a ruler

 

Or a tape measure.

DIY - Measuring Stock Tire Diameter

1. Stand, sit, or squat in front of one of your wheels
2. Get out your measuring device (ruler or tape measure)
3. Try to remember back to your grade 3 mathematics class when you learned about diameter
4. Just measure from one side of the tread, across the tire and wheel to the other side of the tread
5. Write it down on a piece of paper (you can use a pencil or a pen for this)
6. When buying your new tires, hand the piece of paper to the salesman and let him know that you want the new tires to have the same diameter

 

Or you could use a tire size calculator and stay indoors!
http://www.1010tires.com/TireSizeCalculator.asp

 

That too.

 

Yes a DIY for the lazy

 

Here's a thought, maybe your friend did a few mods he is not telling you about.

 

its all in weight, especially the added weight right at the wheels. If they are heavier then they will slow you down, period.