Kerry's Ideas On Gas Mileage [Archive]

Aurora40

April 14th, 2004, 10:28 AM

The Toyota Prius is one of the most advanced hybrids on the road. It barely achieves 50mpg, and it's tiny with a tiny motor. One thing to realize is that those EPA ratings are even less likely to reflect real-world performance than they are for gasoline cars. And here's why. The Prius has a rating of 51 mpg on the highway and 60 mpg in the city. That should tell you something right there. Why is it that on the highway cycle, when it doesn't have to accelerate much, it doesn't get as good mileage? There are other puny cars that come close to that highway rating without the need for fancy electronics.

Here's the catch. The EPA city cycle is an 11 mile distance. The Prius is able to run on batteries alone for 3.6 miles of that 11 miles... This means zero gasonline is used for that period. This will substantially raise the city mileage rating of the car (that's over 33% of the distance). However, in reality, you can't drive 3.6 miles on purely electric for every 11 miles driven. Why? Because the batteries would not recharge. This rating assumes every trip will be magically started with the batteries at full charge, even if the last trip ended with them completely drained. In real life, this will not happen. Unless you only drive your Prius once for 11 miles, you are very unlikely to achieve 60 mpg in the city.

Regenerative braking is one free way to add charge to the batteries, but assuming 100% of the braking was regenerative (which it isn't) and the recharge was 100% efficient (which it isn't), then you'd only gain enough juice to get you back up to the speed you were at before slowing down (which isn't too shabby, but inefficiencies, aero drag, and rolling resistance make this pure fantasy). So you'd still be getting the 51 mpg (or less) that you cruise at, plus the nominal braking/accelerating distance added to that (as they would be gas-free).

I don't know how long the highway cycle is, or for how much of it the Prius can use battery alone. However, I would bet that in real-world driving, the people who own Priuses probably get mileage in the 40's, which isn't bad, but it hardly makes the technology worth it. There are other sissy cars that get mileage like that, and they don't have a cost premium or the incredible expense of battery replacement hanging over them.

While a 600hp hybrid is certainly a cool idea, and it may get better economy than a 600hp gasonline car, I would take those mileage claims with a huge grain of salt.