Reply to "GT 5 Wheel Offsets"

Coz,

There is a simple formula to determine tire width and diameters. Lets use the stock 285/40/15 found on the 10x15 stock Campy:


You take the first number, which is tire width in mm and divide it by 25.4. This converts the mm into inches:

285 \ 25.4 = 11.22" (inches)

Next you take this converted number and multiply it by the aspect ratio, or the middle number (the middle number is a percentage of the width, in this case 40%):

11.22 x .40 = 4.488" This is your sidewall height in inches, just about 4.5".

Next you take the sidewall height and multiply it by 2 (top and bottom sidewalls):

4.488 x 2 = 8.976"

Next you add in the rim diameter, which is 15":

8.976 + 15 = 23.976 or 24" tall


This formula helps you to determine 3 important things about a tire: width, sidewall height, and overall diameter.


You now know that your front tire is 24" tall. Let's do the back 345/35/15 tire:

345 \ 25.4 = 13.58"

13.58 x .35 = 4.75"

4.75 x 2 = 9.50"

9.50 + 15 = 24.5"

The back tire is 13.58" wide, the sidewall height is 4.75", and the overall diameter is 24.5" tall.

When doing this on a calculator, do not hit the = sign between each step, just keep stringing it along and hit = at the end like this:

345 \ 25.4 x .35 x 2 + 15 = 24.5. You will get your other values along the way.


Note that the sidewall height is 1/4" taller on the rear tire. Use the sidewall height figures to determine how a front and rear tire will relate to one another. I like to see a taller sidewall on the rear to excentuate the girth and the power that eminates from the rear of a Pantera.


To me, this is very valuable information in determining how far I can go clearance wise, and what will look right cosmetically.


Lets apply this formula to a likely 17" tire combo: 275/35/17 F & 335/35/17 R:

275 \ 25.4 = 10.83"

10.83 x .35 = 3.79"

3.79 x 2 = 7.58"

7.58 + 17 = 24.58"

Not bad height wise. It's just over a 1/4" taller on each sidewall, top and bottom.

335 \ 25.4 = 13.19"

13.19 x .35 = 4.62"

4.62 x 2 = 9.24"

9.24 + 17 = 26.24"

This tire is less than 1" taller on each sidewall. Nobody has a problem with this tire on their flared Pantera.


Another thing you can determine with this formula is how the tire shoulder will 'round' with a given rim width. Using your stock tires as an example:

285 x 25.4 = 11.22" across a 10" rim. The difference is 1.22"

345 x 25.4 = 13.58" across a 13" rim. The difference is .58"

The front tire has more width for its rim than the rear. This means that the shoulder of the tire will stick farther out in relation to the rim than the rear. This will give the front sidewall/shoulder a 'straighter' appearance, and the rear a 'rounder' appearance compared to one another. Go and look at your tires and you'll see what I'm talking about. If you subtract .58 from 1.22 you get .64". If the front rim were 10.5" (.5" is as close as you can get to .64" in rim width), you will get a shoulder 'roundness' that more closely resembles the rear.

Applying this to the 17" example, we come up with the same .64" difference:

275 \ 25.4 = 10.83" across a 10" rim is .83"

335 \ 25.4 = 13.19" across a 13" rim is .19"

.83 - .19 = .64"

If I were going with this tire combo, I'd order a 10.5" front rim. To keep the wheel located in the stock location, you would add the 1/2" to inside of the rim. This would increase backspacing by 1/2". Adding the 1/2" to the outside of the rim would keep the backspacing the same a stock.


If you're happy with where the rim sits in your fenderwell, ask whomever you're going to buy your wheels from where they take their backspacing measurement from: Is it the tire beadseat flange, or the outermost lip? Then use the measurements in my previous post to determine your proper backspacing.

I personally would pull a front and a rear wheel off to measure for myself to double check anyone's measurements regarding backspacing. I'm a perfectionist, so I measured mine 99 times (I believe in 'measure twice, cut once'). Like you said, once you order them, they're yours, so taking a little extra time will insure your happiness. There is nothing worse than not being happy with a major purchase, especially when it's a finishing touch!

To measure backspacing, place the wheel street side down. You will need a straight-edge strong enough not to sag in the middle as it spans the rim. The straight-edge should be long enough to intersect the middle, but not so long that it hits the tire anywhere as it rests on the rim's edge. With the straight-edge in place, drop a ruler down to the flat pad where the rim would meet the hub on the car (the flat pad has the lug nut holes, and the hub has the wheel studs sticking out of it). Where the straight-edge intersects the ruler (say 5 1/4") is your backspacing measurement for that wheel. Wheel manufacturers a lot of times only offer backspacing in 1/2" increments, so in this case, you'd have to decide if you'd want the wheel to sit 1/4" in more or out further.

This may seem like a lot to digest, but it's really not. It just takes a lot of typing to properly explain the hows and whys! I'd say good luck, but now luck has a lot less to do with your outcome! Share some pics with us when you finally get them on your car.

Michael