Three vehicles...
Switching from a 25-mpg sedan to a 50-mpg hybrid produces only a modest savings of 2 gallons of gas on a 100-mile trip. Switching from a large SUV to a sedan can produce a much greater fuel savings. | March 18, 2010 | Mark Takahashi

What you are about to read may change your perception of fuel economy. And if you take the time to wrap your mind around it, you will see a much clearer picture of fuel consumption and gas costs. This means the next time you're in the market for a new car, you'll better understand how to choose one that suits your needs.

We're about to introduce you to a new system of measuring fuel economy, one our European brethren have used for years to help them keep a clearer picture of fuel costs.

But first, here's why you need to know this.

The Problem

The current system of measuring a vehicle's efficiency by mpg creates an inaccurate perception about fuel consumption. Because as the mpg rating of a vehicle rises, it appears that it does so proportionately. What this means is if you were to trade in a car getting 12.5 mpg for one getting 25 mpg, it would obviously deliver fuel economy that was twice as good. But if you later traded that car getting 25 mpg for one getting 50 mpg, that exchange would not save you twice as much.

Here's why: Imagine three cars lined up side by side. They all drive 100 miles. Since each car has a different fuel economy rating, they each burn different amounts of gas to travel that same distance.

12.5 mpg = 8 gallons per 100 miles 25 mpg = 4 gallons per 100 miles50 mpg = 2 gallons per 100 miles

Now look at how much gas is saved in each case. Over a 100-mile drive, the 25-mpg car saves 4 gallons over the 12.5-mpg car. But over the same distance, the 50-mpg car saves only 2 gallons over the 25-mpg car. Taken to the extreme, if there was such a thing as a commercially available 100-mpg car, it would only save 1 gallon of gas over the 50-mpg car.

What Does This Mean to You?

Once you understand the concept, it reveals two key points:

• Trading in a gas guzzler for a midsize sedan provides substantial savings.
• Trading in a midsize sedan for a high-mileage hybrid would hardly be worth the expense in fuel (especially when the taxes and related fees are considered).

The new system also reveals three related concepts that are largely unrecognized:

• Fuel-economy improvements from 12-20 mpg are significant.
• Once vehicles get over 25 mpg, the gains are much less dramatic.
• While extremely high-mileage hybrids provide eco bragging rights, they don't save much more gas than midsize sedans.

While it continues to be important to note the mpg rating of a vehicle, it's also worth considering a car's gpm rating. Most vehicles will rate somewhere between 10 and 2 gpm. A gas guzzler netting only 10 mpg would consume 10 gallons per 100 miles, while a far thriftier 50-mpg vehicle would consume only 2 gallons. Clearly, the smaller the gpm rating of the vehicle, the better its fuel-efficiency.

Calculating Fuel Costs

Once you get the hang of it, you can estimate fuel costs in your head. Here's how.

Adding a zero to a vehicle's gpm rating would show the estimated amount of fuel consumed in a month. So if an SUV consumes 8 gpm (8 gallons to drive 100 miles) it will require 80 gallons to drive 1,000 miles — what many people drive in a month. Then it's easy to see that driving 10,000 miles (nearly a year of driving) would require 800 gallons.

Estimating fuel costs would then be a matter of multiplying this number by the price of gas. At \$3 a gallon, the yearly expense of driving this 8-gpm SUV would be about \$2,400. Comparing a very fuel-efficient car rated at 3 gpm, you can see it would consume only 300 gallons a year and the annual bill would be about \$900.