Your trans would have a 1:1 4th gear.

With 29 inch tires, (stock height) 3.55s and a 1:1 gear, you should be turning 1645 rpm at 40 mph.

At 55 mph with that combo, you would turn 2262--

You would need 4.88s to turn 2261 at 40, close to your claimed 2300. 55 would then be 3110

If you have a T19, all synchro 4 speed with granny first, your 3rd gear is 1.79, which would result in 2945 rpm at 40.

If you have a NP435, 3rd would be 1.66 or 1.74, for 2731 or 2863 at 40.

Something is wrong here--is there a tag down on the axle? If so, post those numbers. What trans do you have? What size tire? Are we really talking 40mph?

Once we figure this out, if you are still geared too short, you can either convert to a 5 speed, or change the axle, but let's get a correct base line figure to start. BTW, what tach are you using to check rpm to mph? And does your speedo read correctly?

 

Easy way to lower rpm is a taller tire. Your speedometer will read low but that can be fixed. RB

 

H9 = Ford 8.8 / 3.55-1 / Limited Slip / 3750/3800 lbs. Rear Axle Capacity.