What
@ir3333 said;
and I'll add, what is mildly built?
With typical street gears your cruise rpm can quickly fall too low for the distributor to provide adequate ignition timing and then the expected fuel-economy just never materializes.
With a .69overdrive, and say 3.23s, 65= about 1800 rpm. If you mod your V-can to the max, you might get 24degrees, but more likely is 22.
With a 2800 stall you might be able to run 18/20 degrees of initial. Your D is gonna bring in about .8 degree per 100 rpm beginning at about 1000rpm, so that comes to 6 degrees. . So then 18 +6 +22= 46*@ 1800 .... not too bad, but the engine could be wanting 56* Ima guessing.
now, if mildly built is still at 8/1 Scr and say a 262 cam, your CCP is gonna be fairly low, and so, the bottom end is gonna be pretty weak at 1800, possibly requiring a lot of throttle to maintain speed in Loc-Up. and so, the transfers may be (probably will be) topped out and the mains dribbling. This will be working against your economy plans.
There are two cures for this; one is a bigger engine with more inherent bottom-end torque, or modifying yours to do that, or just cruising at a higher rpm in the first place.
By years of testing, using a factory type distributor, I find a reasonable cruise rpm to be 65=~2400rpm. At that rpm you can set your D to deliver at least 56*. With 27" tires and .69overdrive, that will take 4.30s, for 65=2400. But, the gearing in the A500 is 2.74-1.54-1.00-.69 so the starter become 4.30 x 2.74=11.78, which with a manual trans would be great, but with an automatic, and a 2800, IMO is excessive, altho it would produce a lot of wheelspin .
IMO, the gear of choice with the A500 and 27" tires is 3.91. This will get you 65=~2180, and a starter of 10.71, which is a reasonable compromise and, about as good as it gets.
There are of course other options.
If you really want to cruise at sub 2000rpm, you will need to be able to increase the cruise timing, to get the economy.
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I myself have run a low-compression 318 with a factory cam and a manual trans, at 65= as low as 1240 rpm; but with not nearly optimum timing, it was terrible on gas. This combo had double overdrive and in First-over, this made 65=1740, now with nearly optimum timing, it made better fuel economy there. Direct was 2.76 and 65=2230, and fuel economy with now optimum timing was the best.
But I gotta tell ya, listening to the Thermoquad sing in too high a gear was SOooooo addicting, lol.
So if you want to run 65=sub 2000, for fuel-economy reasons, you will need a stand-alone timing computer of some kind.
And finally, as to a starter gear; with a low-CCP 318 in an A-body, and with a 2800TC/A500; all you need is ~9/1 and that will take a rear gear of 9/2.74=~ 3.23s
I myself would someday like to try a combo like yours but with as much CCP as the combo will take. I might even like to try it with a 273.....and the aforementioned 4.30s, lol.
