I have a 51 Chevy truck, 261 engine, 3.40 gears. it has an A883 4 spd OD like FT is describing, the overall gear ratio in OD is around 2.50, the engines turns 2000 rpm at around 65-70 mph.I have a vacuum gauge,the engine pulls 10-11 inches of vacuum at 70 mph, dips down to about 8 inches to cruise up mild hills. The engine has a 2300 type Holley 2bbl with a power valve opening of 6.5 inches of vacuum.So the enrichening circuit remains closed during normal cruising.Downshifting to 3rd direct at 65 mph gives 2600 rpm and 14 inches of vacuum and a lot of noise,lol..Since carburetor main metering circuits are responsive to air flow through the venturi and not engine vacuum as seen under the throttle plates, the low rpm ,higher load should give better fuel mileage.I haven't checked the fuel mileage on this truck yet, but my 261 powered 37 Chevy PU has a 390 Holley 4bbl, 3.36 gears and a 3 spd OD, runs 70 mph at 2 grand, when driving at 60 mph,1800 rpm, the truck gets about 21 mpg, 2-3 mpg more than when it had a regular 3 spd with the same 3.36 gears.My daily driver 94 S10 4.3 has 2.73 gears and a 5 spd OD,60 mph is 1400 rpm, gets 26 MPG on the highway at reasonable speeds.I know the S10 is fuel injected, but the throttle body injection is similar in operation to a carb.
This works for me cause the Holley power valves are a known value and come in a variety of vacuum openings so you can tune the carb to your siuation.AFB's and Edelbrocks use different color metering rod springs to vary the enrichening opening vacuum signal.
Supposedly the lower vacuum also results in less piston pumping loses and a more effecient engine.
