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MG MGB GT V8 Factory Originals Technical - Original Tyre Specifications

Can anyone help me with some original tyre specifications for any of the following tyres which came on the GT V8 or can someone direct me to were I can find this information: The specs I am looking for are for any of the three makes(175HR-14)Goodyear G800 Grand Prix; Michelin XAS and Pirelli CN36. I have been checking out replacement tyres for my 74 MGBGT V8 and would like to compare some statistics. I want to make sure the speedo and tach read true and since the new tyres are so different I also want to make sure I do not mess up my speedo and tach readings. I know the stock V8 rims are 5" wide but I would also like to know a few other details of the original tyres for compareson. Can anyone help me with some details of the following: 1. the Overall Width 2. Overall Diameter 3.Revalutions per mile

The only number you need to match to maintain the same speedometer reading is the diameter, width is immaterial. Plenty in the archives on tyre sizes. Multiply the diameter in inches by pi, divide 63360 by the result to get tyre revolutions per mile. What does the tachometer trueness have to do with tyre size?
George B.


To answer your questions.
1. 175mm (6.89 inches)
2. 25.02 inches
3. 806 tyre revs per mile.

175HR-14. 175 is the tyre width in mm, 14 is the wheel diameter in inches (HR is the speed rating). The tyre height is about 80% of the width. 80% of 6.89 inches (175mm) = 5.51 making the OD of the tyre 25.02 inches.

“since the new tyres are so different”? Tyres are still sized the same way but because most are low profile you will see for instance 195/60x15, 60 is the aspect ratio so the tyre height is 60% of 195. 195/60x15 = 24.21 inches diameter and 833 revs per mile.


Geoff King

You really have to measure the diameter yourself or rely on the measurements others have made, as no two manufacturers tyres of equal specification are the same diameter. The stated width times the stated aspect ratio is only an approximation. Not that it makes much difference, I believe the original aspect ratio was 78.
George B.

Thanks Geoff & George really appreciate your feedback.

Gary Fraipont


You might want to try this website for information on tyre dimensions.

Good luck

Peter Hills

This thread was discussed on 11/11/2002

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