Is the downhill section a cliff? Google informs me terminal velocity of a car is 200-300mph, so to fall a mile at 300mph, the car will need 12 seconds, so let's round up to 15 seconds to account for the time it's accelerating.

To cover the full 2 miles at an average of 30mph, we need to complete the entire journey in 4 minutes, leaving 225 seconds for the ascent.

We know that the old car was averaging 15 miles per hour, but the speedo on an old car is likely inaccurate, and we only need to assume a 6% margin of error for the car to show 15 miles per hour and cover the mile in 225 seconds. You probably couldn't even tell the difference between 15 and 16 on the speed anyway, but let's say that we also fitted out the car with brand new tyres (so the outer circumference will be more than old worn tyres), and it's entirely possible.

So, let's say 240mph. That's the average speed of our mile freefall in 15 seconds.