Thanks to Bill Maloney and Steven Denis for this information.  

On the sidewall numbers, such as 235R 15 or 7.50R 16, the first figure is the cross section of the tire in mm or Inches. The height, if there are no other figures, is 85%. So on a 7.50R 16 tire do: 7.5" X 85% = 6.375". Since there's rubber on both sides of the rim, double that and add to the rim diameter: 6.375" X 2 = 12.75", 12.75" + 16" = 28.75" tire diameter. 

If the tire were (hypothecically) 7.50 70R 16, the height would be 70% of the cross section. For metric sizes you'll need to do a little conversion. I have Sears LT AT 235 70R 15s on my 88 and they do OK. Zippy around town with decent handling and OK in the rain. One day I may switch to 7.5R 16s to improve fuel economy, reduce highway noise, and improve the ride (this is what folks with US 88s that have switched to 16" rims have experienced). But not today.

> My Goodyear Wranglers, which are wearing like steel, have the legend
 > 'treadwear 220' on the wall. What does the number mean?
 >

the number that you see is part of the universal tire grading program here in the states(stop me if you've heard this)Typically american BTW, "universal" means "us" and who cares about the rest of the world... The tread wear is a comarative measure of wear between tire brands...I think 100 was average..220 *is* very good.... the other markings are for Traction and temperature graded "A" "B" or "C" with A being the best So Mud and Snow tires that have an "A" for traction will likley have a "B" or "C" for temperature....but you could have figured that ...and to have an"A"for traction a "B" for Temp. and a 220 for tread wear,you have to lay out the *big* money.....but you *and* the Bank manager already knew that......