# Fuel Cost Math-Related Question

I drive a 2003 Subaru Forester 2.5 X. I may replace the vehicle this coming spring or summer. My vehicle came with only one engine option. Since math is not one of my best subjects, I hope someone on this forum would be able to help me with this math-related question regarding fuel cost over a 10 year period if the next vehicle that I purchase has two or more engine options such as a V4 or V6. My focus on a replacement vehicle would be fuel efficiency. I would like to get the most fuel efficient engine available for the vehicle that I select if that is an option, but I also would want that engine to have high marks for speed acceleration, being able to move the weight of the vehicle, and towing capability. If the most fuel efficient engine is deemed underpowered, such as a V4, and the vehicle has the option for a more powerful engine, such as a turbo V4 or a V6 ( which likely would burn more fuel), and they have better power ratings, I would consider those alternative engine options. To serve as an example of vehicles that have two engine options, I would like to mention the now-discontinued Lincoln LS. The 2003-2005 V6 model had a highway fuel mileage of 26 mpg while the V8 model had 25 mpg. To me, if the vehicle I select has a V6 or V8 engine option with only a 1 mpg highway difference between the two, they are both very capable engines, and the overall price difference between engine options is not too bad, I might be tempted to pick the V8. However, I wish to avoid jumping the gun. I am sure a 1 mpg difference in fuel burn could add up over the course of 10 years. To better prepare myself if I have a choice of engines, could some one do the math and give me a rough idea of how much more money I would spend on fuel with a V8 than a V6 using the following information below?:

Engines: V6(26 mpg highway)/V8(25 mpg highway)
Fuel tank: 20 Gallons
Highway miles driven per day: 26 miles
When to refill the fuel tank: 1/4 tank
Fuel Cost: \$2.00 per gallon ( I know prices can change over time, but this is just a sample static price)
Years of driving the vehicle: 10 years

Where are you going to get a V4 ? Seriously you are making this way to complicated . Just look at the combined mileage number on the sticker, test drive and decide from there. Also V8 engines are becoming a thing of the past in most vehicles.
And I doubt if anyone is going to do your math for you because you have too many variables.

2 Likes

Keep the math only as complex as is needed:

How many miles will you drive in 10 years?

How many gallons will be needed in car A? (miles driven divided by MPG)

Do the same for cars B, C, D, etc.

The size of the tank and its level when you refill is not relevant.

Compare gallons consumed by each car you are considering. Compare, as in subtract smaller number from larger, i. e. find the difference.

26 miles a day, 7 days a week = 9464 miles a year

Letâ€™s say you do 10,000 miles a year, 10 years would be 100,000 miles.

100,000 miles @ 26 MPG = 3,846 gallons x \$2.00 = \$7,692.00
100,000 miles @ 25 MPG = 4,000 gallons x \$2.00 = \$8,000.00
100,000 miles @ 20 MPG = 5,000 gallons x \$2.00 = \$10,000.00

Each mile a gallon difference would be about \$400.00 over 10 years or \$40.00 a year @ \$2.00 a gallon

Towing? Too many variables there. Weight, frontal area, IMHO vehicle should have a towing capacity 25% greater than what you intend to tow. No facts or sources, just me.

The math is easy, but there are two variables you left out. You mentioned acceleration as a factor. If, in driving the lower powered car, you find it unresponsive, youâ€™re going to be stepping harder on the accelerator, and driving your mpg down.

The second variable is your speed on the highway. The largest factor affecting gas mileage on the highway is wind resistance, which increases logarithmically with speed. Iâ€™ll go prepare a spreadsheet now. Back in a bit.

What you expect to pay per gallon of gas divided by what the expected miles per gallon will be. This yields the cost per mile which should be somewhere around 7-10 cents a mile. Cost per mile times the number of miles you drive a year will give you the cost of gas for the year. Plug any numbers in you think are reasonable expectations but remember the cost of gas can go up or down, and the actual mileage you get may not be the same as the government estimates.

Thatâ€™s why for me and my 50,000 miles a year, gas cost was only part of the math.

There is no answer to your question because no one knows what the cost of gasoline will be 5 years from now. Until you can definitively answer how much a gallon of gas will cost at every point in the future your question is meaningless.

If you want fuel efficiency, buy the most fuel efficient vehicle you like.

1 Like

You just threw the math out the window with this one.

Cars are rated for a given mpg based on gentle, fuel-efficient driving. Youâ€™re not going to see those numbers if you floor it all the time. So, given that youâ€™re interested in speed/acceleration, we now have no idea how much of a lead foot youâ€™ll display on a daily basis and therefore no idea how much more gas you, personally, will use in a car.

The towing also comes into it. Just once a year, or do you haul stuff all the time?

Also, donâ€™t assume an engine is more powerful just because it has more cylinders. Itâ€™s not unusual to find V6s with more power than V8â€™s. Horsepower is not a linear relationship to the number of cylinders in an engine. After all, in the '30â€™s Cadillac had a 16-cylinder engine in a car that did 0-60 in 24 seconds and would therefore be vastly outrun by an 80â€™s 4-cylinder minivan.

Skip the hard math. Donâ€™t overthink this. www.FuelEconomy.Gov will show you the comparison of annual fuel costs on vehicles back to back. Here is an example. Start with the tab that says â€śFind a vehicleâ€ť in the upper right drop-down menus. The rest is all there for you. Read the fine print for details at the site.

1 Like

Fuel efficient + high marks for acceleration is met nowadays mainly with small light cars that have 4 cylinder engines with turbochargers. Many such on the market. That 4-and-turbo is showing up on mid size and even bigger cars, too, at some MPG falloff.

Some of us have different definitions of â€śhigh marksâ€ť when it comes to acceleration.

2 Likes

For sure. The tension between acceleration and fuel efficiency could keep us and more than a few engineers going at it forever! The small volume of a 4 keeps fuel consumption down during most driving; the turbo kicks in when desired for a boost in acceleration. But you and I know that already.