This is a good site to determine how your tie and wheel changes from stock size affect speedo.

http://www.eunos.com/tirecalc/

The 87 Elky came stock with a 205/70/14 tire. The stock tire height in order for your 87 Elky speedo to work properly is 25.3 inches. You can play with rim size, sidewall height, and tread width and as long as you come back to a 25.3 inch tire height your speedo will read correctly. I have 235/60/15 tires on my 87 and it comes out to a tire height of 26.1 inches.

TCI Transmissions also has a wealth of information about speedos and tire sizes. I think it's tciauto.com. But remember, when doing these calculations the rolling radius of the tire is the critical number (the number of revolutions per mile of the tire), not the diameter of the tire, commonly called tire height, or the measured tire height. You will note that there is a slight flat spot on the "bottom" of the tire where it meets the road. The distance from the middle of the axle to the ground is the rolling radius of the tire and is the dimension you need to use when doing speedo calculations. Of course, a good old speedo shop with a machine eliminates all of the variables.

I have an old 2002 copy of the NPD catalog. On page 66 it gives the formula for figuring out what speedo gear should be in the tranny to give proper speedo readings. Found out mine was 5 - 7 mph slower than actual speed of the El Camino. Mine had 15" rims on it when I bought it and this was correct for the tires I had at the time. I'll post the formula if anyone would like the info. I knew there was a reason I saved this issue.