These two cars share the same base, but one produces 90kW (A180) and the other 280kW (AMG A45). So isn’t the AMG three times faster?

The title image is of the Mercedes-Benz AMG A45. The base car is the humble A180 which produces 90kW and the AMG version 280kW. The top speed of the A180 is 202km/h, and the AMG is 285km/h (estimated, electronically limited to 270km/h). Yet the 0-100 times are 8.6 and 4.2 seconds. Mercedes have provided data for their four-engine range of A-Classes, so we’ve put it into a handy table here:

 kWNmWeight (kg)
Top speed (km/h)
0-100 (sec)
AMG A4528047515552854.2

We can see the A45 has more than three times the power of the A180, but its top speed is only 40% or so higher. And the 0-100 time is only a bit better than half. Here’s a graph of the key data:


What’s going on?

Top speed first. There are various forces a car needs to overcome to achieve a given speed. These include friction of the various components and rolling resistance of the tyres, but the biggest factor by far is drag, sometimes known incorrectly as “wind resistance”. The nasty thing about drag is the way it increases, which is more or less according to the speed-squared law. So, if you have drag of X at 30km/h, you double the speed to 60km/h, you get double the drag, right? Nope. You get the square of the increase, so 2 x 2 = 4, or four times the drag. Go from 30 to 90km/h and that’s three times the speed, and NINE times the drag.

This is why a basic, 90kW hatchback like the A180 can do over 200km/h yet the most powerful hatchback on the market, the AMG A45, can’t even come close to doubling that speed with triple the power. Even a V8 Supercar can just about touch 300km/h and that might have around 470kW. The drag increase from 200 to 280km/h is about double, even though the speed increase is only 40%. But wait… the AMG’s power is triple the A180’s, not double? You actually need even more power as speed increases because the drag only measures the force required to resist air moving at a given speed. You actually need to push through the air faster – travel across say a 1km measuring straight in half the time – not just be able to provide the force, so that’s where the extra power comes in.

There are other variables such as slight differences in aerodynamic design, gearing and so on, but the basic physics is that the power required for a given top speed very quickly increases and it’s almost all due to drag.


Then we have acceleration. The AMG is a bit better value here, as it’s more than twice as quick to 100km/h as the A180. But again, aren’t we missing something as it’s triple the power? The answer is no, because acceleration is about torque, not power. We have explained why in detail here. If we look at the torque figure of 475Nm vs 200Nm then that’s a bit more than double for the AMG, and it’s 0-100 is a bit better than half, so it’s in the ballpark.

The AMG weighs around 10% or 160kg more than the A180 and that blunts its power edge. Weight isn’t anywhere near as much of a problem for top speed which isn’t dependent on acceleration. That’s why light sportscars such as Exiges do well on 0-100 tests, but have low top speeds whereas big, heavy, powerful cars are the reverse.

The other limitations on acceleration are gearing, traction and various other factors. Traction is increasingly becoming a problem as road-legal tyres only have so much grip, and can be easily overwhelmed by today’s engines. That’s why some of the fastest accelerating cars, particularly the heavier ones – Nissan GT-R, Teslas, Veyron – are all-wheel-drive.

Entire books have been written about the physics of cars, and the intent here isn’t to recreate that good work, nor is this an attempt to quantify all of the myriad factors which have to be considered for performance modelling. Rather, what we’re trying to summarise for you is that torque and light weight is what you need for acceleration, power for top speed, and drag builds up very quickly which is why large increases in power are needed for relatively small increases in top speed.

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