Tire Sizes

The following letter and response appeared in the "Tech Corr" section of the January 1991 issue of Road & Track Magazine. We thank them for its use.

Published in the Austin-Healey Magazine, March 1991


QUESTION


"I own a 1961 Austin-Healey 3000. The original tire size is 5.90-15 for this car, a size designation that seems to baffle most tire dealers, who are unable to help? What would be the modern equivalent to this size? Don't forget the scant ground clearance of the Austin-Healey.


Rafael A. Margarida

Philadelphia, Pennsylvania


ANSWER


Although it may appear to be quite different, the 5.90-15 size for your Austin-Healey is surprisingly similar to today's designations.


The first number, 5.90, refers to the tire's tread width in inches. The first number in today's designation (for example, let's use 185/70SR-15) indicates the tire's section width in millimeters. Section width refers to the tire's maximum width, which is typically from sidewall to sidewall, not just the tread width.


The second number in the old designation signifies the wheel's diameter, in inches: the last number in the new designation does the same. So for both your car and our modern example, the wheel diameter is 15 in.


But what do those other numbers in the modern designation mean? The second number (70, in this case) is the tire's profile or aspect ratio. That is, the tire's sidewall height with respect to its width, which is 70 percent.


The first letter, the S, refers to the tire's speed rating, which in this case is for speeds up to and including 112 mph. Other letters used are T, for speeds up to and including 118 mph; H for 130mph; V, for 149 mph; and Z, for speeds exceeding 149 mph.


The second letter, R, refers to the tire's radial construction.


So now that you understand modern tire designation, how do you translate 5.90-15 to its modern equivalent? We can estimate that the 5.90 in. tread width will result in approximately a 6.4-in. section width, because of tire bulge. And 6.4 in. is 162.6 mm, which we'll round to 165, a common modern increment. And we can assume the aspect ratio is 78 or 80, as those were the ones used for the period. So the proper equivalent size would be a 165/80R-15.


However, the Austin-Healey 3000 is a relatively powerful car and can make good use of wider tires, if they're available. Because we now understand how modern tire sizing works, it's relatively easy to consider some other sizes.


With the Healey's poor ground clearance, it's important that the tire diameter remain the same. So first we need to calculate the tire diameter for that 165/ 80R 15. We use the following formula (section width x aspect ratio x2) + wheel diameter in millimeters. to give the tire's nominal diameter in mm. So, (165 x. 80 xx2)+ (15x25.4)=645 mm. Repeating the formula for 175and 185-mm section widths, and adjusting aspect ratios to keep the same tire diameter give 175/75R- 15. or 185/70R-15, which gives up only 5mm in tire diameter.


Incidentally, the tire engineer we called for advice on this question used 185/7OR-15s on his own 3000 and recommended making the change to this wider rubber.


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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