I used to run 3.89 gears and a top loader with 215/60-15 tires with my '65 fastback. In my opinion it was less than fun on the freeway. I'd opt for either 3.50 or higher gears or the T5 if I were to do it over again.
Well, I have a 3.55 unit i just pulled out of my 65 2+2.
It ran 2800rpm at 60 mph. I'm stepping up to the 3.80 trak-loc posi unit. Already have the 3.55 unit removed just have to install the 3.80. Car has 4ps toploader now. I have a T5 as well that will go in later. I run 25" tires on street. I probably would not have gone to a 3.80 if I didn't have my T5 already. The 3.55 imho was just barely highway friendly, I get passed by everyone and their brother and even 3 cylinder Geo Metro's... At least when my foot is not in it....
I run a WR toploader speed and 3:70 gears. At 60mph, I am turning 3000rpm with 25" tires. 6000rpm in 4th is 120mph. Formally I had 3:55's and really liked them a lot, but since my car is not a daily driver (a TOY), I really enjoy the extra punch the 3:70's give me. I don't need to cruise at highway speeds for much more than a half hour or so they are not a problem.
Of course if you had a 5 speed, you could have the best of both worlds, assuming you can keep it together. In my case, the torque would kill a t5 quickly so the toploader is a better deal.
I've got a Wide ratio Toploader 4 speed in my 89 Ranger behind the V8. Rear gears are 3.73's. Tires are taller than a Stangs usual 26" mine are 28". At 60 mph it's turning 2687 rpms. With a Stang's 26" tires it would be the equivalent of a 3.53 gear ratio. Multiply the tire diameter times Pi ( 3.1416) then divide the result into 63,360 (inches in a mile) to get the revolutions per mile then multiply that times the gear ratio to get the rpms at 60 mph. To convert to other speeds, simply divide or mulitply using the 60 mph figure: rpms at 60 times 2 gets the rpms at 120 mph. 60 X 1.5 = 90 mph. 60 X .75 = 45 mph. Use whatever figure's appropriate to get the rpms at what ever speed.
I guess it is what you want from a car and what you enjoy. I run 3:89 gears with my top loader. 295/50/15 tires, it revs like crazy and I enjoy it. I have dual flowmaster so its not exactly quiet.
Solid lifter 289 it'll rev all day long over 4500 rpm and no complaints so far.
I have had it on many relatively long road trips and one trip was 1400 km one way and I averaged over 21mpg (canadian gallon) and on many of our little outings I have my wife and two daughters with me.
Having said all of that I will eventually put in a 5 speed, but thats for more top end speed
gtss
yeah, my wide ratio toploader with 3.90 gears....4000 rpm at 80. will do one hella job at chirping tires...but the drone at highway speeds sucks. I have been working on dropping back to 3.50.