How much Horsepower do you need
The mainstream media, advertising, and even stuffy old Consumer Reports continue to emphasize horsepower when they consider the performance capability of a car. But engines are tuned differently for different uses, and comparing a high-torque turbo-diesel with a high-revving gasoline engine using horsepower alone will not give you an accurate picture.
Most of us know that torque is king when accelerating from rest or pulling a boat out of the water, but horsepower is what determines top speed and the ability to climb grades. So to determine whether your car or truck will perform adequately all the way from a stop light to two-lane blacktop passing speeds, you need to know both, and then how much mass those horses and pound feet are going to be called upon to move.
So the best way to gauge the performance potential of a vehicle is to add the horsepower and torque and divide the total by the weight in tons. It gives you a nice bite size number that doesn't need any decimal places, and it gets bigger as the potential improves, as a number that predicts performance should. For shorthand, I'll call it "h+t/T."
How do we judge how big an h+t/T you need? If you are not concerned with street racing, then the primary factor to consider is safety. The insurance industry and law enforcement agencies get all nervous if you start talking about the ability to accelerate as a safety factor. but if you can't get up to the speed of freeway traffic for a merge off a typical uphill on-ramp, then that speed differential is a danger to traffic and yourself.
By the same token, if you are on a two-lane rural highway and get stuck behind a row of motor homes, you don't want to be hanging out in the left lane (right if you're in a British-influenced country) any longer than absolutely necessary.
So where is the h+t/T safety threshold? Well, a typical uncontrolled on-ramp for a rural freeway is a bit more than a quarter of a mile long, so you need to be able to accelerate from zero to 70 in that distance. No sweat, you say? Well, yes, a 70 mile per hour trap speed in the quarter mile is a pretty undemanding test for most modern cars, although a VW Beetle couldn't do it. A Smart ForTwo just squeaks by at 70.1 miles per hour, according to Road & Track.
Since it matches my criterion as tested, I'll use that Smart ForTwo as a benchmark. It's curb weight (what it weighs with a half tank of gas and all other fluids topped off), plus a couple hundred pounds for driver and test gear is 2025 pounds. Its little 3-cylinder engine cranks out a grand total of 70 horsepower and 68 pound-feet of torque. That gives us an h+t/T of 136 to aim for as a minimum.
But hold on! Instrumented tests published for most vehicles don't account for your wife, your two kids and your mother in-law, and the three sacks of potting soil you have in the trunk. Add to that the fact that you may be going uphill, and the published test results start to look a bit less useful. As they say, "your results may vary," and they aren't just covering themselves for the lawyers.
Let's take a plain vanilla Honda Civic four cylinder sedan and see what the results are. According to howstuffworks.com its curb weight is 3318 pounds. If you load it up to its maximum gross vehicle weight (that's with all the stuff I mentioned above, up to just short of what the manufacturer considers unsafe), it weighs 4450 pounds, or over a half ton more. So even with 140 horsepower and 128 pound-feet of torque, its h+t/T is still only 120, far below the target Smart Car. There's no way this car is going to make it up to freeway speed with a full load.
There are other cars that miss the cut, like the Honda Fit. The Toyota Yaris just barely beats the Smart ForTwo at 138, and testers say it's faster than a Prius. You could look up any others that you might find questionable. Those are extreme examples, and many cars will do just fine as long as the on-ramp is flat - and you are willing to flog your car every time you get on a freeway. But don't you want a little margin for error? Would you try passing a big rig on a two-lane country road in one?
I'm going to stick my neck out and advise that any car that can't hit 95 in the quarter doesn't give you the flexibility in daily driving to keep up with traffic, safely merge from a slow moving freeway to a faster one, and pass on a country road. That works out to an h+t/T of just about 280.
