Tire and Rim Calculator?

[MPDano]

I was wondering if one exists online. I am trying to figure out whether I can use a lower profile tire when the width and diameter are the same. This is not 928 specific but for my wifes X5. The X5 normally uses 235/65/17 tires, I am looking at some 235/55/17 or possibly 50 or 45's. Anything exist out there that I can check if this will be safe.

[MPDano]

Have Google will travel. Found an awesome online one that give a lot if important information.

http://www.1010tires.com/TireSizeCalculator.asp

[devilinblack]

I don't see why it wouldn't be safe, by it'll throw the speedo off, won't it?

[BB500]

On the safety side, I've seen complete idiots in superlarge SUV's rolling on (30?) inch wheels with a low as a 25 series tire!!!

[Danglerb]

Originally Posted by MPDano

I was wondering if one exists online. I am trying to figure out whether I can use a lower profile tire when the width and diameter are the same. This is not 928 specific but for my wifes X5. The X5 normally uses 235/65/17 tires, I am looking at some 235/55/17 or possibly 50 or 45's. Anything exist out there that I can check if this will be safe.

When you change one of those numbers, you are changing the width or diameter.

Found this on the web ...

Tire sizes are expressed in the format WWW/AAXDD. WWW is the tire’s section width, measured in millimeters. AA is the aspect ratio or profile of the tire, which expresses the tire’s height as a percentage of its width. X is a letter indicating the tire’s internal construction. DD is the diameter in inches of the wheel that the tire is intended to be mounted on. An example of a tire size written in this format is 225/50R16.

This particular tire has a section width of 225 millimeters (8.86 inches). Section width is measured from the widest point of the outer sidewall - the side with the tire sizes and brand name that faces out from the vehicle - to the widest point of the inner sidewall when the tire is mounted on a specified width wheel. Narrower tires have lower section width numbers.

The next number is the aspect ratio. Tire sizes separate the section width from the aspect ratio with a slash. This tire has an aspect ratio of 50, meaning that the profile, or sidewall height measured from wheel to tread, is 50% of the section width. This number is also called section height; the higher the number, the taller the sidewall.

[MPDano]

Yes, if you punch in your numbers on that site. It states for safety to stay within 3% of the original tire. It then talks about risking brake issues, not sure why. Also, yes the Speedo will no longer be accurate.

[MrGST]

Tires are not that complicated but you do have to keep several things in mind when changing sizes, aspect ratios, and diameters.

1. To maintain speedo accuracy, you do need to stay within 3% of stock diameter
2. Tires have a recommended range of rim width that they can be safely mounted on, see the tire manufacturer's data for this range; Tirerack posts these specs on all tires they sell
3. You must use a tire with at least the same load range as originally specified from the car manufacturer

Braking performance is directly proportional to tire contact patch, if you change tire size from what the manufacturer recommends, brake performance can change dramatically. In practice, as long as you stay within the 3%, you should be fine.

If you use a much smaller in diameter tire, yes, your speedo will read incorrectly but also your mileage will accrue faster than normal...this may affect you $$$-wise if you lease and must stay under a specific yearly mileage....just another thing to keep in mind.

[psychII]

http://forums.rennlist.com/rennforums/3463237-post56.html

...this may be useful.

edit - maybe not. I just reread your post and see it's for an X-5 rather than a 928. Sorry.