# mathematical formula

Could use some help regarding determining efficiencies of fuel mileage. For example, a car gets 25 miles per gallon (MPG) city and 35 on the highway at \$2.00 per gallon for regular 87 octane gas v. a car that gets 30 in the city, 40 on highway for \$2.40 for high octane (93) gas. It probably will break down to a cost per mile. Then if I was to calculate this v. 50 mpg at \$2.50 for diesel…Am thinking to purchase, possibly a turbo gas or diesel… thanks

where
Where do you live? In the northern US diesel can be at a disadvantage since it can gel when cold. Also availability is not as easy since many stations do not handle it.

That aside, diesels are great engines.

Other considerations

– Last Updated: Jan-28-15 9:55 AM EST –

I can't imagine buying a vehicle based solely on fuel economy. Comparing the cost per mile to operate each vehicle is simple math, but is that really all that matters?

Vehicle A at \$2.00 per gal. costs 8 cents per mile city and about 5.7 cents per mile highway.

Vehicle B at \$2.40 per gal. costs 8 cents per mile city and 6 cents per mile highway.

Vehicle C (the diesel) at \$2.50 per gal. costs 5 cents per mile.

Other cost considerations

– Last Updated: Jan-29-15 3:00 PM EST –

I just posted this on similar topic that cropped up within another discussion. You don't only have to consider the cost of driving, but the cost of the whole car, spread out over some defined number of miles. That includes the extra cost of a diesel engine compared to a gas engine. Just as the extra cost of diesel fuel cancels-out much of the benefit you get from the diesel's better mileage, the extra cost of the diesel engine does the same. Therefore, you have to drive quite a distance before the cost benefit of better mileage kicks in. I've heard it can be 100,000 miles or more before the savings in fuel cost per mile catch up with the higher purchase price of a diesel engine in the case of pickup trucks.

How do you figure this out? I'd have to sit down scratching my head for a while to do so for a particular vehicle and defined driving distance, and I'd be scratching my head for a lot longer to come up with a formula that can be applied to various cars. I think it's probably too complex to worry about if calculating fuel cost per mile is already difficult for you. You might get lucky and discover that someone out there has a webpage made for this purpose!

If saving money is the main issue, simply buying used is sure to save far more money than you are ever likely to save just by choosing a car that gets better mileage.

Diesel and Hybrids

– Last Updated: Jan-29-15 11:27 PM EST –

Diesels also have some periodic maintenance, as far as I understand, related to the sooth and other stuff in diesel they need to clean from their exhausts, which is not needed for gasoline engines. Make sure to add that cost too. Also, in the city, the difference will be much smaller than on the highway.

With hybrids, they are very dependent on how you drive them. I can get 60mpg or 35mpg on my daily commute, depending on how heavy my right foot is (I have a Honda Insight, my wife has a Prius - both behave similarly).

I did the same calculation when I bought the Insight. Only, I was comparing it with a Prius (both new). The insight was pretty much the cheapest Honda on the lot worth considering (save for some stripped-down Fits). At \$18.2K out the door it seemed like a good deal for a daily commuter and boat hauler for me (just don't drive it up long steep hills - no power). The Prius was \$5-7K more out the door and promised 10mpg extra on the highway and in the city. My spreadsheet came-up with something like 80K-100K miles to break even on fuel savings alone. But, the Insight turned out depreciates much faster compared to a Prius, so if I were to sell after only a few years, the more expensive Prius would have been the better deal (not to mention, "more car"). On the other hand, if I keep the cars long enough, so the selling price will be next to nothing (sold my last Camry for \$3K, bought new 12+ years earlier), the fuel savings would be what would add up nicely. My first Prius I bought well-used for \$6K, sold it after 70K miles and 3+ years for \$4K (gas was \$4 a gallon then) - it was exactly 2x more economical than our Camry at the time, so it pretty much paid for itself in fuel cost savings alone! As said above - buying used is the best bet for saving money, but that is not for everyone (buying used from a car dealer usually does not save you as much, buying used from a private seller could be risky).

I have VW Sportwagon TDI diesel
I’ve had it for two years with 22,000 miles, I love it, great gas mileage without sacrificing power. It is a 6 speed automatic and gets about 40-42 mpg highway (around 38 mpg with racks and 2 kayaks on top) and 28 mpg city. Diesel fuel is more expensive compared to regular gas in the winter because its price is more tied to heating oil than gasoline.

I can carry two 16’ plastic kayaks or two 21’ surf skis.

Negatives - a little small and not great for Northeast winters.

I used to visit the gas station every week with my small SUV but now only have to fill up every 2 or 3 weeks.

try a formula calculator

– Last Updated: Jan-30-15 7:13 AM EST –

such as Edmunds True Cost to Own, for a start:

http://www.edmunds.com/tco.html

There are others, google "cost to own vehicle".

It's not an exact science, especially with new models, as one doesn't know what repairs to anticipate. This is where guideboatguy makes a really good point, and I also agree with magooch.

The mileage calculation, I'd just keep it simple and identify the annual average cost per gallon of each of the two fuel types as one factor.

Check out how much oil is needed
during a change. My 5.9 needs 12 quarts. It also needs two batteries. Diesel isn’t always easy to find – at least in smaller towns and can cost much more than premium gas. A big bummer is that some gas stations there might be only one or two diesel pumps or they are located by the commercial trucks. I sometimes have to loop around the station looking for a green handle.

Sounds like a truck

– Last Updated: Jan-30-15 2:58 PM EST –

Based on the MPG figures, I'd guess that the OP is considering a small car, not a truck. It will probably have an oil capacity of 5 quarts or less. In any case, even if the diesel version requires a few extra quarts, is a few more dollars every several-thousand miles enough to sway anyone's decision about what car to buy? That little cost will be eclipsed by several others (drop-in-the-bucket reasoning).

The whole reason for higher oil capacity is to allow longer oil-change intervals, which of course isn't necessary for cars or personal trucks. That's why even your truck engine doesn't hold much oil as diesels go. A medium-duty truck with an engine of similar displacement as yours would likely hold 30 quarts or more.

It’s a Dodge Ram. One other thing
though, which is important, you might be limited to a dealership for service since many smaller shops don’t work on diesel engines.

30 quarts?
Talk about a major oil change.

Another consideration …
a higher octane recommended fuel may not be necessary. I have two cars that call for 93 octane. They both run fine on 87, one since 2008 and the other since 2011. The computers in modern cars will adjust automatically for the different octane. The salesman actually told us that the lower octane was fine to use. It is true that the mileage might be slightly better if we used higher octane in the cars, but we are talking about maybe 1 mpg better which is essentially a wash with the higher fuel cost.

thanks for the input
Many of the 4-bangers are getting ±38mpg and my Toyota SUV gets 27-30mpg average with a 6. Was contemplating a pickup and would like the pulling power of a diesel’s torque and better mileage…but now must consider the extra cost factors involved that have been mentioned in response to my post. I currently do most of the usual maintenance stuff myself, but if additional costs for materials (extra qts. of oil and more frequent changes) probably off-set the savings. If I get a car, some turbo versions get better mileage than normally aspirated versions, until one puts the “pedal-to-the-metal”. Have been calculating the differences in cost per mile by : the cost of a gallon of gas divided by the average of miles per gallon to get the cost per mile. I live in central west Florida and usually put close to 200,000 miles on my vehicles. Thanks again.