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Vacuum Gauge Fuel Economy

Last weekend I moved a bunch of my belongings using a 10’ U-Haul Truck and a 9’ trailer for my motorcycle. On the driver’s side A pillar was a fuel economy gauge that I assume was based on engine vacuum. There weren’t any numbers on it, but there was a green area, a red area, and a red and green striped area in between.

The truck, which had a Chevy chassis, didn’t have cruise control, but I discovered it had something almost as valuable; it was governed not to go above 75 MPH. Since most of my trip was through 70 MPH zones and 65 MPH zones where people tend to speed like crazy, I put the hammer down and cruised along at 75 MPH for most of my trip.

When I did this, I noticed something strange. When I passed through 60 MPH construction zones, I appeared to get worse fuel economy attempting to hold the speed between 60 and 65 than I got when I put the hammer down and let the governor hold my speed at 75. In fact, the difference was significant. The trip was from Florida’s Treasure Coast area to South Florida and back, so the only hills to speak of were overpasses; the route overall was very flat. When I manually operated the throttle at lower speeds, the fuel economy gauge was in the red, but when I put the hammer down, it was in the green. Even uphill with the pedal mashed to the floor at 75 MPH, the fuel economy gauge was in the striped area, appearing to get better fuel economy than I got at lower speeds on flat land.

Here is my question:

Was I really getting better fuel economy at 75 MPH than I was at 65 MPH, just because the governor was controlling the throttle at that point? Could a computer-controlled throttle operate that much more efficiently that I got better fuel economy at 75 than I did at 65?

…or is it more likely the vacuum gauge was feeding me false data?

Considering the fuel bill for the 200 mile round trip was almost $60, the reading on the vacuum gauge really gave me an incentive to speed. If the truck had cruise control, I would have set it at the speed limit and been content to reach my destination a few minutes later.

I don’t know why the gauge acted like that. Perhaps you were in a different gear at 75 mph vs 60 mph? I used to ride along with a relative in his Olds 442, and it had that sort of gauge. He said it was to help improve fuel economy, but if that’s what it was for, he sure wasn’t looking at it … starting off after every stop sign … vrroooooom!! … down to the stop would go the gauge … lol …

It’s just a vacuum gauge with pretty colors on it. The more you accelerate, the lower the vac reading. It wasn’t really calculating fuel mileage, it was just telling you where the throttle was. Once you hit a constant 75, it would back the throttle off and you’d be cruising. These were common on some cars back in the 70’s or at least on the higher end cars that relatives had. Fun to look at but never paid much attention to it.

Air filter restriction gauge . . . ?

By my calculations that means just under 12 miles a gallon. A 10 foot box van towing a trailer (even with just a cycle on it) getting 12mpg sounds about normal to me. A half-ton pickup towing an 8 foot trailer got me about that when I helped a friend move across 3 states.

our 2015 civic has a fuel economy gauge on the dash. maybe civic owners like to keep tabs on the figure? our long term avg is 34mpg. i do reset it every few months when i drive car which is rarely as it is not my main car. i just put on winter tires and could keep track of mileage but its not really a big deal once winter rolls around. i expect mileage to drop

Maybe Corolla owners are different from Civic owners about the mpg figure. I checked the mpg on my Corolla one time, when it was new, 34 mpg, and never have checked it again in 26 years.

Lots of cars these days come with a digital dashboard that lets you monitor fuel economy. My mother’s Jetta has that feature.