Rear gear recommendation for highway.....

I like sub 2000 rpm so 2000/3000 x 3.91 =2.61s
Add 15% to go plus 15% faster equals 2.22s
Thats what you need 2.22s
These will run at 2.22/3.91 = 56.8% of your current rpm so .568 x 3000 =1703 at 55/60 which at 75 would be 75/57.5 x 1703 =2222, unless your TC slips down there, or your gauges lie.

I like 1600@65, but that's just not doable without an overdrive.That would be [email protected]

Put a vacuum gauge on that teener, and rev it up slowly. When the vacuum peaks, that is the minimum recommended cruising rpm.This is the rpm at which the engine first becomes efficient. Cruising on either side of it will cost fuel. The higher side usually costs more. If it has a long slow vacuum curve like a stock teener, then almost any hi-way gear will work.
If you have a nasty cam that won't vacuum-peak until 2800 rpm,buy a Neon. Or earplugs
If you have a 3000TC that slips 10%,buy a Neon.Or stock in an oil company.
Good luck finding 2.22s
On the flipside 2.22s would destroy the teener performance. First gear would be good to over 75 mph, but the car is basically gonna be a turd, cuz it will feel like it's starting in second gear.More like 2nd and a half compared to those 3.91s. The engine won't even wake up until 50ish in first.
So what to do?
Well everybody has there own experiences. If the teener is making a lotta cylinder pressure, then 2.76s are the old standby.
But if the built teener failed to get hi-compression pistons, then 3.55s are the standby gear.
In-between is in-between.
140psi will pull 2.76s off the line, but it won't be particularly quick with a stock-type stall.
140psi will pull 3.23s pretty good, off the line.and
140 will be peppy with 3.55s.
If the built teener can't muster at least 130, it needs to be re-"built".
If it cranks 165, hallelujah, you got yourself an engine.Put 2.76s in and a 2400TC. With 28 inch tires,this will get you around 2500@75. The 2400TC at 165psi, will get you off the line reasonably well, especially with shorter tires like about 25s. That would be equivalent to a gear change from 2.76 to 3.09s
If the teener only makes 130psi, it needs those 3.91s to be peppy, so you will really really miss them when they are gone.