Originally Posted by zacknolden
Cars only use their rated horsepower when they are accelerating, which is not too often compared to the time spent going at a steady speed; so acceleration (and therefore peak power) hardly counts at all when it comes to gas mileage.
The typical 4 cylinder develops peak horsepower at much higher RPM than a V-8, so the baseline overhead/losses at typical idle and cruise speeds may be actually lower than half. (less than half the friction area at the rings; smaller, lighter valves)
He asked what the average acceleration of a 4-cylinder car is...That doesn't have anything to do with peak power, gas mileage, or 8-cylinder vehicles.
Also, torque is what makes a car accelerate, horsepower tells you how long it will accelerate.