Friday, February 15, 2013

Is Premium Gas Better Than Regular?

So you pull up to a gas station. And we all know how expensive the gasoline is these days. You get presented with choices: regular gas, premium and (most of the time) midrange. The price difference is several dollars per tank. But in the percentage terms it is very low: 2-5%.

The question is do you need to shell out slightly more money for premium gas? Will premium gas make you car run better, prolong its life or even decrease fuel consumption to the point of paying for itself?


There are two distinct opinions on this matter. One is - your manufacturer, based on the engine's compression ratio, chose a gas grade for you (it is clearly printed in your car's manual) and if you use higher-grade gas, you just waste your money. In addition, some say that gas station's owners sometimes mix gas of different grades (to earn money or for convenience).

Another opinion - the higher-grade gas has more value - more cleaning additives, less engine wear and lower operating temperature, higher performance.

The truth is probably somewhere in between. First of all, the simple rule "the higher compression ratio, the higher-octane gas should be used" no longer applies. There are cars with 9.8:1 compression ratio, that require “premium”, and there are even 13:1 engines that run fine on “regular” – a lot depends on ignition timing, presence/absence of turbo- /supercharging, direct injection vs. port injection and bore/stroke ratio.

For newer cars, there are two types of electronic injection control:

1. Electronic engine control system uses preprogrammed "air-fuel mixture maps" based on engine revs, throttle position and other factors. In this case your choice of gas does not matter much (note - NEVER use gas of grade lower than minimum recommended in your manual!).

2. If knock/pinging occurs, knock sensor commands the engine control module to retard ignition. This causes lower performance and higher fuel consumption. It happens when your overload your engine (mountain driving, towing, etc.) or use lower than needed grade of gas.

Therefore, my opinion is:

1. If you drive an expensive car, the manual of which requires or recommends premium gas, it doesn't make any sense to try to save on gas - get the best grade available.
2. If you drive high performance car (and actually rev it up to redline), especially with low per-cylinder displacement, see #1. Engines with low per-cylinder displacement use generally higher compression ratios and rev high easier.
3. Again, never use gas of grade lower than minimum recommended in your manual!
4. If you you feel excessive engine vibrations/knocking/pinging, try higher-octane grade. It may also save you money. Knock sensor retards ignition when knocking occurs, remember? Also your engine will live longer and provide a bit more power.
5. Concerned about financial part most? Perform several tests with different types of gas.

Fill the tank with regular gas (notice odometer reading), drive the car until the tank is almost empty, fill it up again (take note of odometer reading again, and number of gallons). Your consumption in MPG (miles per gallon) on regular will be (New Odometer-Old Odometer)/Gallons. Now divide it by a price per gallon – the resulting number represents your “money consumption” in miles per dollar. Do the same with midrange and/or premium.

Maybe the premium is the right choice for you?

I suspect that the “minimum required” octane rating, specified in your manual, will be the best choice – at least money wise. I performed the test on my 2000 Mitsubishi Galant 4-cylinder and found no improvement in fuel economy if I switched from regular (87) to Premium (91). In fact, the opposite happened - the fuel economy worsened from 24.5 to 22.5.

Although this is not the indication, that 87 gives me better mileage (maybe I drove a little bit more aggressively this time, reassured by the "premium gas :-) ), it is a clear sign that there is no advantage in using premium, when the manual recommends "regular" 87 gasoline.

Later, I did some tests with my 2004 Infiniti G35. Its manual "recommends" premium gas, 91-octane or higher, for maximum performance. But it also permits operation on regular (87 octane) gas. I used to fill up with the premium gas all the time, but later switched to regular. The result? Same mpg, but a little bit less power and detonation in lower rpm/high load situations.

Bottom Line

Read your manual and do the following:
1. If your objective is to save money, use the lowest grade allowed in the manual.
2. If you need the very last horsepower or if you do a test and see that it is actually cheaper to fill up with premium, use premium gas.

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