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2002 Saturn SL1 getting great mileage--but why?

I recall a Car Talk episode in which a guy called in to say that he was getting amazing mileage from his car. He mentioned that he was also getting an error code from part of the intake/exhaust system that had to do with preventing pollution. Well, deja vu! A few weeks ago I got an error code on my 2002 Saturn SL-1 that Pep Boys decoded as the secondary air intake something or other. Shortly after that, it went away. Not long after that I started keeping track of my mileage, and I find that it’s regularly 40 to 46mpg. Granted, ALL of my miles are freeway (I only use the car for trips to visit my folks). Should I be getting this kind of mileage? Is it possible there’s something going on with the secondary air intake something or other to cause this?

Rest assured that no malfunction is causing you to get good gas mileage.

I doubt your great gas mileage has anything to do with a problem with the emissions control system. I will note that it is not unusual to get that kind of gas mileage out of a small car that is kept strictly on the highway, especially if you are obeying posted speed limits. I have driven cross country in a couple different vehicles and can tell you I got great gas mileage out of the cars I was driving, because it was all Interstate. On one 2000 mile trip, I was driving a '06 Grand Caravan with a 3.3L V6 and got 28mpg, driving 75-80mph. It would have done better if I were driving slower. Same trip, same speed, in a '95 Grand Am GT with a 3.1L V6, I got 36mpg. Strictly freeway makes a big difference. Those vehicles, in mixed driving, typically got 21 and 27mpg, respectively.

My question arose because in the episode I recall Tom and Ray telling someone, in essence, that if certain parts of the anti-pollution system in a modern car stop working as they should, then it can result in better gas mileage, although at the cost of putting more pollutants into the air. In other words, we sacrifice MPG for the sake of making our cars less polluting. So while normally a malfunction would be bad for mileage and everything else, in a case like this, it might be causing better MPG due to not choking something off, if I may put it that way.

Altering or removing emission controls on modern vehicles will only decrease efficiency, in terms of both MPG and pollution. Modern, 96’ and newer, vehicles are precision computerized machines that have been engineered to perform at their maximum efficiency when all components, mechanical and electronic, are functioning properly. I’m glad that your Saturn is doing so well.

Thank you to all who responded. Since the great mileage appears to be because of the type of driving I’m doing (all freeway) I will rejoice in the benefits, and if the check engine light ever comes on again for the secondary air intake something-or-other, I’ll have it looked at as a separate problem.

Cars are engineered to deliver the best performance and MPG within the constraints of only using a stochiometric air/fuel ratio (14.7:1) and emitting exhaust gases within EPA limits.

There was always improved mpg and power available when running with a slightly richer mixture. Cars were routinely built running with 10% more fuel up through the mid-70s, when the EPA halted that practice. With today’s improved combustion chamber mixing, I would guess that 10% number has dropped considerably, but I don’t believe it’s 0%.

Back when I had my Saturn SL-2, A friend of mine had an SL-1, and it had pretty long legs. I think it ran around 2500 rpm at 70 MPH. Wouldn’t surprise me if it got at least 40 MPG on the highway.

Emissions is reduced in today’s cars using the available technology to burn the available fuel as completely as absolutely possible. That reduces carbon monoxide and unburned hydrocarbons to almost a nonexistant state in the exhaust. It also means that you get the absolute maximum benefit of the potential energy stored in the fuel…which means you get better mileage. Anything that causes any of these technological wonder systems to not operate effectively cannot improve gas mileage…because it must also adversely affect the ability of the engine to burn the gas as completely as possible, meaning you lose more of the gas’ energy out the tailpipe than you should.

Said another way, reduction of emissions also improves gas mileage. Both are products of complete combustion of the gasoline. And it is the modern technology used to reduce emissions that enables both to happen.

But I’m happy for the guy in the CarTalk episode and for you too.

Although it’s been years since I studied this and it was on much older technology, I still believe there’s opportunity for better MPG with a slightly richer mixture. See my reply above.

It used to be, prior to today’s computer controlled engines or the 1975 mandate to use a stochoimetric ratio, that cars were routinely run with 10% more fuel. That gave better MPG and performance. Anything richer than that would cause MPG to begin falling. Note that power would keep increasing up to about 16% richer. Beyond 16% richer, power too would begin falling off.

The reasoning behind the above behavior was that oxygen was the precious commodity in the combustion chamber with the short amount of time to do the mixing of the mixture. The extra fuel increased the chances that every molecule of oxygen would have an adjacent fuel molecule next to it.

Today’s engines have greatly improved combustion chamber designs for improved swirl and more complete combustion. I have to believe the combustion hasn’t reached complete in today’s engines and possibly never will. They are constrained by using a mandated stochiometric A/F ratio and exhaust emissions.

I welcome any new information on how today’s engines may have changed the reasoning I stated above.