Quote:
Originally Posted by bkrell
Doesn't that make sense or am I wrong?
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Good question. I don't know. What I'm saying is that the math is deceptively complicated.
Here's an example of what I'm talking about. Consider this comparison:
A) Travel up the hill at a steady 25 mph
B) Travel up the hill at a steady 30 mph
Contrast that this with comparison:
C) Travel up the hill at about 24-26 mph, but with the speed slowly dropping
D) Travel up the hill at a steady 30 mph
When an instrument like the SG looks at C, it's going to give you a deceptively optimistic result. Why? Because it's calculating mpg based on fuel use and distance, but it's not taking into account the fact that you're losing speed. The momentum you're losing was paid for previously, by gas you burned a little earlier. And now you're relying on that momentum, instead of fuel, to propel the car. That's why the car is slowing down.
The reverse happens when you're accelerating. The SG will give you a deceptively pessimistic result. It's going to tell you that you're burning a lot of fuel relative to the distance you're traveling, but it's failing to take into account that some of the fuel you're burning is being used to create momentum in the vehicle. It's like putting money in the bank. In a few moments, you'll be relying on that momentum.
When you add speed, you're adding kinetic energy, and when you climb a hill, you're adding potential energy. Either way, you get this energy back later. But the SG is not taking this into account, if you're just looking at an instantaneous reading. That's why you have look at a broader interval of analysis, like a trip.