So you want to see REALLY how fast you are going.
Its a very simple formula that you can figure out with just a little bit of information. First of all you need to find out a few things. You will need to figure out your gear ratio which is the rear sprocket divided by the front driver or sprocket. (i.e. you are running an 11 tooth driver on your clutch and an 88 on your axle gives you an 8 to one gear ratio.) Next you will need to measure the circumference of your right rear tire. (Measuring the right rear makes this formula work even if your running stagger on an oval kart.) The last bit of information you will need will be the top RPM you turned while on the track.
So lets say that you are running an 11-81 with a top RPM of 13,900 with a right rear tire size of 34.125. We now have the information needed to figure your real top speed.
Take 81 divided by 11 which gives you a gear ratio of 7.36. Next divide your tire circumference by your gear ratio. 34.125 divided by 7.36 = 4.64. This is the number of inches forward your kart travels for every RPM of the engine. Now take the number of inches forward, 4.64 and multiply that by the top RPM. 4.64 x 13,900 = 64496. Now we must turn our inches forward into feet forward which is accomplished by dividing 64496 by 12. (12 inches in one foot) This gives us 5374.6666 which rounded up is 5375. Then we divide that number by 5280 which is the number of feet in one mile. This gives us the number of feet our kart travels per minute. 5375 divided by 5280 = 1.018. Then multiply that number by 60 (60 minutes in one hour) and we get 61.08. This is our top speed we obtained during that track session. Below is the formula written out so you can save it and prove to your friends that you are really going 61 MPH.
Tire circumference divided by gear ratio, multiplied by top RPM, divided by 12, divided by 5280, multiplied by 60 = MPH!!!!! It's really not that hard now is it? If your still having trouble figuring this one out, e-mail me your tire circumference along with your top RPM and your gear ratio and I'll figure it for you. E-mail to email@example.com
This formula is very useful to use when you want to know how much top speed you gain or lose when changing gears at the track. You might find that it helps you figure out the best possible gear for each track you race at. It has definitely helped us!!!
Good luck with your racing,