I came across this and found it very helpful..........thought I would add it to the site.
To ensure that you get the correct highway mount or clamp set for your application you will need to find the diameter of the tube, to do this take a tape measure or soft ruler of some sort measure the tube's circumference (distance around). If you do not have a flexible ruler or tape handy, take a piece of paper and wrap it around the tube in question, then mark the start and finish points, now take the paper off of the tube and measure the start to finish lines.
If you get 3 13/64" circumference, you have a 1" diameter tube.
If you get 3 1/2" circumference, you have a 1 1/8" diameter tube.
If you get 4" circumference, you have a 1 1/4" diameter tube.
If you get 4 11/16" circumference, you have a 1 1/2" diameter tube.
The formula to figure this out is as follows: Circumference divided by 3.14 (pie) will give you the Diameter (distance across tube or bar). An example of this is as follows: tube circumference of 3.50" is divided by 3.14 (pie) and comes out to be 1.1146 (round up to 1.12) convert decimal of 1.12" to fraction of 1 1/8.