You want taller gear for the highway I see,I ran 3:90's in my big block '69 with a 4 speed and was pretty happy with them,not too low like 4:10's but still plenty of jump off the line.
What gears do you have now?
3:73's or 3:55's might be better if you do a lot of highway driving.
What 4 speed do you have,that makes a difference as well.,wide ratio might be better for highway driving and close for city,where do you spend most of you driving time?

 

You can run a 3:55 to 3:73 with a 12 bolt .and or 3:42 gears with a 8.5 10 bolt With a 26-8 tall tire.

 

either 4 speed has the same 4th gear ratio of 1:1 so no difference in final gear RPM's between the two at any speed

if fwy driving is a strong use than go with a higher gear. Use one of the online RPM calculators you enter your tire size and tranny type and select a given gear ratio available for your rear end and it will tell you what RPM at what speed you will have. Make a determination based on that info

 

Then what is the point of having a wide ratio trans?

 

The RPM the engine best work at .And what you want the car to do. Like a road course likes a close ratio gear box.Because there is limited RPM drop .And wide ratio box works good for pulling heavy loads .

 

If you're using highway gears (3.55 or higher), I think you might be happier with an M20 wide ratio trans. It has considerably lower 1-2-3 gears compared to the M21 close ratio model, which will give you better off the line performance.

 

the term "close ratio" and wide ratio" mean on a close ratio the gear ratio (numerically) is closer together from gear to gear vs a "wider" ratio from gear to gear.

For drag racing and quicker acceleration shifting through the gears you want a close ratio

Most want a close ration trans

 

2.20 would need a 4.56:1 rear gear; 2.52 would be a 3.90:1 ratio.

 

OP

use this, or any other one online, to determine what RPM your motor will run in 4th with whatever tire size you have or will get and decide what rear end gear ratio provides the RPM's in 4th (1:1) that is acceptable to you

I had 3:73's which was great for stop light to stop light but got old if wanting to sustain 65, or higher, mph for a hour or so on the FWY so I went to 3:08

Overdrive transmissions are certainly the win win. I have 4:10's in my T56 car and am under 2k RPM at 80 mph in 6th with a 27" tire.

Do the math and make a choice on what RPM at FWY speeds is acceptable to you

calculator
https://spicerparts.com/calculators/...rpm-calculator

 

My thoughts exactly for a highway geared car.

Right I understand that but he asked about highway gears not drag gears so wide ratio was my thought with tall rear end gears.