The "215" number is the width
The "40" is the percentage of the width that equals the sidewall height.
The "17" is the rim size.
An Example is:
A 225/50R15 is 8.8" wide with a 4.4" sidewall
A 225/60R15 is 8.8" wide with a 4.8" sidewall
The only difference between the 2 tire sizes is that the sidewall on the 60 series tire is larger which makes the overall diameter of the tire bigger. The bigger/smaller the diameter with respect to the stock size of the tire affects the speedometer readings.
So on a 17 X 7.5 wheel you could put a 225 tire on it. The max you could go safely would be a 235 tire But the unsupported portion of the tire will affect handling slightly.
To get the exact size of the tire and to see how much tire sizes affects your speedo go the the link below. Type in the stock size and the size you want to go to. It will give you all the measurements and just under the diagram it will tell you how far off the speedo will be.
http://www.secret-secret.com/turbo/DIY/tirecalc/turbotire.html
BTW ~ If you have an 87 SC as your ID shows, The stock tires should have been 215/60R14.
If you were to put on 225/40R17's the speedo would read 0.2% faster. So at 100.0mph the speedo would show 100.2 mph.
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[This message has been edited by Oreif (edited 02-28-2001).]
