i found a couple good ones that some may like to use...the secondlink compares truck sizes to metric too. if someone wants to go to a metric size i found that a 265/75-15 is nearly idendical in all size aspects as a 31x10.5x15

http://www.discounttire.com/dtcs/infoTireMath.dos

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

 

You should also add that they need to go to a site like:

http://www.miata.net/garage/tirecalc.html
^- To see the effects of going up or down to far in size.

http://www.redrock4x4.com/tech/tire_calc.php
^- Metric to Standard converter or vise versa.


Just figured i'd add my $.02, since when i went up in tire size my speedometer became off by a few mph.

 

both links show how far the speedo is off when you change tire sizes...in the case of switching from 31x10.5x15 to 265/75-15's the speedo is only 0.6 mph too fast