Vacuum and gas mileage

Hey folks! I remember hearing about vacuum gauges and how they can help you get the best MPG. Remember the old AMC cars with vacuum wipers? When you were “on it” the wipers would just about stop and off the gas they worked normally (or did I get that backwards?) Anyway . . . anybody care to comment, confirm or bust this myth? Can you use a vacuum gauge to show you how to drive better/achieve better MPG? Rocketman

Ancient history, so my memory may not be that good. Back in the late 1960’s a friend tapped my manifold and installed a vacuum gauge and told me to keep the reading high…that is a light foot on the accelerator. I have no idea if it ever really worked since I never kept any records, but the theory seems logical…an even light acceleration would save fuel.

Yes indeed, it will greatly improve your mileage if you keep the needle in the green zone. Acceleration is what destroys gas mileage and a vacuum gauge will let you see it happen…People with the hottest brakes get the worst gas mileage…


The way to control gas mileage via your driving habits is to drive as if you have an egg in the middle of the passenger seat. The vacuum gage just gives you a direct way of monitoring the position of the throttle plate relative to the volumetric demands of the engine, which is basically an air pump trying to draw air in past the throttle…the faster the pistons are moving up and down is the greater the demand…so that you can drive as if you had an egg in the middle of your passenger seat. Drive as if you were an old man in a Buick. Forget the vacuum gage.

Some GM cars way back when had a color coded vacuum gauge that was mounted in the factory instrument display.
Keeping the needle in the green band meant you were getting the best fuel economy.
My parents had a 78 Caprice with this feature, although they generally ignored it.

It could help one become acclimated to feathering the accelerator pedal while keeping the needle in the peripheral vision and eventually it would become a habit with no need to look at the gauge.

I’m glad my current car does not have one; it would irritate me to no end to see my wife burying the needle in the red all of the time. :slight_smile:

Some GM vehicles of the 70’s had this type of gauge in the dash. A friend had a Chevy stationwagon with this gauge.


I submit that trying to improve gas mileage by using a vacuum gage is complicating the simple. I have a foot at the end of my ankle. I know what the foot is doing. Push pedal hard, mileage goes down. Push pedal soft, mileage goes up.

My friends here probably find it unusual for me to be taking this direct an approach. But jeeze, nobody needs a vacuum gage to know whether they’re driving more gently.

If you wanted to win the Mobilgas economy run, you had to have one…Professional drivers would get twice the fuel mileage normal drivers got…

The more you stay off the gas pedal, the higher the vacuum. You can learn how to give it a little less push and improve the economy. The fuel injection system is more economical unless you need power, then it will just dump the gas in. If we stay reasonable and stop the mad accelerating we can save some gas.

I fully agree with all the above posts.

What’s ironic is that your engine is running at maximum efficiency at wide open throttle, where efficiency is in terms of how much work is being done for the amount of fuel it’s consuming (very different from best gas mileage). I remember this back in my internal combustion engines class.

The two main reasons for the increased efficiency are 1: The open throttle minimizes much of the vacuum losses inherent to gas engines, and 2: The flame speed for a denser more tightly compacted mixture in the cylinder burns faster than that in a less dense mixture.

The above seemed odd to me until we computed it and then measured it on our small dynamometer.


Several problems with WOT operation is that most cars do not need that much power all the time. Another is that most fuel systems enrichen the mixture at heavy throttle settings to forestall detonation and provide fuel cooling of the flame temperature. If you have worked on Holly carburators you know about ‘power valves’. Even electronic fuel injection goes to ‘open loop’ on WOT and enrichen fuel and retard ignition for knock control. Keeping the vac gauge between 20 and 10 inches provides great economy. So when you are on a hill you may need to shift down early and limit your climbing speed for better economy.

That’s why a lot of gas mileage records were set by using a “pulse and glide” driving style. The driver accelerates the car at the engine’s maximum efficiency operating point (pulse), then switches the engine off to coast for a long ways (glide). Then restarts the engine and repeats the process. Because the car burns so little gas this way, the engines are often wrapped in insulation and the radiators partially blocked to keep the engine from running too cold.

Not exactly and certainly would not work with a modern consumer engine.

In the cars in question the engines were designed to run at their maximum efficiency at WOT. Few engines are designed for that. They also were very small compared to the weight of the car so at WOT they accelerated slowly. In addition they did reduce engine cooling as that is wasted heat. They ran those engines far hotter than the consumer engines at that time. After all then engine only had to last one trip. Modern engines are far different and the same techniques will not work. One thing has been incorporated into modern cars. With better manufacturing and better temperature control they often operate at higher temperatures than consumer engines of that time.

Why would you want to accelerate slowly? Accelerating a 3000 pound car to 60 mph takes 360,745 ft-lb of energy whether you accelerate like a 18 wheeler or do a jackrabbit start. Thinking it takes less energy to accelerate that car to 60 mph by accelerating slowly is a lot like thinking that your car insurance will be cheaper if you make the minimum monthly payment instead of paying for it all at once. Me? I’m in the pay now and save later camp. If you got to accelerate to a given cruising speed, you might as well do it using the engine at its most efficient power setting which is probably about 2/3 throttle at the peak torque rpm.

