Rear axle gears

Hi I made a 5 speed trans for my Hornet that has a 3.25 first gear and .75 or .63 overdrive (I have .63 in there but have an adaptor coming to put on the .75 5th gear)
I got an AMC 20 rear from a 70 AMX and it's 3.54 gears
I was taking it for a test drive today and I think I could benefit from 3.31 or 3.08 rear
The tires are 26.5" wide on the ground but the effective diameter is 25" I would guess since it looks about 12.5" from center of the wheel to the ground.
In 4th gear, at 65MPH, with 3.08 rear gears, I have 2691RPM
Is that about right?

Also I've heard overall first gear ratio for the street is good around 10:1 - 3.08x3.25 is 10.01

I want to make sure I have this right because it's a pain to change the gears in that 2 pc axle AMC 20

Oh this is tricky. My stock Eagle 3spd is being replaced with a 5 speed with the 4.03 1st gear and 0.76 OD combination. My STOCK gearsets are 2.72 and I believe I may benefit from 3.08 gears. I have no way to test this until everything is reassembled, which might be a very long time the rate this has been going. I might be interested in a lift kit later on and possibly bigger tires but mine are 28's and yours are less? I have 16" rims, are yours 15s? You should be able to bolt up bigger wheels and be done with it, no gear swapping involved. Is this wrong?

Also, my overall ratio looks like 2.72*4.03(1/2.6) which comes out to 10.96 high and 28.50 low. The OD makes an already high 2.72 fly up to 2.06. The lowest offered ratio in these diffs was 2.35 and it was NOT with an overdrive transmission of any kind, pretty sure it was either a Borg Warner or Chrysler automatic. Anyway, this is all after junking the factory anemic 258 for a EFI 285 and possibly M62. What's going in front of your gear masher?

Thanks you guys
I started with putting the AX-15 in the Hornet because it's tougher than the T5 and I wanted to be different
The 3.83 first with 3.54 rear was way too low
I figured out how to put the Toyota Supra gears into the AX-15 for better ratios including the 3.25 first gear so that is working better
The wheels are tall Radial TAs on 14x7 steel ralley wheels to fill up the flared fenders nicely
The engine is a hot 258. Ideally I want to take it on road trips. Good freeway cruise speed would be nice as well as being able to use first gear. I'll drive it around a little more and keep tuning the carb to get a feel for how to make it feel better. The carb isn't completely dialed so it's feeling sluggish
The T5z has the 3:1 1st and .63 overdrive so it's pretty close to that right now but I have an adaptor coming to put in the .75 overdrive of the R154 if that seems necessary but it's pretty far from going on the freeway as the suspension needs a full rebuild
If I do a 3.08 to get the best first gear ratio, which 5th gear would I want - the .63 or the .75?

This is going to take some tweaking. It's my first build and I am pretty sure I made it way too complicated with the whole hybrid transmission thing...

Don't forget the weight of the car as this will also play into trany & rear gear ratios.

When I did the WC T5 in my factory T10 Javelin I knew the T10 first gear was a little to high for my liking, slip clutch too much with the stock 3.54 rear gear.
The WC T5 I used has a 2.95 first and just moving the car in & out of the garage with no clutch slipping.

Lighter cars can get away with a higher first gear to get it moving and same with OD to keep in moving. Think same with larger motors over smaller ones.

What was nice for me was knowing how my car was with the 3.54 rear gears and T10 and then knowing what the T5 had on how it would work out.
Dave ----

uncljohn wrote: If you aim towards gearing your car for 32 mph/1000 rpm in 5th gear your final drive will be pretty much on the money. My choice? 3.08:1

That's an interesting way to do it
Axle gear wheel MPH 5th gear RPM
3.15           26"     32      .75           977
3.31           ..        ..        ..           1027

3.15           25.5" 70     .75           2179
3.31           ..        ..        ..             2290


Seems like consensus is centered around 3.15