[DaBlue1]

15/43 offers the best balance of acceleration and top speed provided you are maxing out rpms in every gear. Your rpm speed will decrease about 750 rpm in any given gear, but you will see an increase in top speed.

stock gearing 14/45 @ 11000 rpm, 6th gear = 89.3 mph
custom gearing 15/43 @ 11000 rpm, 6th gear = 100.2 mph

 

[DaBlue1]

It is recommended to change the chain when changing sprockets.
11800 rpms with a 15/42 will get you 110 mph in 6th gear. The challenge is getting the bike tuned enough, to turn the gears in 6th gear to past 11500.

You can find a 42 tooth sprocket here. http://www.motorcycle-superstore.co...7704/21483/PITEM/JT-520-Steel-Rear-Sprocket-2004-Kawasaki-Ninja-250R-Parts.aspx

 

[DaBlue1]

@zx9r; You should be going about 73 mph in 6th gear @ 9k rpms. 65 mph in 5th gear (actual speed). The speedos are known to be off a little, but not by so much. They are usually about 5 mph faster than the actual speed.

 

[DaBlue1]

@zx9r, if your tach is reading correctly at low rpms but you suspect an error at higher rpms, you may have a tach problem or IC Igniter problem. see this thread- http://www.250r.net/forum/showthread.php?goto=newpost&t=1449

Lately, I've been seeing a lot of riders with improperly adjusted clutches, having rpm issues as well. One has had to replace the clutch within a year of having the bike. A properly adjusted clutch lever makes a the difference in the world. The clutch lever needs a little bit of slack in it.

If you do a 15/39 combo you will have to shorten the chain. The stock chain is way too long. Also you will have to at least shim the needles and remove the snorkel for a little more low rpm umph. The smallest rear you should do with the stock chain is 15/41. Keep in mind that the bike will have a hard time reaching red line in 6th with these combos unless you re-jet or put on a full exhaust. Even then the gains maybe minimal.

 

[DaBlue1]

@zx9r, There is a way to check your rpms externally. You can use an inductive pickup clip accessory on a digital multimeter. The clip attaches to the spark plug wire. If the multimeter is good, it can be set up to convert electrical pulses into a rpm format. With this I believe you will be able to tell if there is a different reading between the tach and actual spark output. The trick would come in trying to mount the multimeter and take a reading while riding at speed.

This is what the dealer should do as well, however since the bike is new and stuff isn't working 100%, the dealer needs to fix it all and a free oil change!

 

[DaBlue1]

The bike will not pull a 15/39 combo to redline in 6th with out some serious mods;
Racing carbs
Shaved heads
Polished valves and seats
High compression pistons
Advanced timing
and
Nitrous
That should get you a little past 121.854 mph and a new Official Land Speed Record for a Modified Ninja 250.