I’ve seen comments about an inverse relationship between gas mileage and gas prices – as the price goes up, folks comment that mileage is dropping. It is political – someone pushed the idea that alcohol was a renewable resource, Congress voted, and now nearly all of our regular gasoline has ethanol added.
Like most things in life, mathematics explains what’s going on, how to figure out how much ethanol is in your gas, how far gasoline will put you down the road, and what ethanol will do to your gas mileage. You don’t need to fasten your seatbelt – the research is complete and available online. So, let’s look at the two substances. My little chart is in gallons – though we could change to liters as we cross the 49th parallel, and I’m just pencil-whipping regular. (everything is standardized at 68 degrees Fahrenheit.)
Gasoline (regular) 6.1 lbs 114,100 BTUs
Ethanol (E100) 6.58 lbs 76,100 BTUs
Methanol (M100) 6.60 lbs 56,800 BTUs
Obviously, with a quality scale, I can weigh a gallon and figure out how much is gas and how much is alcohol. It’s going to be harder to figure out whether the alcohol is ethanol or methanol – the weight of each is fairly similar. It gets a bit more complex when we notice that different grades of gas can vary from 5.9 lbs per gallon to a high of 6.5 lbs per gallon. We’ll use the normal 6.1 for our calculations.
Still, we can figure out that 9 gallons of gas will weigh 61 lbs, and when we add a gallon of ethanol, the total weight will be 67.58 lbs . . . or 6.758 lbs per gallon of E10 blend. Still, in order to keep gas prices down, our President has ordered that the 10% alcohol can be raised to 15%.
Now here are the critical calculations:
9 gallons of gasoline provide 1,026,900 BTUs. A gallon of ethanol provides 76,100 . . . so a gallon that is 10% ethanol yields 110,300 BTUs – which is 3.33% less energy than the 114,100 BTUs that a gallon of pure gas produces. We’d expect that reduced energy to yield an equivalent drop in gas mileage.
If we go to 15% ethanol – which Joe Biden has approved (to end the high prices I guess) the math works easily – 8.5 gallons of gasoline provide 969,850 BTUs, 1.5 gallons of ethanol provide 114,150, for a total of 1,084,000 BTUs. Divide that by 10, and we’re looking at 108,400 BTUs – a full 5% drop in energy potential, and probably a similar result on gas mileage.
You can work the values of ethanol blend gasoline prices at the pump when you know what the blend is – my old Talon likes premium . . . $5.52 today and 93 octane. Octane measures how stable the fuel is as it burns – I pay more, but I don’t really get any BTU increase. But the ethanol blend just doesn’t let the high compression engine run well. E15 fits in at 88 octane, so I won’t be using it if I can help it.