Is it cheaper to drive a natural gas vehicle or a gasoline hybrid???

Which is the better deal? Driving a car running on natural gas or gasoline? I have heard that it's a lot cheaper to drive a natural gas vehicle. They run on Compressed Natural Gas (CNG) T.Boone and others say it's cheaper. But I need more. I need to see the numbers. So I set out to find the answer myself. It should be an easy task. I have a mathematics degree, some free time, and access to the internet. I can do this.

Step 1
Decide what to compare: I decided to use a natural gas vehicle that is now in production. The Honda Civic GX, and the most popular fuel efficient gasoline car, the prius. Now that was EASY.

Step 2
I need to find out which one has better gas milage. So I go to the Honda website and get the numbers and BAMMM! I’m half way there. Well not so easy. According to the Honda website, “The GX engine achieves 24 mpg GGE city and 36 mpg GGE highway/ 28 GGE combined (Gasoline Gallon Equivalent).* ” My first thought was, “Yes! That was easy too!” But wait, what the heck is GGE?? Oh I see it’s the Gasoline Gallon Equivalent. I guess someone created this equivalent “thing” so that they could still give you an “MPG”. DANG :-( I just realized that I need a step 2A. I need to find a way to compare Natural Gas to gasoline. That means I'll need a conversion factor for GGE to BTU. Does it exist???

OK so after step 2 I have:

Step 2A: To find a conversion factor. According to my research, 1 GGE of natural gas equals 114,100 BTUs. And to think I was worried about step 2A. That was easy. Wait ... DANG! DOUBLE DANG! I just realized something. Before I can go onto step 3, I need to make another decision. I have to decide if I will convert Honda’s mpg GGE to mpBTU (miles per BTU of natural gas) then use the price of natural gas, which is traditionally charged by BTUs, and then figure out $ per mile for both the GX and the Prius! This is a lot of converting. Hey, I learned about this in Algebra. Proportional reasoning! I move on...BUT WAIT! In my search for the cost of Natural Gas prices, I realize that the stations that pump natural gas for NGVs(Natural Gas Vehicles), charge by the GGE. SSSSSSWWWWWEEEEEEEET! They did the conversions for me. I averaged the cost of a GGE of natural gas in San Diego using the website and came up with $2.67. For the math nerds like me, the median price was, $2.59.

ON TO STEP 3!!!!!!!

Step 3:
Find a fuel $ per mile cost for both vehicles and compare. I think I'm close :-) Prius: 46 miles per gallon combined (according to Toyota website.) According to the Energy Information Administration EIA the average cost of a gallon of gas is $3.955. So you can expect to go 46 miles per $3.955. Using 6th grade mathematics that gives us a unit rate of 11.63 miles per American Dollar. 11.63 MPD Whew. One down and one to go! Honda Civic GX: 28 mpg GGE combined as we already determined. And the cost of CNG averaged $2.67 GGE in San Diego. I use some straight forward Algebra skills and... we get a unit rate of 10.48 miles per American dollar. 10.48 MPD

AND THE WINNER IS ... The Prius, by a bumper.
Prius: 11.63 MPD Miles Per Dollar
Civic GX 10.48 MPD Miles Per Dollar

But wait!!! One last thing. Here's your disclaimer: There is actually a lot more to this comparison. If you purchase a CNG vehicle and fuel up at home, I believe there are some great tax benefits that you can take advantage of. There are other dynamics as well. If America switches to Natural gas powered vehicles, that will drastically increase demand which would mean much higher prices for natural gas. But that’s a topic someone else can tackle. I have to go pick up my daughter from school.


Charles R.

Credentialled Math Teacher

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