Regear Rear End

[Don C]

You can a very good idea of your ratio by jacking up the rear and turning the rear wheels and counting the driveshaft revolutions.
https://www.7173mustangs.com/thread-can-...-from-this?

https://itstillruns.com/calculate-rear-a...88196.html

From the sounds of it you must have 4.10 or lower gears.

[mbrew2]

Agree with  Don C, someone has already swapped gears in your car. I bought a 68 shelby GT 500 that the PO had pulled the 428CJ and put in a race-built, high winding 302 out of a race jet boat. Big Bearing toploader. He said he built it for street racing and was fairly successful in our little town. Car was definately strong runner but always was screaming on the highway. I swapped in an old school stroked 427FE and Tremec 5-speed. When I changed rear end gears I found that the car had 4:56 welded up rear end. Changed to 3:89 gears and with the Tremec gearing, first gear was still almost useless. Old 427 came up so quick in revs that 1-2 shifts were almost immediate, but it was great on the highway with the overdrive. Used it a daily driver for about 5 years but eventually had to swap the mechanical 2-4's out for a single 4. Had put a 22 gal tank in it but it still took me approx 55 gals fuel to travel 325 miles. 

Point is, you have to decide how you want to use the car. Highly suggest going to a 5-speed and there is lots of information on appropriate gearing available here and on other sites. When choosing, be sure to consider gear ratio in the trans itself to get a better idea of what the effective ratio of 1st gear is going to be.

[barnett468]

If it has a posi, jack it up so both rear wheels are off the ground then spin 1 tire 1 full turn and count the driveshaft rotations.

If it has an open diff, jack 1 rear tire off the ground and spin it 2 full turns and count the driveshaft rotations.

.

[icejawa]

We have 4.10 gears and a differential locker, oh boy. Now I just have to get the damn car to start in this below 0 weather. Probably doesn't help that I never degreed the distributor I threw in there

Is it bad on the engine/transmission to drive on the freeway with 4.10s? Or just bad gas mileage

[73pony]

I will say this 4.10 gears, 4 speed, and thekind of power you are putting down, I would have to own stock in a tire company as I would not be able to help myself.

[jpaz]

I will say this 4.10 gears, 4 speed, and thekind of power you are putting down, I would have to own stock in a tire company as I would not be able to help myself.

And a u joint company too, lol

[OMS]

We have 4.10 gears and a differential locker, oh boy. Now I just have to get the damn car to start in this below 0 weather. Probably doesn't help that I never degreed the distributor I threw in there

Is it bad on the engine/transmission to drive on the freeway with 4.10s? Or just bad gas mileage

It will be 50 On Monday or Tues next week
I think I have a set of 350's ...

[barnett468]

1. We have 4.10 gears and a differential locker, oh boy.

2. Is it bad on the engine/transmission to drive on the freeway with 4.10s? Or just bad gas mileage

1.

2. Well, it can cause it to run hotter than it otherwise would, plus it will wear it out faster simply because it is turning over more than it otherwise would with numerically lower gears. I would definitely use a high quality oil.

[icejawa]

I use Valvoline vr1 oil and additive for the cam in there, already replaced the u joints with Spicer heavy duty offroad joints, same ones I use in my jeeps. I don't see any reason to bring it on the highway but I do have to be at the Carlisle mach meet coming up (I forget when exactly). Guess I'll trailer it up there

[Don C]

There is an option, a Gear Vendors overdrive will give you the best of both worlds, tire shredding acceleration when the light turns green and a decent cruising RPM, plus 8 forward speeds.
https://www.gearvendors.com/hrford4sm.html