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The Gas Experiment
In April of this year we took ownership of a new (new to us, it is a 2002) car. The old (a 1996 model, different brand) car had a 1.9L 4 cylinder engine and was manual and the suggested octane level was 87 or regular. The new car has a 2.4L 5 cylinder engine with turbo and is automatic with a suggested octane level of 91, generally the highest level of octane. I know the previous (and only) owner of the new (to us) car and he always just put regular or 87 octane in the car and never had any problems with it (i.e. no engine knocking). So that is what we have been putting in the car since we took ownership.
Over the summer I tried to do a little bit of looking into what the different octane levels mean and if I really should be looking at a higher octane level. I wanted to know if there would be any benefit to using higher octane. There are claims on various discussion boards that using higher octane for engines that it is recommended for actually "improves performance". Ok, whatever. If that means for the extra 5-10 cents a litre ($3-7 bucks a tank) I get to shave 0.5 seconds off my 0-100km/h time I don't really need that. If however that means that I actually get better mileage with higher octane and my overall gas cost per kilometre driven is pretty close to the same, then I am willing to spend the extra $$ at the pump. A cleaner, longer lasting, smoother running engine might also be a side-benefit but my main motivation will be fuel cost per kilometre. If the cost is within a margin of error (say 5% now but I reserve the right to change that on a whim) then I will switch to the higher octane.
So in order to test this out I need to keep track of the gas that I put in, the kilometres that I have driven and the octane level I have been using. To try to reduce the level of noise in the results I have decided to run 3 tanks of each octane level (roughly 87, 89 and 91) and take the average of those tanks to give me the average mileage (litres per 100 kilometres) and average cost per kilometre.
How do I determine how many litres I have used to drive the distance I have driven? It isn't an exact science since I don't run the tank until the car stalls and since I don't put exactly the same amount of gas in the car every time I fill up. The best I can do is guess. So I try to fill the tank to close to the same level every time. I started this experiment by filling up the tank and resetting my trip odometer. The next time I filled up I wrote down the trip odometer reading and I wrote down how many litres I put in the tank. The mileage for that tank would then be litres in/(kilometres driven/100) to give litres per 100 kilometres.
The Results So Far
I have completed 3 tanks at the 87 octane level. Here are the results from that round of fuel consumption:
Tank 1: 64.504 Litres and 688.1 km driven for a 9.37L/100km average
Tank 2: 64.016 Litres and 598.3 km driven for a 10.70L/100km average
Tank 3: 60.004 Litres and 626.3 km driven for a 9.57L/100km average
Total for the 87 octane level: 188.524 Litres for 1912.7 km driven for an overall average of 9.85L/100km.
Tank 1 of 89 octane is being consumed now. Stay tuned for updates.