Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Using Instantaneous MPG indicator to control gas usage

Hello! Love your show! I have a Certified PreOwned 2008 Acura RDX SUV with SuperHandling-AWD & Turbo. I love it! It is so refined and drives like butter! But the vehicle is rated as getting a lousy 17mpg. It does suck gas especially when the turbo is engaged. I have noticed the Instantaneous MPG gauge in the trip computer that has a range of 0-40mpg. Instead of heavy foot acceleration and hard breaking, I’ve discovered that by gently pumping the gas throttle intermittently, I can take advantage of inertia and increase my mpg up to 23-24mpg! It does not matter if I’m in city street or highway driving. I’ve learned to refine this gentle “pumping” while maintaining consistent speed wherever I am. My question is: Is this continuous gentle pumping of the throttle hard on the engine or turbo? Is it bad for the car in the long run? I’ve heard of using this technique in hybrid cars, but what about performance vehicles?

You should not be continuously pumping the throttle.
Learn how to apply the throttle gently, and then–gently–apply more pressure or–gently-- ease off on the pedal as needed.

Make believe that there is a raw egg between your foot and the gas pedal, and the object is to be so gentle with throttle application that you don’t crack the shell of the raw egg. This will yield far better gas mileage than continuously pumping the gas pedal.

My car also has an instantaneous read-out of mpg, as well as average mpg for each fill-up. The range on the instantaneous read-out goes from 0-99 mpg. If I am going downhill on an expressway, without any throttle application, it will usually read anywhere from 90-99 mpg. When I have to climb the next hill, it drops to somewhere around 17 mpg. When the car is idling, it reads–obviously–zero mpg.

However, on level roads where I use the “raw egg technique” and also anticipate having to slow down for curves, traffic, construction, etc, I am able to average better than 24 mpg. If I was to continuously pump the gas pedal, I would undoubtedly lose 2 or 3 mpg.

BMW included mpg gauges on their dashes for over 20 years. The first were simple vacuum gauges, the later were more sophisticated computerized instantaneous mpg indicators. I’ve found them useless as far as helping my driving technique. Average mpg trip computers are more useful IM0. It really does not matter what your mpg was during the last city block – how you did over the last 200 miles is more important. If you want to get crazy, google “hypermiling.” Just drive sensibly and enjoy the ride. “Pumping” the accelerator is the worst driving technique.


Yes… My technique is similar to your “raw egg” one, hence the “gentle pumping” description. I get the same results as you in similar situations. After an acceleration, if I see the mpg gauge going below 20mpg, I GENTLY ease up on the throttle and use inertia to increase the mpg usage. Then, when the gauge is close to the 40mpg end I either keep my foot exactly where it is or ease up on the throttle even more… But if I notice a reduction in speed… then I gently increase throttle again until the mpg gauge dips just below 20mpg… then I repeat the technique. As you have stated, I can’t do this all the time, especially on steep uphills where more throttling is required just to overcome downward acceleration.
This has changed me into a Sunday driver and not try to race everyone on the road unless extremely necessary to get to my destination.
But my question is more about the internal mechanisms: Is this bad for the turbo in the long run? I don’t want to be saving a few dollars on gas each week if it means a costly turbo repair later.

Due respect, but the technique you described is nothing like VDC’s. VDC is accelerating when necessary and slowing down when necessary. As you described it, you are constantly accelerating and slowing down in order to ride the “inertia” wave that you think is being created. In actuality your theory is off for two reasons: 1: if it really took less energy to accelerate than you retain when you coast (as would have to be true if your theory were plausible) then you would have come up with a perpetual motion machine, which is physically impossible. 2: instant and average mpg indicators, even on Acuras, are notoriously inaccurate even when you drive normally, which is what the computer is expecting you to do. You certainly aren’t getting a genuine mpg reading when you play tricks on the computer like that.

