Quote:
Originally Posted by WorkinLonghorn
I remember back in the 70's after the first oil embargo, you could purchase "Mileage Gauges" that were just vacuum gauges actually. I'm not sure but I think a couple of vehicles actually came with one stock (correct me if I am wrong). So maybe higher vacuum equates to better mileage to some extent. I always heard from the old guys that for an old car/truck with no overdrive, ~45 MPH was the sweet spot. "Modern" cars with OD, ~55 MPH but I guess gearing plays a big part too.
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I have one of those fuel efficieny gauge in my garage.
Higher vacuum = less air being pulled in = less fuel
the 45 / 55 mph thing has a lot to do with aerodynamics. I've heard 48 MPH is where that starts. Our trucks aren't aerodynamic, so it has a larger effect than it would on more modern aerodynamic vehicles.
Gearing changes the cruise RPM. Lower RPM is typically better as that is less times fuel is pulled into the cylinders, but it also depends how optimized the engine is at that RPM.
Brake Specific Fuel Consumption is used to measure how much power you make for the fuel used. It can be used to determine where your engine will cruise most efficiently.
https://en.wikipedia.org/wiki/Brake-...el_consumption
Modern cars (just about anything in the last 15+ years) have EFI, computer controlled spark, and variable valve timing. This allows them to optimize the engine over a wide range. Without VVT, the RPM range is more limited. You can still optimize the ignition and fuel at your cruise RPM.