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Vacuum gauge for my car

Fuel economy is a concern for me and it’s one of the reason I still drive an old civic hatchback. Nothing on the market right now is doing as good as my car, while being cheap and practical (4 seats). Something I miss from hybrids car is an instantaneous fuel consumption readout.

Fuel flow meter is not something easy to fit on old cars. You have to mess with fuel lines and you need to place sensors on both feed and return line from the tank because of injection.

I recently installed a mechanical vacuum meter. This meter plugs between the engine and the throttle valve, measuring how much the throttle valve is opened/closed. Maximum vacuum is reached at idle, when the valve is closed. The vacuum is generated by the intake cycle of the engine. When the throttle valve is completly opened, vacuum reaches 0, meaning atmospheric pressure is available for the engine (no restriction).

These are the readings I get from the meter:
Idle: 22 inHg
Cruise: 15-12 inHg
Accelerating very gently: 10 inHg
Acceleration normally: 5-0 inHg

In recent rides, I try to drive-by-vacuum instead of driving-by-speed. I slow down in steep climbs, but I try to limit the vacuum to values above 10 inHg. The most dramatic change in my driving are accelerations. I though I accelerated gently, but I often bottomed the gauge. Now, when traffic allows, I stop accelerating at 10inHg. This is really gentle, but eventually reaches the cruising speed.

I hope this will help me reduce consumption even more. I’ll post the results here later this summer.