Hi all,

I have a 2018 KLX250 with the EFI (instead of carb) setup and I'm thinking about swaping out the front 14-sprocket for 15 since I will be doing a little more street riding than dirt so I'd like to lower my RPM's.

With that said, will my RPM and speedo now need to be recalibrated? And if so, how and what is the best method possible? Thanks all.

 

Speedometer is from drive off the front wheel and it's already about 10% low and it will still be 10% low 15/42 gearing so no nothing needs to be recalibrated. You might want to read through this post https://www.kawasakiforums.com/forum/klx-250s-71/whats-your-gearing-combination-chain-length-42553/ your 106 link stock chain should be ok but the link I gave will tell you what chain others have used with different gearing

 

Your top speed will probably go down.

 

I thought my top speed would actually go up. And my torque will go down. Since I'm going from a 14 to 15. RPM's should be lower though.

 

Depends on how hard you are willing to rev it. It takes energy to push the air at higher speeds and there is a limit where the power cannot overtake the opposing force of pushing the air. Plus there is a point where the engine is most efficient. Usually around torque peak. With the KLX250 the range is between about 4000 rpm bottom and around 6000 rpm top. Working in that range around 5000 rpm is likely the sweet spot. If you lug too low it isn't as efficient as letting it rev. This isn't to say putting along in town at 2500-3000 should never be done, but you don't want to accelerate hard from there, the power curve just doesn't support it.

We ran into this with Gold Wings in the mid-late 80s, riders would complain about poor mpg, we'd learn they were chugging around at 2500-2800 rpm. Honda informed us that best range was between 3000-4000 rpm, even if it meant downshifting. The engine efficiency dropped off to the point that it was worse at 2800 in top gear than at 3300 in fourth. I personally can attest to that having owned two 83 GL1100 standards. I also experience that with the KLX650. In normal riding I seldom drop below 3500 rpm, reving upwards of 4500 rather than lug the engine. I get around 60 mpg on the 650 regardless of how or where I'm riding. Back roads with a lot of third/fourth gear stuff or on an open road in fifth, within a mile or two per gallon of 60.

Actually chugging around out the bottom of the power range can be rough on the engine too. Much easier to spin a bit more than pushing a tall gear. Ask any bicyclist and they will tell you the same. There is a range for good performance and it isn't particularly good to work either end too hard.

I did a 15/39 sprocket set, the SF does 14/39, and could cruise at 70 easily on the flats, but wouldn't shift to 6th until around 60 mph. If a grade was encountered I'd have to drop a gear. Only so much power in that 15 cubic inch engine now.

 

Wow. Thanks for the explanation. Makes sense, thanks!

I'm 5'11" and 170ish. So i'll probably have to take it out for a few rides before I decide if I want to mess around with gearing. Most of my riding is just in and around town (I live in Virginia) but once in a while I'll probably make a longer trip (mainly off high-way) into Washington DC or Maryland if the weather is nice.