You’re right, I changed my mind halfway through and realized that I’ve only noticed this in the northeast.
Well that’s surprising. I thought it was in the double digits, but I guess my memory failed me. It certainly seems steeper, being able to coast so fast.
On a hot summer day (say 70F ) the air density is only about 57% the sea level value, thus aerodynamic drag is reduced by over 40% for a given speed. But since drag increases as the square of speed, the net result is that you’d coast around 1/3 faster at 11,000’ MSL and 70F than at sea level and 65F.
So if you coasted to 115mph up there, you’d only make 86 at sea level.
Oh yes, the power of a normally aspirated, fuel injected engine is down by that 43% as well, so the uphill bits seem all the more steeper.
Carburated engines do far worse, as the mixture goes way rich.
Interesting. I hadn’t thought about the decreased drag. My car is already good on aerodynamics. I was still accelerating pretty well at 115, but after I hit my speed limiter at 109, the engine died and I didn’t enjoy flying down a downhill curve without power brakes or power steering. So anyone who’s interested in seeing what happens when you exceed your speed limiter in neutral should just read my post and forget about trying it. Alright, I’ll quit going off on a tangent in this thread now.
Is there any place online I could find these formulas?
I’d be really curious to work them myself and see how various roads with known traits work out.
Your engine SHUT OFF when you hit the speed limited in your car?!? That is unbelieveably dangerous, especially on a road like 70 goming down from Eisenhower. What kind of POS are you driving?
Glad I don’t have to worry about it. My car’s top speed is limited by drag. Right now, about 180 mph. 
It’s a 94 Thunderbird LX. The speed limiter works by completely cutting the fuel injectors. Apparently Ford didn’t program the EEC to make sure the car was in gear before doing it. To be honest, I doubt many car’s computers do. How often do people roll down the Rocky Mountains in neutral past the speed limiter? Maybe I should write them to let them know about this glitch.
I can’t find anything about what the actual grade is, but I-35 coming over the hill and descending into Duluth, MN is one I remember as pretty steep (and twisty, too). And a favorite spot for Highway Patrol to hide, to try to make their quota of speeding tickets from people suddenly on this steep downhill road.
It’s probably not as steep as the old Highway-61* was, though. Anybody know just what grade it is?
- Yes, this is the Highway-61 of the Bob Dylan song.
True, but Dylan is referring to the southern end, where it’s the Blues Highway, perhaps more than the northern end, where it passes near his birthplace.