buttuming that you measured your low-speed mpg correctly enough for a valid mpg estimate), your car has a combination of features that make its best mpg speed harder to predict, compared to modern cars.
AFAICT, your car is carbureted (in an uncertain state of tune) and has a 3-speed automatic trans. Does the engine operate with closed-loop mixture control? Does the trans have a locking torque converter clutch? I'd guess "no", in both cases.
If your car does not have a locking TC clutch, and-or it runs "open loop", it is *very* different from the modern cars that people like Ed White and myself have been talking about, and cars like Dave's.
In the years that I've been discussing this topic on r.a.d., I've specifically excluded old cars (esp. with automatics) because there's no way to tell what those old powertrains are doing without adding a lot of instrumentation.
Anyway, when you get a modern car, even one as modern as Dave C's, don't expect it to get best mpg above 60 mph (or at 40% of redline, for that matter).
Expect best mpg at the lowest speed at which the powertrain will run in top gear with no powertrain slippage (locked TC clutch, or manual trans).
BTW: I don't see anyone recommending that people actually drive at best-mpg speed. My time is worth too much to me, so I drive faster whenever safely possible. -- Chuck Tomlinson
