Why do we make consumer vehicles capable of going double the speed limit?

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(I’m in the US where the speed limit on most highways is usually 65 or 70 mph)

The speedometer on my car tops out at 140mph. Aside from getaway drivers, who would ever need to drive that fast? Is there something about engine design that makes it too hard to have a max speed of 90/100 without affecting the overall function?

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I bought the whole speedometer I’m going to use the whole speedometer.
But really. Many vehicles do govern out at a speed. Fords for a long time were 112 I believe. I owned a few. My Acura seems to get to 120 no issue. I like to open them up every now and then when it’s safe.
They want to keep their customer base happy. And people would just disable it anyway.

Vehicles designed for commercial use can have speed limiters turned on in the ECM so the truck or van won’t go past a certain limit.

However the gauge on your car goes up to 140 doesn’t mean the car can physically or safely go that fast. Even on a closed track. It’s mostly for psychological reasons and so manufacturers can reuse gauges.

It’s more about the power that a vehicle can deliver somewhere between its top speed and the speed limit. If you test drive a car that accelerates poorly from a merging interstate lane, you’re going to drop it pretty quick for a model that can jump to attention when you want it too. The fact that it can top out at a high speed isnt the design intent, rather a byproduct.

My car only went up to 140 on the dash because when you switch it to metric it makes more sense. Top limit was 104 mph

It has to do with gear ratios. You would use the highest gear at highway speeds to keep the engine rpm’s low for fuel efficiency.

You would not want your engine revving at 6,000 rpm for the car to go 75 mph. This is a relatively normal highway speed. They make it closer to 2000 rpm by adding a larger gear. This way, the engine runs at a more efficient point of the power/torque curve when you are at the expected speeds, but the car could go faster than it is probably safe to operate at rpm’s higher than they would want the engine running at over a length of time.

As others pointed out, the numbers on the speedometer mean nothing; I have a 5.0 Mustang where the speedo goes only to 85 mph. I have had it faster, the needle just keeps going

Some people don’t care to follow the speed limit. Also, people can take their car to the track.