Can someone tell me exactly what the middle number means with respect to tyre size.

At present I have michelin radials, and it says on the tyre that its a tubeless radial - 165R15.

How does this differ from a 165/75R15 or a 175/70R15. I am asking which of the three are positively compatible with the User Manual guide of 5.90-15" .

Sunil

>> EDIT - See what happens when you start a post, go away for a few minutes (which turns into an hour or so), then come back and finish? Looks like Blue Horse and I agree on tire stuff... they're pretty much identical posts. :) <<

The 165R15 Michelin, labeled in the more modern convention, would be a 165/80R15. That middle number (in this case the "80") is the aspect ratio... sidewall height divided by cross section width. In other words, the sidewall height is .80 of 165 mm. The cross section being 165 mm therefore makes the sidewall height 132 mm. To calculate the nominal tire diameter in inches, convert the 132 mm to 5.2 inches, double it to 10.4 inches, and add the rim diameter (15 inches) for a overall diameter of 25.4 inches. Installed rolling radius will be something less than half that, due to tire flex when loaded.

Anyway... the 165R15's, while slightly wider in cross section, are virtually identical to the original 5.90-15 diameter of about 25.4". Incidentally, my source for the diameter of a 5.90-15 is the Coker Tire online catalog. They also list 3 different brands of 165R15's (Michelin, BFG and Coker), all at 25.3 to 25.4 inches in diameter.

The 165/75R15 and 175/70R15 tires would be quite a bit shorter than the 5.90, at apprx 24.8" and 24.7" diameter, respectively. The 175 would obviously be significantly wider, as well.

Gary L
--
1971 142E ITB racer, 1973 1800ES, 2002 S60 T5

The middle number is the tire's aspect ratio---sidewall height as a percentage of casing width at the widest point (NOT tread width, as many people think) Tires with no middle number have an implied figure of 80 or sometimes 82. This was the standard profile for many years, and it was felt that only variations (such as 70 or 60) needed to be designated. Therefore, the common 165 R 15 is actually a 165/80 R 15 when spelled out in full.

The 165 R 15 is a good replacement size for 5.90 15 or 6.00 15L (the L of which stands for "Low Profile" in the old American system of size designation--these had an 80 or 82 aspect ratio--earlier types had even taller sidewalls) The lower profile tires tend not to look right on a PV, as the sidewalls are too short. This also affects the speedometer reading.

To make it a little clearer, the numbers in a 165/60R15 would break down like this:

165 = tread with measured in millimeters.

60 is the hight of the sidewall in relation to the tread width: 60% of 165, or 99 mm high.

"R" is radial, 15 is the wheel size.

You'll also see other letters in the label - those relate to speed rating. If you're going to drive really fast, look into speed rated tires.

Other things you will want to know when tire shopping: the wear and traction rating. I don't recall exactly how those are expressed, so compare the wear ratings between tires so you'll get the best tires. Again, if you're driving really fast or on twisty roads, a higher traction (or adhesion) rating is better.

try: http://www.tirerack.com for more information.

Steve

The 165/75R15 is the equivalent tire. The 175/70 is wider but squattier so
it has approximately the same outside diameter as the old 165R15. The middle
number is the "aspect ratio" and the smaller it is, the squattier the tire
is in relation to the distance from the tread to the rim.
--
George Downs Bartlesville, Heart of the USA!