• Apr 7, 2010
Gearheads the world over have always talked in terms of horsepower. The bigger the number, the better we like it. The type of engine and its output are always one of the first statistics covered in any test drive or car review, because we want to know!
But our fixation on horsepower is kind of a meaningless metric. Just because a car has a big engine, or big output, doesn't necessarily mean it will perform well. Conversely, a car with a small engine and modest output can actually be a blast to drive.

It all depends on how much weight that engine has to lug around. And that's why, as enthusiasts, we should be looking at the power-to-weight ratio of a vehicle to get an idea of its potential performance. That is far more revealing than knowing the horsepower or torque.

It's an easy calculation. All you have to do is look up the curb weight of a car and divide it by the horsepower that the engine puts out. Or if you speak metric, you can divide kilowatts into kilograms.

I know you purists out there will deride me for calling this a power-to-weight ratio, instead of weight-to-power. But saying "power-to-weight" just trips off the tongue more naturally. Besides, it's easier to remember how many pounds an engine has to lug around, rather than trying to figure out fractions of a horsepower per pound.


John McElroy is host of the TV program "Autoline Detroit" and daily web video "Autoline Daily". Every week he brings his unique insights as an auto industry insider to Autoblog readers.


Here's an example of what I'm talking about. The 2011 Ford Mustang with the new 3.7 liter V6 is rated at 305 horsepower, one more horsepower than the 304 hp that comes out of the Chevy Camaro's 3.6-liter V6. One more horsepower, you might say, who cares?

Well, the base Mustang is nearly 300 pounds lighter than a comparable Camaro. That means the Mustang has to lug around 11.3 pounds per horsepower, compared to the 12.3 pounds in the Camaro. Or, to write it as a ratio, the Mustang is at 1:11.3, while the Camaro is at 1:12.3. That is a significant difference, and indicates the Mustang will easily blow the doors off the Camaro despite having only one more pony under the hood.



Here's another example. Which vehicle would you think accelerates faster? The Ram Power Wagon with a 5.7-liter Hemi V8 that pumps out 383 horsepower? Or a Nissan Altima hybrid with 198 horsepower (combined with its electric motor)? According to Motor Trend, the hoary Power Wagon takes 8.5 seconds to accelerate from 0 to 60 miles an hour. The milquetoast Altima hybrid does it in 7.1 seconds. See? It's about power-to-weight, not raw horsepower.

My staff put together a simple little chart that lets you estimate the probable 0-60 time a vehicle might generate, based on its power-to-weight ratio. It sure is a lot better predictor than just knowing the horsepower.


We're never going to give up talking about horsepower. It's just ingrained in our psyche. But by adding power-to-weight ratios to our discussions we can gain a lot more insight into the cars we're talking about.

###

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    • 1 Second Ago
  • 23 Comments
      • 4 Years Ago
      Thanks John for reminding us the power to weigh ratio thingy. But really, most of the time power is just too sexy to resist. I can go all day of talking about horsepower without uttering one "weigh" word.
      • 4 Years Ago
      Thanks for writing this John. I wrote an article exactly on this topic a while ago. I get a little bit more technical but in addition to torque, I also did bring up the weight part which people tend to forget.

      http://uh2l.blogs.com/realitydriven/2007/02/the_myths_of_po.html
      • 4 Years Ago
      John,
      While your point is quite valid, you are seriously over-simplifying the issue. If you incorporate the old "all things being equal" statement, then yes, power to weight ratio is pretty much a guaranteed way to measure the dynamic capabilities of virtually any vehicle. Unfortunately, there are many variables that come into play, and these variables can heavily skew the calculations. Quite possibly one of the best examples of this is what ultimately killed the Mercury Marauder's chances of rekindling the love so many had for the Impala SS. Although the Marauder had a significantly better power to weight ratio than the Impala SS (13.79:1 vs. 16.23:1 respectively), it was a dog on the street, and at the track. Ford had a great idea, and although the Marauder is hands down, a better car in virtually every way, than the Impala SS, their choice of drivetrain components, killed the car's appeal for most.
      Carlos
      • 4 Years Ago
      Weight will only keep going up due to increasing safety standards until the costs and maintenance costs of composite material go down. Until then it is up to the aftermarket or special versions of cars to keep a vehicles weight down.
      • 4 Years Ago
      I can't be the only one who thought this would be discussion of torque rating vs. horsepower rating. Even so, good points made about power to weight ratio.
      • 4 Years Ago
      Gotta love the Ariel Atom.
      1400lb.
      250hp.
      0-60 < 3sec.

      It goes around corners well too.
        • 4 Years Ago
        Or the Caterham RS Levante, 1200 lbs 500 hp out of a 2.4 Litre V-8 reving to 11,000 rpm. I have always had a blast driving small cars with relatively small mills. I have had a 1969 MG Midget, Honda CRX, MGB, Datsun 510 with a 2.0L aluminium truck engine. I would love to either put a V-8 into a MGB or a Miata one day. Weight is everything; if you have a 2000 lb car with 200 hp, to maintain the same performance you must add 10 hp for every 100 lbs you add in weight to another car.
      • 4 Years Ago
      No this is not news but then again it's always a pissing contest with manufactures. The 2 most fun cars I ever owned were a 1965 Lotus Elan S2 and my '92 MX5. Both had about 25% over stock HP, and with their light weight and agility were an absolute blast to drive.
      • 4 Years Ago
      thank you captain obvious.
        • 4 Years Ago
        Eh, you'd be surprised how many people have tunnel vision and only consider horsepower.
      • 4 Years Ago
      I don't want to be harsh but I'm kinda wondering who the target audience of this article...? I don't think people on AB need to be reminded that weight matters... "in the world". Or am I missing something?
      (In every single comparison of current pony-car trio the importance of weight difference is emphasized.)
      Probably even Brian knows that. O.o
        • 4 Years Ago
        @watcher

        Chip on the shoulder much? You speak of a senario where v8 guys get all pissy over tuners, but it sounds more like a tuner guy is getting pissy over the v8's.

        get over yourself
      • 4 Years Ago
      hp/ton or kW/tonne is neater, you get a metric that has bigger = better and you don't need to muck around with fractions

      for kW/tonne:
      most passenger cars score around 90-110 kW/t;
      most performance production cars/hot hatches etc score about 150-200 kW/t;
      Astons, Jags, Porsches, BMW Ms & MB AMGs score around 200-250 kW/t;
      Ferraris, Lambos and the new McLaren zip around 300 kW/t;
      Paganis, the Veyron and the McLaren F1 move at ~400 kW/t;
      absurd things like Koeniggseggs and Zonda Rs come in over 500 kW/t;
      the Caparo death trap develops 912 kW/t
      • 4 Years Ago
      Power to weight is why tuning eco-boxes and compacts is so popular they already have relativly low weights and can be tuned rather easily. The other thing paying attention to weight also will tell how a car will handle I.E the Mustang being lighter will handle better then the Challenger, Even with out dated suspension. And also why an MX-5 is a hoot to drive.
      • 4 Years Ago
      This is preaching to the choir here on autoblog, but it's worth spreading the word to the world at large. There isn't enough knowledge about weight.
        • 4 Years Ago
        yeah, my first thought was DUH,
        then the hyundai commercial. duh duh duh
        • 4 Years Ago
        I guess the news has not reached some people. There was always some dipstick in a Viper, Vette, Mustang, Muscle car etc. that wanted to play while I was just minding my business on my large displacement sportbike. I did not mind turning them into a speck in the mirror from time to time. Most people have no idea how quick a modern motorcycle can turn 25mph into 175mph.
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