with just 8 or 10 lug nuts as well!
With my old age, I missed it this time...
OK old man (U said it first), the final drive is the second to the last thing that determines how many rpm you are going at a given speed. The tires are the last thing.
So... Talking about changing to a 3.76 from 4.08 is really like talking about going from a small diameter tire to a larger diameter tire.
Using a rpm calculator
http://www.ringpinion.com/Calculators/Calc_RPM.aspx lets show some comparisons:
4 speed trans w/ 4.08
Tires: 185/60-13 (21.7" tall)
Trans: 4th gear ( .959 to 1)
Speed: 60mph
RPM:
3635
Changing to
4 speed trans w 3.76
Tires: 185/60-13 (21.7" tall)
Trans: 4th gear ( .959 to 1)
Speed: 60mph
RPM:
3350
Changing tires to 205/60-13 on 4 speed trans w/ 4.08
Tires: 205/60-13 (22.7" tall)
Trans: 4th gear ( .959 to 1)
Speed: 60mph
RPM:
3475
Changing tires to 205/60-13 on 4 speed trans w/ 3.76
Tires: 205/60-13 (22.7" tall)
Trans: 4th gear ( .959 to 1)
Speed: 60mph
RPM:
3202
Make more sense now?
For further comparison, my '74 has 205/50-15 tires on the back and in 4th gear I am spinning 3415 rpm at 60mph. So, with only a tire change, I am close to the same as if I was running a 3.76 ring and pinion and 185/60-13's.