Break even point in buying gas



I shop at King Soopers in NoCO (N.Colorado) for the fuel points which can deduct 10 cents to 1$/gal. Hypothetically: gas @ KingS cost $3.00/gal w/ a 30cent/gal fuel pt deduction. I was on my way to a mtg in Wyoming and decided to wait to fuel up. Gas in WY was $3.3o/gal @ a non-discount station. So I filled up there saving my pts for later in the month @ KingS. I noticed an increase of 5mpg with the WY gas fill up.
How can I calculate whether I should buy gas in WY at the higher price, but increased mpg or at KingS? This is rainy day math and hypothetical, so driving conditions, wind (WY has some terrific tail winds), etc. are not part of the answer, OK? TIA



Your increase in 5 mpg is not due to the gas, but probably (1) your driving style or (2) a miscalculation or (3) the pump in WY shutting off too early.

If all things were equal, simple high school algebra would solve the problem.

Just gas up with the cheapest gas you can find on the way, and you’ll come out ahead financially.


I’ll bet you got a load of much better gas. Probably with much less, or no ethenol.

Also consider that the super cheap gas is that way for a reason !
now you know.


I vote for the Wyoming tailwind…


The best way to measure this is cost per mile over several tanks of gas. Many years ago I was on the road a lot and ran stats on each tank of gas. There was a lot of variability tank to tank even from the same station, so I got averages over several tanks. Weather, traffic, driving style have a big impact on the mpg, sometimes more than the brand. One thing that did come out was Gulf premium, while more expensive, gave consistently better mpg and the cost per mile was less. This was before Chevron (I think) bought Gulf, no telling what the gas is like now.


when I drove my oldtruck with a 390 on the highway a lot, I often would buy high test gas. when it was within 20 cents a gallon of regular I calculated that I came out ahead as I got much better mileage with the high test. at that time I was driving to MA from DE a lot and had opportunity to get accurate mileage info


You’re paying 10% more for gas in WY, but 5 mpg is (probably) greater than 10% better mpgs, so WY gas costs less per mile driven.


If the mileage can be repeated tank after tank you can consider that the fuel has some quality that others don’t but that’s very unlikely. The slope of the lot at the gas pumps can make a big difference in the amount of fuel that can be pumped into the tank. And as stated there can be a great difference in the shut off mechanism in the dispensers.


I tend to agree with Caddyman about the wind and especially so since you make mention of a strong tailwind.

Some years back I attended a Subaru service school in San Antonio, TX and drove a new demonstrator down there with almost the entire distance being a north/south interstate.
The demo got about 30 MPG going down there on Sunday with little wind.
Coming back on Friday the headwind was howling out of the north. Fuel mileage on the return trip was stuck right around 21 MPG.


So, are the two towns at the same altitude?
In truth there are way too many variables to make any sense of your difference in your calculations, including the sensitivity of the automatic shutoffs of the different pump nozzles. And if the Colorado town at one end is 5,000 ft above sea level and the town at the other end is 100 ft above sea level, you were going net uphill in one direction and downhill in the other.

But, hey, it’s always fun to try to puzzle these things out.


they are in two different states and the ethanol content may be quite different


I’d be curious to know if you are buying the WY gas, and then driving around in WY and checking your mileage, or heading right back to CO? Are you doing all of this along the front range, or driving to Douglas or Casper on the WY gas, and then back to Sterling or even Wray or Burliington? The reason I ask is, your car’s computer will make the fuel/air mixture leaner the higher you go. Hypothetically speaking, you might get 35 MPG at 7100’ Above Sea Level near Laramie, and only 30 MPG in Denver at 5280 ASL, or 28 MPG at Burlington which is about 4100 ASL.

The other factor is the uphill nature of your trips north, and downhill nature of your trips home.


I did enjoy reading all the comments buTT did I say, “This is rainy day math and hypothetical, so driving conditions, wind (WY has some terrific tail winds), etc. are not part of the answer, OK? TIA”?
@ DocNick I am interested in the algebra to solve the hypothetical problem. Would you be so kind & send me the algebraic equation(s) ?? TIA


^ You need the increased MPG (as a %) to exceed the increased sale price (as a %). It’s that simple.

If you forgo $3.50 gas for $3.80 gas, the expensive gas has to increase MPG from 35 to 38 to break even.


Trouble is, you’d be basing the equation on the assumption that you’re getting an additional 5mpg with the WY gas, and there are too many unknown variables involved.

if you really want to know if it’s worth the drive, you’ll need to calculate with at least three tankfuls what your MPG is with the WY gas and do the same measurement with the CO gas. To do this, you’ll first need to purge the tank of CO gas (run to almost E, which I don’t usually recommend), and then do the same for the WY gas.

Once you have the actual difference in MPG (and I doubt if there will be one), then the variables become the difference (MPG1-MPG2) multiplied by the miles necessary (Mn) to make the trip to get the gas, multiplied by the difference in the price (P1-P2).

(MPG1-MPG2)*Mn *(P1-P2)
MPG1 is CO gas
MPG2 is WY gas
Mn is the trip distance (miles to CO station less miles to WY station)
P1 is the gas price in CO
and P2 is the price in WY

But before you do anything you need to determine the actual differences in mileage, and doing the test runs back & forth is the only way to eliminate
a) the difference in altitude (which causes an uphill vs. downhill difference as well as affects your engine’s operation as the ECU compensates for changes in altitude)
b) the difference in road conditions
c) that wind effect you mentioned
d) variations in the gas pump automatic shutoffs.
Oh, and you’ll need to do all the test runs in comparable weather.

If you want to calculate everything without test runs, start by looking up how much energy it takes to raise the weight of your car the difference between the altitude of the CO town and the altitude of the WY town. The look up how much energy is in a gallon of gas, look up the efficiency of your engine (how much of the fuel energy is actually converted to torque (it’ll be around 15-20%), and the fuel energy multiplied by the efficiency figure times the energy necessary to raise the car the difference between the two towns will get you started.

If you get that far, post and we’ll go over the effects of altitude changes on fuel metering, and its effects on efficiency.

Note: this is the simplest I can get it. Sorry.


the same mountainbike your comment about finding the break even point for gasoline almost made me curl up in the corner and cry. It reminded me of my first class in hexadecimals. It’s easy now but you have to put your mind into a whole new direction to learn it. I’ll be OK after I eat breakfast…hopefully.


Who dumbed down the amount of comments on this forum,seemed like it used to be a considerable fraction of what the repair section used to be,now it seems like now,its configured like people seldom post here,what gives?


LOL, missileman, I’m not sure if I should apologize or just crawl into a corner and hide!
I wrote the post at 1:03AM, so can I plead insanity?