tach reading

Greeting to everyone.. I have just purchased my first vette at age 64. I'm 64- not the car. The vette is a 90 model, convertible- 6 speed manual trany, and runs great. My question however is about the tach, which just doesn't seem right. It pegs out much to easily. At 66 mph it reads about 3600 to 3700 rpm. Also the readout for fuel miles per gal. seems too read about 5 miles per gal.to high. In reality, what should be about 30 mpg reads closer to 34.5mpg. Any help is appreciated..  martybelairman------