word of caution for anyone using Microsoft Excel spreadsheets.
When you change the value in a cell the new calculation does not take effect until you click in another cell, any cell.

M F Anderson

Itís not correct to use the Outer Diameter of tire for calculations. Because of the rolling tire deformation itís necessary to use for calculations so-called ęDynamic DiameterĽ of a tire. Some manufacturers give this parameter in their data sheets (more often data sheet has parameter of Revolutions per mile (km). If you know this itís easy to calculate the dynamic diameter.
As a sample:
BF Gooddrich 155/80R15
1. The tire OD = 629 mm
2. Revolutions per km = 522
3. Dynamic diameter = 1000000mm/(522*3.14) = 610 mm
You can see 19 mm difference between OD and real rolling diameter of the tire.
Nikolai Skliadnev

Where did you find the information of the 24.8Ē (628 mm) diameter that you use for calculations?
Nikolai Skliadnev

I am surprised that the dynamic diameter would actually be the smaller of the two.

I would have thought that the dynamic diameter of a tyre at speed would be greater than when it is at rest because of centrifugal force throwing the tread outwards?


Colyn Firth

Thanks Mick, I wrote the formula the wrong way round (sorry Brian) I should have written D=C/Pi
I think I need to go back to school to do an algebra refresher :-)

Colyn Firth

Yes Colyn,
You are right. The dynamic diameter is greater than static diameter (when a car is at rest). But the static diameter and the outer diameter of the tire are not the same.
When a tire is at rest itís compressed and the static radius and outer radius of the tire are different.
The information which I found for the 5.60-15 tire in one of the car manuals:
OD = 651 mm
Static Radius = 304mm (D=608mm)
Dynamic radius = 309 mm (D=618mm)
Nikolai Skliadnev


A tyre can have a range of diameters based on what the wheel is doing, off the ground, on the ground at rest, at slow speed, and at high speed (centrifugal force).
I got my starting diameter from the MG factory data for the original 5.60-15 cross ply tyre. They gave it at 17.2 MPH per 1000 RPM. I then worked backwards to get the diameter.
This all academic with modern radial low profile tyres.
What each person needs to do is to drive at their normal speed, note the RPM and note the speed with a GPS or using road markers.
Then they can work out the diameter to use.

M F Anderson

Thank you Mick.
Itís a good staring point for further calculations if you know a real rolling diameter of a tire. I couldnít find this information before in order to calculate speed when changed my original style 5.60-15 bias tires to modern radial tires.
This is why I asked you where the information received from .
Nikolai Skliadnev

Put a chalk mark on the tyre and the ground and roll the car along for a full revolution of the wheel and measure between the chalk marks----rolling circumference
William Revit

This thread was discussed between 04/10/2017 and 06/10/2017

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