Quick answer: The car likely can go 10 mph faster with no increase in horsepower. If it weren’t for this governor thingie in the way.

The governor (it used to be a mechanical device) is one of the parameters programmed in the onboard engine management system. Essentially, it starts to throttle back the engine power as the vehicle speed approaches the programmed limit, which in this case is 142mph. More horsepower won’t get you past this artificial barrier.

So, if you could hack into the car’s firmware, you could find this top speed parameter and set it higher. Some people make a living doing this, believe it or not. Heard of performance chips? Any respectable seller of aftermarket BMW parts sells these performance chips, and they alter lots of other engine performance parameters as well. Shift points, air/fuel ratios, spark timing, and such. Generally you get an increace in hp, but that’s not really my point.

'Nother quick answer: you only need to raise the hp at the top end of the rpm range.

Back to the problem. Let’s assume that the top speed was limited at something like 185mph. But the real-life, good day with primo fuel, low humidity, new air filter and everything - top speed was 142 miles per hour. At this point you’ve reached an equilibrium - i.e., the forces pushing the car forward (engine power)are exactly equal to the forces acting against the car, mostly wind resistance.

A few real pieces of data and some bold assumptions can get us going, but the answer is still elusive. One trouble is wind resistance doesn’t act linearly, but it is a squared function IIRC. And another little twist involves the hp/rpm curve, and the rpm/speed curve.

For any given gear, the vehicle speed increases linearly with engine rpm; e.g. 2nd gear at 4000 rpm = 40 mph and 2nd gear at 6000 rpm = 60mph (my old Z-car would do this). Hp though, is generally linear only in the early and mid rpm range, and starts to flatten out some before finally starting to DROP at the higher end of the rpm scale. So really, you get the hp peak well before your engine rpm is maxxed out, and hp falls with increasing rpm.

So then, drive her up to 142mph and check the rpm reading. (in my old Z-car, this would be 7100rpm, - in theory anyway) Then look up the engine hp on the hp/rpm chart at 7100 rpm. Probably a good bit less than the 197hp spec.

Now is when I run out of real answers. I’d try plotting curves of hp/rpm and speed/rpm on the same chart and see what happens at 7100rpm. Then I’d extrapolate the speed/rpm curve another 10mph (7600rpm, in theory, in the Z)and follow the hp curve another 500rpm. How much did the hp drop over this interval?

I would boldly assume this is the hp deficit you seek. A first aproximation anyway.

BTW, I was going to be the first response to this post, but by time I got done, several others had already posted. Sorry if lots of this is repetitive.