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
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
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.
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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