Let me make a case for the “jackrabbit start”. Let’s say the car weighs exactly 3000 pounds and has a frontal area and coefficient of drag that results in the car needing 15 horsepower to maintain a 60 mph cruising speed in still air.

Driver A gets in the car and accelerates like grandma taking an entire mile to get up to 60 mph, then cruises at 60 for the second mile, and then spends the last mile gradually slowing down to zero. The first mile of the trip takes 2 minutes to cover, the second mile takes one minute, and the last mile takes another 2 minutes for a trip time of 5 minutes. His moving average was a pathetic 36 mph. His engine had to generate 360,745 ft-lb of energy to accelerate the car and then had to make 15 horsepower for one minute, that’s 495,000 ft-lb of energy and then the car coasted the last mile with the engine using whatever amount of fuel an idleing engine uses up in the 2 minutes it took to cover that distance. That’s a total of 855,745 ft-lb of energy that the engine had to generate.

Is there a more efficient way to do this 3 mile trip at a 36 mph moving average? I think so.

Let’s use only 1/10 of a mile to accelerate to 38.4 mph and use the last 1/10 of a mile to slow down. This also results in a 36 mph moving average. Accelerating to 38.4 mph only requires 147,761 ft-lb of energy. How much horsepower does it take to maintain 38.4 mph? Since 38.4 mph is .64 times 60 mph, the horse power needed would most likely be 15 hp X .64 to the third power or about 26.2% of 15 horsepower, which comes out to 3.93 horsepower. We cover the middle 2.8 miles in 4.375 minutes so we have to generate 3.93 horsepower for 4.375 minutes. That comes out to 567,394 ft-lb of energy. That comes out to a total of 715,155 ft-lb of energy, a difference of 140,590 ft-lb of energy.

I don’t accelerate like grandma when I drive for this reason. It’s more efficient to accelerate briskly to a lower cruising speed and achieve the same moving average. I drive a 2008 Toyota Yaris five-speed non-hybrid car EPA rated at 29/36. So far, I have never gotten less than 40 mpg and my best so far is 42 mpg in mixed driving.

What really wastes gas is not jackrabbit starts, but pointless acceleration, jackrabbit or not. Any acceleration that does not increase your moving average is pointless acceleration, i.e. accelerating towards each and every red light.

What a vacuum gauge measures is the suction in the intake manifold. As you close off the fuel/air supply by keeping the accelerator closed, you should increase the vacuum. As you push down the accelerator pedal, you open the throttle plate, thus decreasing the suction. However, the suction is caused by the movement of the pistons in the cylinders. Therefore, the faster the engine is turning, the higher the potential for increased vacuum. On the early carbureted engines, it was possible to time the engine with a vacuum gauge. For a set throttle position, we would increase the timing and watch the vacuum gauge go up. It would reach a point where it would go no higher–after this point, the gauge would become unstable. We would then retard the timing just to the point where it was at its maximum, but a hair more retard would decrease the vacuum. This point was supposedly the optimum point to set the timing. We could also check the compression with a vacuum gauge. With the vacuum gauge connected, we would short out the spark plugs one at a time. On a particular cylinder, if the vacuum dropped less than the other cylinders or not at all, this was the weak cylinder. The object is to get the maximum vacuum reading for a given throttle plate position. I’m not certain how helpful a dash mounted vacuum gauge would be. I do know that I rode in a school bus that had vacuum gauge that looked like it was factory equipment. The bus was equipped with vacuum assisted power brakes, so I suppose that the driver could monitor the vacuum available in a reserve tank to the power assist. This was back in the early 1950’s, and many buses didn’t have power assisted brakes. I also rented a U-Haul truck back in 1971 that had a vacuum gauge. According to the instructions on the dashboard, the vacuum was to be at a certain point before driving away. This was to insure that the power assist unit on the brakes would function.

You are correct about the windshield wipers if there is no vacuum booster on the fuel pump. If you were traveling 60 miles per hour and released the accelerator, there would be more vacuum available to operate the wipers. Converesly, if you were in high gear and floored the accelerator at 15 miles per hour, you would decrease the vacuum and the wipers would stall. The AMC cars that I owned that were made in the 1960’s as well as the GM cars I owned that were made in the 1940’s and 1950’s had a vacuum booster on the fuel pump. This maintained a more steady vacuum to let the wipers perform more reliably. My 1950 Chevrolet pick-up truck, however, did not have a vacuum booster section on the fuel pump. I had to often let up on the accelerator to make the wipers move. Every time I shifted gears and let up on the accelerator, the wipers would fly across the windshield. The wipers, in effect, are doing the same function as a vacuum gauge.

I’ve never understood why electric wipers were an option on many cars. Cars that had the optional wipers had a simpler fuel pump without the vacuum booster section. It would seem to me that it would be just as cheap to have the simpler fuel pump and an electric motor to run the wipers.