4 speed to 5 speed

Topping out my 03 YZF 450 powered shifter kart with the tallest gearing available. Short tires need a five speed to break 100. Can this be done? the motor is strong and a tranny swap would get it there. I searched for tranny swaps only to find YZ to WR conversions no 4 speed to 5 speed conversions.

The '03-'06 WR450 trans IS a 5 speed, and while it's a wider ration than a YZ five speed, the the trans will bolt in.


Other TT'rs have used 400/426 transmissions, but these are not bolt-ins, and require some custom fitting and machine work.

You should also note that the top gear ratio in the 4 speed is 1.050:1 vs. .095 for the YZ 5 speed. That's a 9% difference or so. OTOH, top gear in the WR is about .085, if I recall correctly. Then too, you can use the primary drive gear set from an '05, and make an overall change in gear ratio equivalent to adding a tooth to the front.


Sounds like a couple of options may be available. Ideally maintaining the close ratio 4 speed while adding an overdrive 5th sounds perfect. Can the 4 speed gears be combined with the 5th gear and 5 speed drive and main axles to create this type of tranny with out having to narrow the 4 speed gears.

Thanks again

Can the 4 speed gears be combined with the 5th gear and 5 speed drive and main axles to create this type of tranny ..(?)
Nope. Not in any way. If you want to put all the work into the thing to get an earlier 5 speed into the unit, you can then mix those gear sets (the YZ/WR426 gear sets) to some extent, but adding an over-tall high gear to the box is putting a wide ratio gap at the high end, right where you want it least.

Here are the ratios of a WR450 box and a YZ426 box:











Thanks Grayracer

I ll look closer as it gets colder.

Check out the YFZ 450 quad gears they have smaller wheels that may be a better choice for the conversion for you

It's my understanding that the quad transmission is wider, and that the parts won't interchange, but that may not be the case. Take that as a precautionary statement, not as hard fact.

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