gps'd my 2006 speedo, still uncertain

[xorbe]

(Clarification: not that I am concerned about top speed, just that when I am going the speed limit, seems like cars are ripping by me. I guess I can get someone to double-check from their car. I am inclined to believe the gps numbers below.)

Found out stock sprockets are 14/45 stock, but my used bike has 15/42. Bike has brand new stock oem tires. Bike has less than 8K miles.

According to gearingcommander dot com, 1-4 gear top speeds @ 14K rpms should be roughly 39, 57, 72, 88 with stock everything.

With 15/42, it suggests 45, 65, 83, 101.

My GPS claims 40, 58, 74, 91 (speedo shows 10% high btw compared to this GPS) with the bike pegged at top rpms in each gear.

Why is gearingcommander so far off of the GPS readings? How can I double check my GPS readings / speedo error percentage? Possible clutch slippage?

[Alex]

Trust the GPS. Your tach is probably off.

[Motofool]

Originally Posted by xorbe

..........Why is gearingcommander so far off of the GPS readings? How can I double check my GPS readings / speedo error percentage? Possible clutch slippage?

You can try this one:
http://dmitriy.levchenkov.com/misc/n...pm/index1.html

Air pressure of your tires can make a big difference.
Your rpm's are read via number of sparks - electric tachometer, which may introduce some error as well.

The proper way to measure your actual speed is placing road markers at known specific distance and time your ride at constant speed between those two points.

When doing that, your rpm's will not matter, as the speedometer-odometer only reads the speed-travelled distance of the front tire via number of turns.

[xorbe]

That's why I took the readings at redline ... the ecu should be pretty accurate about 14K rpm spark removal vs whatever is on the analog tach (it shows a bit over 14K actually). Nah, that 2nd website also shows 15/42 as 97 mph in 4th, which is still notably above my 91 mph gps reading. Double checked the rear tire just now, heh.

Update: well, my brand new fancy car's speedometer and the gps match exactly, so the bike's speedo really is 10% low. Don't know why the bike's real speed differs so much from the suggested chart values, oh well! The tach would have to be wrong by 11% which seems like a lot (redline of 12600 in reality?)

[DaBlue1]

Originally Posted by xorbe

... the ecu should be pretty accurate about 14K rpm spark removal vs whatever is on the analog tach (it shows a bit over 14K actually).

It's not going to be 100% accurate either, unless you've had it calibrated, which no one does. There will always be a small margin of error.

Go out and actually measure the circumference of your rear tire and then plug those numbers in on the charts and see what you get.

You can also try this chart as well. http://faq.ninja250.org/speed/#noredir