At 11:41 PM 7/7/99 , you wrote:
>how do I know whether I have the 15x7 or the 15x8? Does the second number
>matter much? I know nothing when it comes to tires, so what is the 50 in
>295/50/r15 mean?
The first 2 numbers (15x7 and 15x8) are rim sizes; the second number
(295/50/r15) is a tire size.
As far as the rims go, that's easy. The first number is the height, and
the 2nd is the width (both in inches). So, a 15x7 rim is 15" tall, and 7"
wide. 15x8 would be one inch wider than the 15x7. To tell what rims you
have, as far as I know, you can just go outside with a tape measure or
ruler and measure them.
The tire size is a bit more involved, but not too bad once you know it.
The first number is the tire width in millimeters, the second number is the
sidewall height, (expressed as a percentage of the width; 50 is .50, 75
would be .75, etc.), and the last is the inside diameter of the tire, in
inches. (the "r" just means its a radial, I believe)
So...
A 295/50/r15 tire is 295mm wide, with a 147.5mm sidewall, and the inside
diameter is 15 inches wide. Say you wanted to find the height of this tire
(a common practice to reduce spedometer error when selecting a new set of
tires). First you have to convert all the units to standard; there are
approximately 25.4 mm's in an inch. So...
295 / 25.4 = 11.614"
This tire is 11.614" wide. To find the sidewall height, we need to
multiply by the percentage/aspect ratio (the 2nd number).
11.614 * .50 = 5.80"
This tire has a sidewall height of 5.8".
We already know the inside diameter of the tire; that one was given to us
(15"). You have to count the sidewall twice, obviously, since a
measurement from the bottom to the top of the tire will cross the sidewall
twice. So, the overall height is:
5.8 + 15 + 5.8 = 26.6"
Keep in mind that if you were to go outside and measure the tire while its
on the truck, it would be shorter than 26.6", because the weight of the
truck is pushing down on the tire, making the bottom part flatter than the top.
(If you'd prefer not to have to go through all these calculations by hand
for a bunch of tires, you might find the DML tire and wheel calculator
handy: http://www.cs.fredonia.edu/~stei0302/www/dakota/tirecalc/) Its a
good procedure to commit to memory though; useful when going out tire
shopping and you don't have Internet access. :-)
-Jon-
.--- stei0302@cs.fredonia.edu ------------------------------------.
| Affiliations: DoD, EAA, MP Race Team, NMA, SPA, USUA. RP-SEL |
| '96 Dodge Dakota v8 SLT CC (14.58@93.55), '96 Kolb FireFly 447 |
`----------------------- http://www.cs.fredonia.edu/~stei0302/ ---'
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