Thanks for confirming it. I guess my speeds are slightly higher cause my l&m halogen bulb gets 30 lumens/watt vs. 20 lumens for standard halogens plus the custom reflector really throws the beam far. I'll adjust the curve that calculates the speeds and adjust it down a little.
The reason why the speeds are slower for the city is because there's a lot of ambient light in the city. So your eyes aren't dilated as much. The result is that they're not as sensitive to light and you don't see as far, plus dim street lighting usually casts an even masking orange glow over everything that your lights have to power through. It's easier to see the road with less wattage without a lot of lights distracting you.
Another thing to keep in mind is the glare on the road from oncoming cars, the moment a car rounds a corner in the opposite direction, your visibility of the road can drastically be reduced from 150ft down to less than 20.