1 wheel on the ground. 1 wheel just off the ground. neutral of course. make a mark on the tire at 5 o'clock & turn tire a bit to 6 o'clock to take the backlash out of it. turn tire 2 revolutions (back to 6 o'clock twice) as you count d shaft revolutions. That'll b your exact ratio (ie 3&1/4 turns is 3.23 as Scratch said)

live every 24 hour block of time like it's your last day on earth