As to whether its hard on your vehicle, probably not terribly so, although you’re likely to wear your turbo out faster since it will be spooling up and down constantly. But it is hard on the guy behind you who wishes you’d just set a steady speed and stay there instead of playing the constant-speed-change game. For that reason your practice is also dangerous to other drivers…

Yes… hypermiling is the word I was trying to remember! Thanks!
I have both: Instantaneous MPG and AVG MPG indicators. I feel that if one controls the Instantaneous MPG by using the technique above, then the AVG MPG will follow in the same direction. The problem with AVG MPG is that it (obviously) does nto give you real time information for the driver to adjust GENTLE throttling accordingly. As I mentioned in my first post, it does not matter if I’m going 35mph on city streets or 80mph on the highway. The same technique works. My question, which I hope Click and Clack takes note of, is it bad for the mechanical parts of the turbo/engine in the long run?

There is a hypermiling technique (“pulse and glide”) where you run the car up to a higher speed, then coast for a while, then run it up again. This can work because if you take your foot off the gas and you are going fast enough, the computer will cut off the fuel injectors and you really will continue to run on inertia alone. The time spent coasting more than makes up for the lower mpg you get while accelerating. But to really make this work you need a scan tool or something to tell you when your injectors are turning on and off so you will know the best speeds to start and stop. And the speed range that I have heard of is in the 45-65 mph range. It does not seem likely that you are going fast enough in town at 35 mph that your computer would cut off your injectors. I think it is much more likely that you are being fooled by an inaccurate computer estimate. Both the instantaneous and average mpg calculated by the computer use the same engine parameters, and they are probably using manifold pressure or rpms and not necessarily direct fuel metering. The only way to be sure is to compare the computer’s average mpg to a real mpg figure calculated by miles driven over gallons filled.

I would think that constantly engaging the variable flow turbo would cause more wear & tear, and subsequently a very expensive repair bill. Please don’t hypermile the RDX. Trade it in for a CRV if you’re really concerned with gas mileage.

What’s VDC?
And Who said I was proposing converting the car into a perpetual motion machine? I’m not that arrogant to assume it’s possible…yet. Also, I do not have judgements on Acuras or any other car manufacturers. If the option is available, why not use it to it’s full advantage? I’m learning here. That’s why I (hopefully politely) started the forum.

LOL… Man… I do try to be careful when writing factually on posts. I said in my original post “while maintaining speed wherever I am”. Let me clarify: I can feel the car reaching a peak velocity before beginning to slow down… that’s when I gently apply the throttle again. This allows a maintained speed, so the guy behind does not notice (or get annoyed). I feel that this technique is not easy to master. But if one can, I have noticed an increase in mpg. I too get annoyed when I notice the car in front of me changing speed.

VDC is the guy you replied to.

It will never be possible to convert a car or anything else into a perpetual motion machine without altering this universe’s laws of physics, btw.

Regarding your judgment on Acura, I didn’t say you did have one. But most Acura drivers, myself included, buy them because they consider them to be a damned good car that has a higher level of technological refinement than most other vehicles commonly available. Couple that with the fact that you are using your mpg indicator to judge your vehicle’s mpg, its therefore reasonable to assume that you consider your mpg indicator to be accurate. It is not. None of them are. The only reasonably accurate mpg indicator you can use is to fill your car and figure out how many gallons you used since the last fillup, and how many miles you went since the last fillup (and even this is not completely accurate due to vagaries of fuel expansion rate at differing temperatures and vagaries of pump shutoff sensors).

If you are pumping the throttle, as you said you are, then no matter how slowly you pump it, it will make you speed up and slow down, even if you don’t really notice that you are speeding up and slowing down. You said that yourself: “I can feel the car reaching a peak velocity before beginning to slow down…” which therefore means you are not holding a “maintained speed.”

Cars, especially modern cars, are optimized to use minimum fuel while cruising - meaning, maintaining a steady speed. Constantly changing the throttle setting means you never actually cruise, which means you never realize peak mileage.

