Matt, I certainly wouldn't argue with your experinece, but if it takes X amount of Hpower to go Y mph, I would think the only significant advantage to the lower RPM would be reduced pumping forces.
I love low RPM torque and drive an older Golf Turbo Diesel, but that's a different kettle of fish.

 

You are correct, but lowering the rpm doesn't erase the horsepower. One could say that the best meat of power for the 250 is in the 6-7k rpm range. Cruising at 70mph, stock gearing is around 7.5k rpm? (its been a while since I had the 250). Moving the rpm into a sweeter spot in the power band CAN increase mpg and make cruising better. With the klx it does (of course uncorking helps)

The xt225 on the other hand, has a super tall 6th and gearing it up is detrimental to the cruising speed, makes it essentially a slug

 

depends what you whant.

A 15t sprocket (considering that the 250S is a realy slow bike) is ideal if you ride your bike in highways and motorways.. But then IMO you dont buy a klx in order to make big highways trips, you buy a ZXR 250 to do this.

 

Hey, I was going to order some sprockets, 13 and a 15, from rocky mountain. They don't list a 15 for my 2012 250S but they have a 13. I thought the KLX300 sprockets were interchangeable so I looked up a 13 for the bike and its a different part number. Can anyone confirm if a 07' KLX300 15T will work on my 12' KLX250S?

THANKS!

 

This has been a source of great confusion for racers on both two and four wheels for a long time.

Motors, generally, have a power band that relates proportionately to RPM [lower RPM, lower horsepower... higher RPM, higher horsepower]. Think of horsepower as the ability of the engine to continue spinning at the same speed under resistance. At low engine speed, you are more able to stall it by going up a steep incline than you are at higher engine speed.

Decreasing the front sprocket size by one tooth is equivalent to increasing the rear sprocket by 2-3 teeth.

Imagine riding at very slow speed (rough trails, large rocks, steep hills). As you try to power over a rock or climb that hill, the engine speed decreases, and maybe threatens to stall. By changing the gearing, you can get the bike to rev a little higher at the same speed. You aren't changing the horsepower of the engine, but you are in effect accessing the higher end of the power band at the same bike speed.

You can also think of this as adding a gear below first (and removing top gear). If you've ever been riding along in first gear and thought "Gee, I wish I could downshift" then you want to try a decrease of one tooth in your front sprocket, or get a larger rear sprocket in the 2-3 extra tooth range.

I wouldn't recommend making a change to gearing more than this at one time. Take it in phases. Maybe try a decrease in the front by one tooth, try it out, then increase the rear by two. You'll be surprised at the change just this much can make.

Hope this helps!

 

Funny, I frequently find myself accelerating on the road and trying to upshift into seventh gear. I've got an SF and one of the very first mods I did was fit a 15T front. But I ride almost exclusively on the street (including highway and Interstate), so am unfamiliar with the challenges faced by S owners offroad. I think my SF has sufficient power to pull the 15T, and would even consider larger if I was confident it would fit.

 

I'll dispense with the "that's what she said" comment (oops too late?) and share that I'm rather novice at riding. Did not get started until about 3-4 years ago, chasing my son around the yard on an XR100 with him on his CRF50. Only last year did I try my hand at a bigger bike (1989 XR250R) and only last week found myself with a dual sport. I primarily ride trails with my son now, and find myself enjoying the challenges of rough trails. More trees, more logs, more boulders, more challenges. I was riding the XR at nearly stalling speeds in first gear, and found a one tooth change in front made just enough difference to make me feel more confident in really rough stuff.

 

DING DING DING we have a winner!!!


Except that's what the clutch is for, and any affect in gearing to riding the street can be counteracted by using the clutch at low speeds.

 

I have a 15T since I mainly do street riding. I've also found it adequate for my modest off-road adventures. I max out at 80 mph on the highway and get about 50 mpg. It has more then enough torque to get off the line before cars, which is all that matters for me.

 

I really wish I could get behind this "mileage" thing these road-riding guys keep talking about. I fill it before I ride and every once in a while pop the lid and go "oh crap, I'm almost empty!" and head back to the trail head!

I try not to ride the clutch. As slow as I go, I'd be on it all day and wear it out in a month.