To figure out miles per hour multiply the engine RPM by the wheel diameter in inches and divide this by the Gear Ration times 336.
or
MPH=RPM* wheel diameter (in inches)/ gear ratio* 336

This is the most accurate reading that you can get. I got this from a site www.macdizzy.com neat site i suggest you give it a look

 

This was covered in the Yamaha forum by me.
http://forums.atvconnection.com/thre...B=1&Tid=208312
It goes into detail on how to find your "Rear Gear Ratio".
You also have to know what rpm your bike will pull on top to be accurate. For instance if a Raptor could pull to the 9000 rpm rev limit it would be doing 92mph. If it does the 75mph they say it does then it would be turning around 7300rpm. You also have to consider tire growth and I have no idea how much the tire grows.
Oh, by the way its Tire diameter not Wheel diameter.

 

How does that work with mine, cause I got a W/B rev limiter and to my understanding it will take the stock 9000 rpms and make it go up to possibly 11000. Same theory apply?

 

yes same theory applies, this theory applies to everything

 

Need4Speed- This is Great!!! Apply this theory to the meaning of life. Let me know what the answer is!!! I am holding my breath...LMAO!!!

 

It does work on anything for finding MPH.
You must understand though that most bikes wont reach the rev limiter in top gear. By solving the equation for RPM you can find the RPM your turning if you know the MPH.

336 X Rear Gear Ratio X MPH
RPM = ____________________________
Tire Diameter

You can find your Rear Gear Ratio my looking in your manual under Specifications...Multiply Primary Reduction Ratio X Secondary Reduction Ratio X Gear Ratio (Top Gear)

 

well does anyone know the gear ratios on the 400EX? just wonderin since i cant find em anywhere, but that cant be extremely true can it, if you kept the same rev limiter but bored it and put some different size sprockets it would change it wouldnt it? so does this apply to stock machines or anything?

 

If you change the sprockets then you change the Secondary reduction ratio which is the Rear sprocket tooth number divided by the Front sprocket tooth number. If you have changed the tire size then change the diameter in the equation.
It always works if you plug in the right numbers.
Bore has nothing to do with it. A bigger bore will probably pull to a higher rpm but probably not to the rev limiter. Look in the back of your manual under specifications where it gives the weight and stuff, there should be a section on the transmission ratios and primary and secondary reduction ratios.

 

if you want to know how fast your actually going
on your stock machine just buy a low price G.P.S.
they are very accurate!
loafie