Distance vs. Speed

[forty2j]

Hypothetical question:

My commute is 30 miles. TomTom tells me it will take 60 minutes, with traffic.
TomTom then finds a faster route. 45 miles, 45 minutes.

Which uses less gas?

[Varmint]

Calling in sick.

Short answer: Can't say. Too many variables and not enough information.

Long answer:

In the first scenario, it takes 2 units of time to cover 1 unit of distance. In common terms, you'd be traveling at an average speed of 30 mph.

With the second scenario, you need only 1 unit of time to cover 1 unit of distance... or 60 mph. (In the real world, this would mean many speeding tickets.)

Both of those are merely averages. Getting stuck in traffic could mean stop and start acceleration with a few bursts at high speeds. This is much less efficient than steady state driving at 30 mph (barring any hypermiling techniques). But if we assume the averages are both for steady state driving, you'd have to figure your fuel consumption per mile for those speeds, then multiply it by the total distance traveled.

For example, at 30 mph (steady-state), I probably get somewhere around 50 mpg. This is a measure of efficiency, not consumption. It translates to 1/50th of a gallon per mile. Multiply that by 30 miles and I'd use 3/5ths of a gallon to travel that distance.

I get about 44 miles per gallon steady state driving at 60 mph, which translates to 1/44th gpm. Multiply that by the 45 miles total distance and you get just a hair over 1 gallon of fuel consumed. That makes it more efficient to travel the route with the shorter distance.

But this is a completely unrealistic comparison because you never get to drive steady-state like that.

[human668]

Do those two routes have equal amount of up hill and down hill?

[forty2j]

Originally Posted by human668

Do those two routes have equal amount of up hill and down hill?

Yes. Both routes are across the nearly-flat areas of NJ, with little incline / decline on each. The sharpest incline is probably a bridge.


Also, the 30 mile route is of the stop-and-go variety, with probably half the route being steady state and half being a pain in the leg.

[Varmint]

Gotta drive 'em both and record your economy score for each drive. Then come back and we can run the math.

[human668]

Assume you can do the 45 miles route by using 1 gallon.
Assume the half of 30 miles route is about the same condition of the 45 miles route, so you use 1/3 gallon. And you finish this 15 miles in 15 minutes.
Then there is 15 miles of 45 minutes stop & go traffic. You have to average better than ~22.5mpg in this section so you use less than 1 gallon in the 30 miles route.

There are just too less data and too many assumptions. You really have to drive them both to see the difference.