How much power does your car have? Do you brag about it? If you drive a powerful car -- or even a powerful car for the class of car you drive -- you do. A guy at the office just bought a new Honda Civic and he was bragging about the horsepower even though he was more interested in the fuel economy. The real question, however, is how much power you really have. And by that I mean how much power gets to the road.

Vehicle manufacturers rate power at the output of the engine because that is the easiest to measure and generally the most reliable measurement until now, perhaps. Just a few years ago, the manufacturers decided to get more honest (with the urging of the Society of Automotive Engineers) with their ratings. In the past (read the muscle-car sixties), the horsepower was measured on an engine dynamometer with no accessories at all bolted to the engine. These days, they are. Those accessories are the alternator, water pump, power steering pump (if hydraulic power steering is used), etc. plus real exhaust systems with emission controls attached. Again, we have repeatable tests. With modern manufacturing tolerances, the engines that actually get delivered to the cars should meet the rated numbers. Yes, Ford did have a problem with high powered Mustangs a few years ago not getting the rated power, but that never should have happened.

The real question is just how much power actually gets to the road. Older "road" dynamometers used rollers that drove paddles in a tank of water. The power measurement was made by measuring the increase in water temperature. Those old roller dynamometers were difficult apparatuses. The contact of the tires with the rollers was problematical. With small rollers, the tires wore at an alarming rate because they got really hot. Bigger rollers meant bigger equipment. Another problem was keeping a high powered car sitting on the dynamometer. You don't really want the car jumping off when it is reaching maximum power when the measurement is being made inside a building.

With roller dynamometers, measuring the power of an all-wheel-drive car became even more difficult. Now you had to have two of the systems that could be easily moved because unless you were always measuring the same car, the different wheelbases meant that the separation between the rollers needed to be changed.

Another much smaller problem with these engine dynamometers is the fact that both wheels on an axle are spinning a single roller system, so that the differential between them is never exercised. This is generally not an issue because most people just want to know what power gets to the wheels in a straight line, not going around a corner.

Enter a Swedish company, Rototest. They make something called a "chassis dynamometer". That means that the wheels are removed from the car and the system is attached to the hub. Four of them are used for an all-wheel-drive car. The systems are really electric motors that have a bunch of sensors on them and run into a big resistor to dissipate the power.

This system is very clever because it allows engineers to experiment with the drive systems. For example, you can ask what happens if one driven wheel is on a surface with no traction (disconnect the load resistor).

Rototest has done a lot of research to figure out who really has the bragging rights. They have tested more than 500 different cars since 1995 (right, those are European cars). You can see the results at www.rri.se. They have concluded that on the average, about 90 percent of the rated engine power gets to the wheels. Do you think you are shortchanged if you don't get all the quoted engine power at the road? Maybe. But the most interesting thing is that Rototest has discovered a few cars that actually deliver more power to the wheels than is quoted in the catalog. This may be for marketing reasons -- or insurance reasons. It also just might be because the cars that Rototest has evaluated are usually press cars. Even today, apparently, some press cars are more equal than the cars that you get from the showroom.

Note that European Union laws state that a manufacturer must measure the power developed by the engine (on an engine dynamometer) and quote it in their catalog. The real power that the engines develop that are delivered to the customer cannot vary more than five percent. This is why Rolls-Royce now specifies engine power, rather than the former "adequate" quotation.

The real losers in the engine power to wheel power losses seem to be the all-wheel-drive vehicles. One quoted is the Range Rover Sport Supercharged. It's European quoted power in 2005 was 385 bhp, but delivered just 301 bhp to the wheels. That's an 84 horsepower loss, or 22 percent. The Audi A6 Avant V-8 4.2-liter automatic for 2005 is quoted at 330 bhp, but delivers 265 bhp to the wheels. Here, the loss is 65 horsepower or 19.6 percent lower. A car we are more familiar with, that isn't all-wheel-drive is the Chrysler 300C. It's European horsepower in 2005 was 335 bhp, but delivered just 278 bhp to the rear wheels. That's off 57 horse power or 17 percent. The new Audi RS4 that everyone just loves (well the car magazines, anyway) is quoted at 413 bhp for the 2006 model and delivers 345 bhp to the wheels. It's loss is 68 horsepower in the drive system or 16.5 percent.

What about the cars that deliver more than quoted horsepower to the wheels? They don't have as big an increase as the losers have a decrease. The European 2006 Mazda 6 MPS (that's our MazdaSpeed6) is quoted at 256 bhp and actually delivers 264 bhp to the wheels. That is a 7 horsepower increase or 2.7 percent. Another one that we see in the U.S. is the MINI Cooper S (an old 2005) was quoted at 168 bhp, but actually delivered 169 bhp. That isn't much of an increase, just one horsepower, but still it is an increase. However, something that small probably is within production tolerances, so one would assume that you probably would get close to the quoted horsepower at the wheels in your MINI.

Their big loser so far is the European Nissan Primera 2.0 with the CVT transmission. Note that Nissan is a big supporter of CVTs and even their CEO Carlos Ghosn has said that their efficiency is close to a manual transmission. The measurements don't support that. The CVT transmission absorbed about 23 percent of the engine's power.

An interesting comparison is the Audi S6, an all-wheel-drive, V-10 car. It loses some 82 horsepower of its quoted 429 bhp by the time the power gets to the wheels. That's a 19.1 percent loss. It's competitor, the rear-wheel-drive BMW M5 loses just seven percent of its quoted 500 bhp between the engine and the wheels.

There are many details provided for the cars that were tested. The 2007 BMW 335i that was tested showed 282 bhp at the rear wheels where 302 bhp is the specified number. BMW specifies the maximum power at 5800 rpm, but Rototest found the maximum power at the wheels at 5610 rpm. Torque is similar story. The measured torque is 284 lb-ft at 3009 rpm, and BMW specifies it at 400 lb-ft at just 1300 rpm. An interesting byproduct of the measurements are speedometer accuracy. I've heard that BMW speedometers tend to be optimistic and the test confirms this. Since the European cars have km/hr speedometers, I've done a conversion. At a real 56 mi/hr, the speedometer is reading 2 mi/hr fast. At 81 mi/hr, it is reading 3 mi/hr fast.

Here's the story for a real American car, the 2006 Chevrolet Corvette Z06. The measured at-the-wheels power is 471 bhp at 6307 rpm. Chevrolet specifies the power at 506 bhp at 6300 rpm. The measured torque is 439 ft-lb at 4808 rpm, with Chevrolet's specification of 470 lb-ft at 4800 rpm. Here the measured power and torque more closely match the specified rpms than BMW's do. The Corvette speedometer is even more optimistic than the BMW's. At 56 mi/hr it is reading 4 mi/hr more and at 81 mi/hr it is reading 6 mi/hr more.

So it isn't all it might seem when you see a huge quoted power in your car's catalog. Maybe all that power isn't really getting to those drive wheels. One pet peeve of mine is the aftermarket sale of performance enhancing equipment. First of all, does that stuff really deliver the power increases that they advertise? And second -- and most important -- is a five horsepower increase really worth $1500? It wouldn't be to me...