HOW-TO: TOP SPEED TEST! Changing gearing and tire sizes

Yamaha’s YFZ450R has already won a couple of our shootouts. It handles great, gets around a track quickly and is fun to ride. However, when you get the machine away from the track, it is limited by its top speed. The YFZ450R and YFZ450X have nearly the slowest top speeds in their respective classes.

We wanted to find out if we could get some more mph out of this machine with some simple bolt-on parts. Before we started, we installed a complete Dubach exhaust system. If you own a YFZ450R or any other sport quad, chances are you have installed an aftermarket exhaust system.

The meat of our speed search will be in the form of gearing changes and trying different rear tire sizes. We used ITP Holeshot XC tires mounted on ITP aluminum wheels and Renthal sprockets both front and rear.

In the chart are the top speed results of each tire/gearing combo. In the notes column, we explain if the quad’s engine even had enough power to pull the taller gearing. Or, we noted if the stock rev limiter kicked in, meaning even more speed was available if the engine control unit was modified as well.

In the end, we used the aftermarket sprocket choices of 15/36, because bigger than stock countershaft sprockets help gain speed, and smaller would improve response but hurt top speed numbers. On the rear end, smaller sprockets improve speed. Another thing you want to keep in mind when setting up your quad for higher top speeds: a smaller rear sprocket adds a little more ground clearance, which is, for the most part, a good thing.

We didn’t bother testing the 37-tooth rear sprocket since the changes are not dramatic when only going up or down one tooth at the rear. A simple rule of thumb is that one tooth different on the front countershaft sprocket is equal to a two-tooth change at the rear.

If you have your slide rule handy and like a math challenge, you can figure out your theoretical top speed without even starting the quad. The formula is (R x C)/(G x 5280), where R is engine rpm at redline multiplied by 60, C is the circumference of the rear wheels expressed in feet, G is the overall gear ratio (the primary, final and fifth-gear ratios multiplied together) and 5280 is the number of feet in a mile. There are two reasons we actually went into the field to measure this with our handy GPS. First of all, most motors won’t pull redline in top gear. Second, tire circumference actually grows at speed, effectively gearing your quad higher as the wheels spin faster.

The bottom line is that a number of factors influence speed. Countershaft size, rear sprocket size and wheel size all come into play, as well as power. Every quad is different, but here’s what we found with our Yamaha.

RUN GEARING TIRE SPEED NOTES
NUMBER (RATIO) SIZE
STOCK 14/38 20″ 74 mph Does not hit the rev limiter, gets to tops speed quick.
RUN # 2 14/38 22″ 75.1 mph Does not hit rev limiter, goes through 1-3 well, but 4&5 don’t pull very hard.
RUN # 3 14/36 22″ 74.9 mph Does not hit rev limiter, spread between each gear felt much longer than stock.
RUN # 4 15/36 22″ 77.1 mph Does not hit rev limiter, spread between each gear felt the longest.
RUN # 5 15/38 22″ 75.4 mph Does not hit rev limiter, rear end felt skatey.
RUN # 6 15/36 20″ 74.9 mph Does not hit rev limiter, strong out of the hole and felt the best on the track.