To be rather blunt, though hopefully still polite, it seems that you came in here looking for validation of your theory, and you aren’t very open to comments which oppose it - which is why you have argued with everyone who has responded to you. To put it succinctly, and answer your question in a very direct manner, yes, it will harm your vehicle by causing premature wear on the turbo. And while you are harming your vehicle, you are not realizing the mpg gains you think you’re realizing because the technique does not work.

Chuckle… My previous car was a 2002 CR-V. I thought an upgrade was due. You are correct, the CR-V gets 20mpg compared to the RDX’s 17mpg, but the drive experience is night and day! The RDX drives like smooth velvet and accelerates 0-60 in 6seconds when needed. The CR-V is more of a workhorse car. But if I can experience both a refined ride and considerate gas mileage… why try not to?

Under normal turbo usage (non hypermiling), I would think the egagement is harder because it’s a burst of speed although less frequent than “gentle pumping.” I was hoping that an expert mechanic or a turbo design engineer would chime in.

Thanks all! This has been a nice forum!

Thanks, StrongDreamsWaitHere!
I concur that it’s easier to hypermile in the 45-65mph range. At slower speeds, I do notice the instant mpg gauge going up and down more quickly, thus the technique is harder to use. I have noticed an overall increase in total miles driven per full gallon of gas using this technique compared to not paying attention to using inertia.

Overall, whether or not my assumptions are confirmed, I’m kind of excited by this hypermiling technique because it shows real results. If I had this on my older cars, I defintely would have taken advantage of it then also. I’m just excited to see the technology being applied… even if there may be inaccuracies with the readings. I have a friend who bought a new 2010 MDX, but he did not have this intant mpg function even after we carefully searched the trip menu. So it makes me wonder why the RDX specifically came with it… and mine is a used one!

Thanks again, I appreciate yours and EVERYONE else’s comments! Peace! -ldpasion

DFCO, or Deceleration Fuel Cut Off, is a recent innovation. On older cars the coasting technique would not have worked even if they had instantaneous mpg read-out. I’m not even sure all cars have DFCO today. For more info google “pulse and glide”.

I think you should drive gently, watch the displays less and the road more.

Nice… you and Shadowfax should go bowling together, since you both enjoy creating negative banter. You have no priviledge in knowing how safely I drive to make such an idiotic advice. This forum is intended for pragmatic, intelligent discussion.

Thanks to everyone else who gave constructive comments! Peace! -ldpasion

i don’t know about in town but on the highway I get the best mileage just using the cruise control. So I will keep my thought to my self about all this hypermiling stuff. I believe in paying attention to the road, and not to whether or not I am accelerating or decelerating too quickly or slowly.

We only have what you tell us, which is that you’re pulsing the throttle as the vehicle slows down and speeds up. Those are your words, not ours. So in your own words, you told us you drive like an idiot. You have to expect when you come on to a forum populated by car geeks and driving enthusiasts, that some of us are going to take a dim view of the highway shenanigans you fool around with in the name of higher mileage.

You’re right. This forum is intended for pragmatic, intelligent discussion. When were you planning to start participating in that?

“This forum is intended for pragmatic, intelligent discussion. When were you planning to start participating in that?”

Most likely that will happen around the same time that he learns how to spell “privilege” correctly. I won’t even comment on the grammatical mistakes in his “intelligent” discussion.

I’m sorry!
The Devil made me say that.

Anyway–this appears to yet another case of someone who seeks validation of his…unique…theory. When he does not receive validation, he takes umbrage. We see this type of behavior time and time again, unfortunately.

Two comments.

Pulse & Glide is a well known technique. To do it right and to gain higher mileage is a lot of work for very little gain. Under some conditions it can result in reduced mileage. It may also be hard on your car and even a minor repair will wipe out most if not all the gain you might have accumulated.

The one thing I would worry about is the traffic around you. You can aggravate drivers around you and frankly I find that a poor idea.

One final note: On board mileage computers are seldom accurate enough to properly report the results of P&G. They are generally calibrated for the average driver.

How does a 2.3 Turbo I4 manage get worse mileage than my supercharged 4.6l V8. That must be an amazingly inefficient AWD system.