Calculating Final Drive Ratios

by Jason Lavallee

Over the years one thing has always pissed me off. Those people who are seemingly from the planet of the galactically stupid!! You all know the guys I'm referring to don't you? The tall tale guys who say they did 290kph on a 1982 shit-box etc,etc...

The Idea for this article came from friends who inquired as to how theoretical top speeds of bikes are calculated. And I'm sure you can all remember the old cycle magazines that would give the theoretical speeds in gears at redline. I'm going to proceed to show you how to calculate these speeds for your bike and put an end to gearing myths! It really is simple to do.

One of the most important things to remember is to watch your conversions!!! Also don't be a lazy S.O.B. and round off all over the place. Try to use at least 4 significant figures O.K.

Lets start, first things first, check your owners manual for :

1. Engine redline (max Rpm)
2. Primary drive ratio
3. The transmission gear ratio you want to use
4. Final reduction ratio (sprockets)

After this you'll need to know the circumference of your rear wheel You can calculate it but it wont be very accurate and can add major error. C = 2 (3.141592654)(radius of your wheel) Radius of wheel = (17 inches)(25.4) = 431.28mm

Example Tire size times aspect ratio (190mm)(0.5)(2) = 190mm (431.28+190)/2 = 310.9mm C = 2(3.1415926534)(310.9)
C = 1953.442312mm then convert to km so multiply by 10^-6
C = 0.001953442312 km per rev of the rear wheel

Instead of doing this I 'll give you some standard circumferences. This data has been derived from manufacturer specs.

• 190/50 zr 17 your circumference is = 0.002009929 km
• 180/55 zr 17 has a circumference = 0.001967509 km
• 200/50 zr 17 has a circumference = 0.002041345 km
• 170/60 zr 17 has a circumference = 0.001947938 km

I'll be using a 1998 gsxr 750 for a step by step example,

Now take redline in rpm and multiply by 60 to get engine revs per hour (13500 rpm)(60) = 810000 rph.

Divide rph by your primary drive ratio (810000)/(72/41) = 461250 rph primary.

Then divide by gear ratio (for transmission gear you want) (461250)/(28/25) = 411830.3571 rph output

Then divide by final reduction ratio (i.e. sprocket ratio) (411830.3571)/(44/16) = 149756.4935 rph wheel

Then multiply tire circumference by rph wheel to get speed in kph (0.002009929)(149756.4935) = 301 kph at redline in 6^th gear on an all stock 98 gsxr

Now for my 97 gsxr the 6^th gear and primary changes are the same but the final drive is 43/16 and I'm running a 200/50 zr 17 so I'm ok to rph output hence my bike is geared for, (411830.3571)/(43/16) = 153239.2026 rph wheel

Tire circumference times rph wheel (153239.2026)(0.002041345) = 312.814 kph

Handy conversions,

• mph times 1.6093 = kph
• kph times 0.6214 = mph

So theoretically my bike would be doing 313 kph or 194.5 mph at 13500 in 6^th

Of course there are problems with this,

first, you have to have the power to pull your gearing(i.e. be able to redline in 6^th )

second even if you see red in 6^th your tachometer could be off

third there is some degree of wheel slip at high speeds (even though you can't feel it)

fourth, if you are not straight up you will shorten your gearing by running on the smaller part of the tire. This is the trick Scott Russell uses to speed up acceleration when he runs tall gears at Daytona.

THE BAD NEWS,

After reading all that I'll go back to my initial pet peeve, galactically stupid people! Its not uncommon to hear someone telling a tall one, and its ok because we have all done it but there has to be a point to stop.

About 4 - 5 years ago I had this guy tell me he could get 220 kph from his radian 600 (which incidently was packed) and I replied yeah sure. Because I thought he was kidding. But to my surprise he took great offense And bet me $25.00 that he could so we went for a ride and I clocked him at no where near 220, yet he insisted my speedo was wrong (not by that much) and would not pay! So I did the prickish and sensible thing by asking him how close to red he was?

He replied "a couple hundred off" so instead of working out the math I got his buddy to ride his bike in 6^th gear at ½ of redline and note the speed. As it worked out he was not even doing 100 kph at half of redline and as I later explained to him, his ratios did not change so he would with his gearing (which his bike had a job pulling as it was) have to be well into red to pull 200 kph let alone 220.

I may have been harsh but that $25.00 paid for a pizza.

So sorry about the long winded story. Back to the tech.

With what we have here we can figure out all sorts of good stuff like what gears to run at a track or approximate how much we want to gear up or down. Just remember its no good to gear too tall or you'll end up going slower because your bike cant get up to speed as fast!

As a rule of thumb gear the bike so you are just close to red in 6^th when flat out for extended periods, that was you'll have a few rpm left to draft your buddy.

Table of ratios and speeds for selected bikes (stock)

I figured I'd also include a handy gearing chart so you guys can start playing with ratios.

Remember if you can change one at a time, or both so as to split the tooth (get a ratio in between) Also remember changing both can save chain cutting by using smaller sprockets with the same ratio, This can also be used to lengthen or shorten your wheelbase a small amount.

GEAR RATIOS

Counter shaft sprocket teeth

Well now that you have the info, go ride like the hooligans you really are!!! Stay head down, throttle pegged, Jason Lavallee

Back