Does anyone know the primary reduction ratio in a WR400?

It appears to be something just over 3 to 1. Does anyone know for sure?

Per my service manual ('00 WR400FM):

Primary reduction ratio: 62/21 (2.952)

Secondary reduction ratio: 50/14 (3.571)

Gear ratio -

1st 29/12 (2.416)

2nd 26/15 (1.733)

3rd 21/16 (1.312)

4th 21/20 (1.050)

5th 21/25 (0.840)

Hope that helps.

yer secondary is yer sprockets!!!


Thanks Ron in SoCal.

Ron C,

Oops! If I use that ratio I get a top speed of 103 (14/50, 110/100-18 tire) at 11,500 rpm, and almost 99 at 11,000 rpm. If your bike won’t go over 90 then maybe you DO have a problem. Are you still running 14/50 gearing? It would take 4 additional teeth out back or one less up front to bring it down to 90.

Has anyone else tried to calculate this? I get 93 mph for a YZ which also seems high. I must be using too large a number for the tire diameter (26.7 inches). I calculated this based on the size (110/100-18) but maybe the only way is to actually measure it.

You probably need to compensate for wind drag. Any NASA engineers out there?

I wonder if things would work out per your calculations on a dyno…

forget this method hick it always works out stupid. the tyre creep is worth at least 5%.


Thanks Taffy, that is what I’m thinking as well. I think. It depends on what you mean by “tyre creep.” Sidewall flex making OD smaller in practice than in theory?

I’m just thinking you can’t arrive at a real-world diameter using the tire size. I think I’ll take an actual measurement tonight to see what is going on…

11,500 Engine RPMs

2.952 Primary Reduction Ratio

3,896 Transmission primary RPMs

0.84 5th gear ratio

4,638 Front sprocket RPMs 5th gear

14 Front Sprocket

50 Rear Sprocket

3.5714 Secondary Reduction Ratio

1,299 Tire RPMs

25 Dupont tires 27" diameter worn to 25"

3.141592 Pi

78.53982 Circumference

101,988 Tire inches traveled / minute

6,119,295 Tire inches / hour

509,941 Tire feet / hour

97 Tire Miles / hour

This leads me to believe that the REV limiter is not cutting in at 11,500. It would be more like 11,000 unless Taffy's 5% tire slippage is accurate. Take 5% from 97 and you get right at 92. That is about what I clocked myself at last night with the GPS after putting the 180 main back in.

And the best way to check tire circumference is to put a piece of tape on the tire and on the ground and roll it till the tape is down again and then measure how far it rolled.

And what kind of surface were you riding on?

DeBarker thanks for your help here.

I was riding on hard asphalt.

Also, when you sit on the bike you compress the tire. Even though the tire may be XX inches in circumference, the actual inches traveled would be Pi times the length from the center of the rear axle to the road surface when you are sitting on the bike.

The only two factors which would change this would be centrifugal force pushing the tire out and slippage on the road. Between these two I don't see how they could make more than a couple of percent difference.

My experience here is that the squash doesn't apply at all at high speeds... The tire circumference (and radius of the rear axle up off the ground with it) is going to grow.

I have noticed this phenomenon on both roadrace and dirttrack and ice bikes... When a tire that it is 3/8" or so away from a part of the bike while sitting still, will rub on it at high speed.

I guess you are just doing this out of curiosity... because the only way to know for sure on the RPM is with an accurate tach.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now