here is my speed chart for my 40. it has 7.50x16's on it with a 4.56 rear. this chart was generated from a web calculator that i have verified with my westburg tach and my gps.
MPH RPM
40 2009
45 2261
50 2512
55 2763
60 3014
somewhere i read where gmc designed for a continuous engine speed to be 2/3rds of max.(cant speak for chevy) so with that being said 2/3rds of 3400 equates out to 2278 which is 45 mph on my truck with the pointer sticking straight up and down on the speedometer. not to say you cant run it higher but i found that all my trucks run their best, sound their best and i can hear the best when i run that 2/3rd rpms. the sweet spot on my 40 is 2450 which is 47.5 but i run 50 on the big roads when i want to get somewhere fast. what is the condition of your engine? mine is freshly rebuilt.
