I drive a 1976 ford f100 2wd, c6 with transgo shift kit, 9 inch rear with 3:73 gears, limited slip, 31x10x15 tires. My rpms at 60 mph are 3000, I feel that it's a little too high, I mostly drive around town but when I do drive on the highway over 65mph it's loud. I was thinking of going with 3:25, I don't know if the 3:50 will make much difference from the 3:73s. Any suggestions?

 

 

Just for clarity, Ford didn't offer a 3.73 ratio in the 9-inch rear ends. The closest you would have gotten to that in a factory ratio would have been a 3.70.

Not much difference from a 3.73 but, will make some difference in your calculations.

 

Lemme find a Ford 9 table.... BRB.

... dagummit. Can't find one.. Oh well, just use the table above for guidance, ya know? Or swap in an 8.8 and then the table will be spot on! LOL.

 

Formula for calculating engine RPM for a given tire height, gear ratio, transmission final drive ratio and MPH.

RPM = MPH X Trans Ratio (in high gear) X Rear End Ratio X 336 ÷ Tire Height.

....or, you can just do it the lazy way and use this handy dandy RPM calculator that will compute the factors for you.

Engine RPM / Engine Speed Calculator

 

AXLE code stamped on the 2nd line-far right of the Warranty Plate, riveted to the left door face below the latch.

If the AXLE code begins with a letter: Limited Slip (L/S). If with a number: No L/S

1973/77: 09 = 3.70-1 without L/S / 1975/77: H5 = 3.70-1 with L/S

 

I had pretty much the same setup you do after I put what I thought was a 3.50:1 locker in my 77. But it is closer to 3.90:1 and it would not hit 3000 rpm until 70 mph. I have since put the original 3.25 trac-loc back in it and it made quite a difference. I think you have something off like the speedo or tach maybe because you should be around 2500 2600 rpm at 60 mph The 31x10.5x15 will make your speedo read slower if nothing else was changed.
When I went to the locker it changed the speedo reading almost back to what it was before I went to the 31" tires which are only 30 in reality. A lot closer to accurate anyway

 

I originally had an open diff with 3:00. When I replaced it with a LSD 3:70 I also replaced the speedo gear. I cant recall how many teeth it had but it was orange and preplaced it with a white one. I might of use the wrong speedo gear that's why its not accurate.

 

Before GPS on every phone, I would check my speed by timing a mile on the freeway.

 

well I downloaded a speedometer on my iPhone to check my speed. My trucks speedometer ended up being off. I'm at 2500 rpm at 60 mph. The speedometer seems to be accurate until I begin to go over 50 mph. thanks for all of your help.