KDX vs KX ratios

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Oct 28, 1999
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#1
I once read a thread posted by C Dave where he mentioned the huge difference between his KDX and a buddys KX, in terms of top end ratios.If I remember correctly, his impression was that the KDX still had a gear left while the KX was WOT in top gear. The KX was race prepped, so I am not sure if it was stock in terms of sprocket ratios.

My question is : Is there a huge difference in top end speed between a stock KDX (13/47 ratio) and a stock KX250. It is interesting that some of you prefer a 13/51 on the KDX. I realize that it depends on terrain. Have any one done any real life comparisons (drag racing) between the 2 bikes ?:cool:
 

Mac

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#2
As far as acceleration I'm sure the KX would smoke the KDX but top speed is totally different, it depends on the drive ratio.

I run 12/47 due to cost but it is almost the same as a 13/51, It takes away from the top speed but makes it a better trail bike.
 
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#3
My cousin has a 97 KX 250, and sure enough, off the line he blows the doors off my 89 KDX. But once he hits 5th WO, I'm just hitting 5th and still got 6th. Top speed on my KDX kills his KX. I believe his gearing is 12/48, where mine is 13/51. His KX is bone stock though, mine has a pipe, reeds, and airbox mod. I was still kinda shocked, I would have thought a 250 would destroy my little 200 in tod speed. It's all a factor of sprockets I believe. I know in Austrailia, the stock gearing is 14/48 or something to that effect, and most can get up around 130-140 km/h.

Dave
 
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#5
I'm running a 14/50, and I can walk away from an MX 250 on top, and my modded engine can hold it's own in acceleration, as well. It's not just the sprockets, the KDX's also have wide-ratio trannies, giving them a much taller top gear than a close-ratio MX bike.
 
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#6
I recently had a chance to run my 00 220 aganst a 00 KX 250 on a long stretch of flat smooth field long enough to get any bike wide open. We found a slow rolling start 1st gear would be the best way. It was pretty close through and up to 4th gear. When I hit 5th I began to slowly pull away and began walking away as it really screamed out. Hitting 6th didn't give me a huge amount of extra speed but was like shifting into overdrive and by then he was quite a few bike lengths behind me, no contest in the top speed department.Both bikes stock except FMF's and Reeds. This is better though, same field, same start, against the mighty Yamaha Raptor. thought I was going to be eating dual dust clouds at top speed....it pulled me by about 1 bike length through the gears. That is until I hit 5th and held it open. Managed to get side by side and pull ahead by a half a bike length with 6th. The Raptor is really impressive if you like torque. Ride one, hit 2nd gear and you'll see what I mean. But I really thought I was going to eat him up through the gears until top speed and then get blown away. Alot of people underestimate how fast this bike really is. Love my KDX
 

dirt bike dave

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#7
Generally speaking, most 250 mxers will top out under or around 60 mph with stock gearing, while KDX's should go about 70-75 mph. As Spanky said, the internal gearbox ratios are the critical factor. You could probably gear a 250 mx bike to pull 95 mph or so, but 1st gear would be like 3rd with stock gearing. Not much fun on the trail.

My ICO reports top speed; on most woods rides with occasional top gear fire roading, my high speed during a day's riding is usually around 45 mph.
 
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#8
I have a '94 KDX 250 registerd and stock except for some porting I am running a 14/48 setup. Last weekend I had the chance to ride with a CR and a YZ both '99 models with pipes ect. And I am proud to say that they only gained 1 or 2 bike lengths on me. And after reading Spanky's post I think I will do a few more mods to my KDX and go and roost the mxers.