Why the Spoiler on the Prius?<br><br>I have been given four reasons for the presence of the spoiler on the Prius. According to Priusman, it improves streamlining by 2-3 mpg. This was easy to test. Having recorded a baseline figure for milage, I took the spoiler off. The mpg rose slightly, but there was definitely no drop in mpg. So much for the hypothesis that it cuts drag.<br><br>Toyota Customer Relations told me that it was there to aid stability by helping to keep the rear wheels on the road. Therefore, it is acting either as a downward-lifting wing or as a spoiler to keep the car itself from generating lift in the rear.<br><br>I will give the figures to two significant digits. The car weighs about 1,400 kg (3,000 lbs) which means that it presses down on the road with a total force of a bit under 14,000 Newtons. <br><br>Assuming that the spoiler is a great airfoil (it isn?t, it has a slot in the bottom and is sharper in front than in back; a good airfoil is the other way around) and granting it a coefficient of lift of 1 (which is very generous), it can provide a downward force less than 90 Newtons at 70 mph. As you slow down the force drops off rapidly. A downward force of 90 Newtons can be provided by putting 20 pounds of groceries in the trunk. Any force the spoiler generates is insignificant compared to the 14,000 Newtons with which the weight of the car grips the road.<br><br>Perhaps the spoiler is there to "spoil" the lift generated by the rear of the car. <br>the area of the car is less than 10 square meters, and an elementary calculation shows that the total lift at 70 mph will be at most 2,000 Newtons, or less than 1/7 the weight of the car. The lift on a car acts more on the front than on the rear (a racing car can be fast enough for the front to lift up and flip the car over backward). How much decrease in this lift could a spoiler in the rear provide? It would, at most, cancel as much rear lightness as putting a pair of adults in the rear seat, and that?s based on unrealistically large assumptions about the lift generated and the effectiveness of the spoiler.<br><br>My family happened to visit the home of a Toyota executive working in the US. When he walked us outside in saying goodbye, he was very pleased to notice that we had purchased a Prius. He was, however, disappointed that I had taken off the spoiler. He told me that removing it upset the special look of the car, and that they had added the spoiler to make the car more attractive. Not a word about engineering. Given the physics of the situation, this is the only explanation I?ve heard that makes sense.

Umm, let's see: the car is more attractive, so they sell more of them, so the Toyota _fleet_ mpg goes up 2-3 mpg... yes/no? ;-)<br><br>Seriously, though, I'm not interested in the spoiler and I _am_ interested in mpg. Can you describe your mpg test & results for us? Thanks!<br><br>&gt;&gt;I have been given four reasons for the presence of the spoiler on the Prius. According to Priusman, it improves streamlining by 2-3 mpg. This was easy to test. Having recorded a baseline figure for milage, I took the spoiler off. The mpg rose slightly, but there was definitely no drop in mpg. So much for the hypothesis that it cuts drag...<br><br><br>&gt;&gt;My family happened to visit the home of a Toyota executive working in the US. When he walked us outside in saying goodbye, he was very pleased to notice that we had purchased a Prius. He was, however, disappointed that I had taken off the spoiler. He told me that removing it upset the special look of the car, and that they had added the spoiler to make the car more attractive. Not a word about engineering. Given the physics of the situation, this is the only explanation I?ve heard that makes sense.

Besides reducing lift, which as you point out<br>is insignificant at non-racing speeds, spoilers<br>can reduce aerodynamic drag by reducing flow<br>separation at the rear of the car.<br><br>I don't know how significant this is in the Prius,<br>but it is another factor to consider.

Before removing the spoiler I was averaging about 44 mpg on each tank. Since I removed it, the mpg has moved to a range closer to 45 mpg. This is not a significant improvement, but it certainly denies the idea that the spoiler increases mileage by 2-3 mpg. If it could do that, then removing it should have shown an easily-seen decrease in mpg.

As you say, the spoiler's function might be to reduce drag, but if the spoiler decreased drag, then it should increase mileage. That's why I tried the experiment of removing the spoiler to see if mileage got worse. As I noted, it did not. There was a slight move in the other direction, if anything. And the car is now a few pounds lighter and the visibility to the rear is improved!

Ok, I'm ready: how do I remove it? Is it just the 2 brass-colored bolts visible thru the holes in the trunk lid? What did you use to cover up any holes in the trunk lid exterior?

I'd be interested in knowing how "scientific" your results are. Over what distance? In what driving conditions?<br><br>Some time ago Priusman (who, sadly, seems to have departed our company) asserted that the spoiler was put on at the insistence of the engineers, over the esthetic objections of the marketing people. So I for one would like to see more than anecdotal evidence that the spoiler has no effect, or a negative effect, on fuel economy.