Did you recalibrate the speedometer when you changed the gear ratio? If not, you're still indicating the same speed as before for any particular engine RPM, but the truck is running faster than the speedo indicates. Do a stopwatch check over a 1-mile run. 60 seconds/mile equals 60 MPH.

"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!