A500/A518 Aftermarket Overdrive Ratios
2300 rpms at 60mph with my 28" mickey thompson radials.
From a fuel economy standpoint, 2300 is actually about ideal. ...... because with the factory type distributor, you can just barely modify it to get close to the ideal cruise timing. Your combo might want 56* cruise timing to get optimum gas-mileage.
You can modify your Vcan for 22* maybe as much as 24*.
Your normal rate of advance will get you about 14* from 1000rpm to 2400.
You can set your basic Idle-timing to in the range of 10* for the stock cam to 16/18 degrees with a typical street cam. Thus the total cruise-timing becomes say
16initial +14 mechanical +22V-can =52* close enough.
Under 2000 it becomes almost impossible.
A typical rate of advance might be .78* per 100rpm beginning at ~1000rpm. Thus at 2000, you might be looking at 8 degrees mechanical. To get a cruise-timing of 56* would take ;
56 -( 8+22)=26* initial. That could work. The question is; will the engine detonate at WOT at 2000rpm with 26+8=34* of power-timing? I suspect it would. To get around that might take a 2600stall or more. Of course now you would be cruising a 2600 plus TC slip; which could total 2700, unless you have a lock-up convertor .......
In terms of getting enough ignition advance;
in all my testing, a cruise rpm of 2250 is about ideal (230*cam).
And I have tried just about every final-drive you can imagine, down to 65mph =1450 rpm . And no I didn't cruise that one at 65, lol; we cruised at from 1700 to 2250.
2250 is the lowest rpm I would cruise at, but it is impossible for my engine to get enough cruise timing at that, so I installed a dash-mounted, stand-alone, spark-delay box with a usable range of 15*. With that unit I can experiment while driving..... and can run almost any ignition curve/any rear gear/any trans.
65=2250 is very comfortable to cruise at.
You may wish to trailer your car, lol.