The dirty little secret about MPG

Edmunds.com:

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 miles
50 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.

Here's a calculator if you're interested.

UPDATE: Honda trades mileage for "utility, affordability" and "fun" in the new Insight. Well sure. My '06 CIVIC still gets almost 38 on the highway and low 30's in town. Got it for about seven thousand less than a hybrid version, and it's a blast to drive. Environmentally-unfriendly batteries aren't required. Low emissions too.

Of course, if you really want to get a lot of mileage out of your Honda...

UPDATE: Cash for Clunkers could turn out to be one of the Obama Admin's most efficient ways to cut CO2 emissions and reduce fuel use if the goal is to get folks from the 12.5mpg to the 25mpg range. Hopefully they won't mess with that...

UPDATE/Bumped: "Hopefully they won't..." Dang. Spoke too soon.

UPDATE: Interesting backstory on the down side of destroying tens of thousands of perfectly reliable $4,500 cars. More here.