Thanks, I will look at Dynobob's graph again.

 

It won't change your speedo. That runs off the speed sensor on you front wheels hub.

 

I just stick out my foot and push the Prius off the side of the road, as I ride by yelling "YEE-HAA" of course!



Fortunately, the top speed of a stock KLX250 is NOT 75 MPH...

On my new bike, I was easily hitting mid 80's, with plenty to spare. I now have my Speedo corrected (to the tenth) and I can still easily get into the mid 80's with it.

Here are some rough numbers for you:

With 14T:
7k RPM = 60 MPH; 8k RPM = 70 MPH; 8500 RPM = 75 MPH...

With a 13T, RPM increases about 600-650... so it would take about 7600 RPM to hit 60 MPH, 8600 for 70 MPH, etc...

 

Thanks rgoers! That helps out alot.

 

I posted this in another thread some time ago.
42/14 = 3 or 3 to 1. This is the stock gearing ratio. Meaning for every 3 full revolution the front makes the rear sprocket will make one full revolution.
42/13 = 3.230 or 3.230 to 1. Meaning for every 3.230 revolution the front makes the rear sprocket will make one full revolution. Or 3.230 divided by 3 (Stock Gear Ratio) = 1.076 or 7.6% gear reduction over stock. So if you are going 40 MPH at say 5000 RPM with stock gearing. With the 13 tooth front the engine will have to be running at 5380 RPM to go the same 40 MPH. 5000 RPM x 7% = 380 + 5000 = 5380 RPM.

So it is a ratio on the RPM change not a set amount like 500, but close.
14t at 60= 7,000 RPM then 13t at 60= 7,532 RPM
14T at 70 = 8500 rpm then 13t at 70 = 9,146 RPM

 

How much will the gas mileage go down with the 13T front sprocket?

 

gas mileage only goes down if you are revving it up to the higher rpm all the time because of your gearing...if you change gears normally...should be same...but, sounds like on the highway you'll be buzzing at 9,000+ rpm and barely keeping up with traffic (not sure what the speed limit is in your area...but, it's 65 mph and 70 mph in california)...not sure if that is good for the engine...i run 8,000 rpm and 8,500 rpm with no problems...9,000+ rpm feels and sounds like the engine is trying way too hard....i wouldn't hold it there for long period of time...

sorry..the 75 mph top speed was what i saw on the specs on klx250s review website...

 

Saw this in another forum. Mods can take this out if against forum rules.
http://klxforum.suncup.net/viewtopic...77bda9115f9f01
I can't make out what it means though.

 

that's the gear ratios...if you wanted to calculate rpm or speed, you need to know the gear ratios....that's what cmott was talking about...

 

When my bike was a stock 250cc ( 2 weeks ago ) a 13 tooth sprocket on my bike ran about 400 rpm higher than the 14 tooth. I'm sure not all bikes are calibrated 100% the same. Just like the speedo's. With the 13 tooth I could get the bike up to 75mph tops with no wind. With the 14 tooth I could only get the bike up to 72mph with no wind. In theory the bike should have been slower, but I actually gained 3mph. My bike had a very hard time in 6th gear with my weight. Now with the 351 there is NO problem. Also I lost about 5 miles before reserve. With the 13 tooth I could get 79 miles before reserve, and with the 14 tooth I could get 84 miles. For only 5 miles less, I felt like I had a little more bike with the 13 tooth because I could maintain speed so much easier on the highway. Your results may vary. I paid $20 for my sprocket and I know some have paid less then $10. It's very cheap to try so give it a shot and see if it's right for you.