Getting 6K from a stovebolt engine is about as likely as finding a chicken with lips. 3500 would be pushing things. What engine are you running, and what brand of tachometer?

The simplest way to get an accurate speed reading is to use a Garmin or TomTom GPS and compare the satellite info to your speedometer, then either adjust the speedometer drive gear teeth up or down or get a ratio-change box from a speedomerer shop.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!