I have 2 basic ways of travelling home each day. One, taking the freeway, takes me 5 miles and 20 minutes. Only about a miles and a half is actual freeway; the rest is access road and side streets, with their various traffic lights, all taking me straight or left.

The other way takes 6 1/2 miles and 10 minutes. There are lights and stop signs. I can go right-on-red at some of the lights.

The discussion arose at to which uses less fuel. I contend that running the engine half the time almost necessarily requires less fuel. Less idling also shouldn't be as hard on the a/c unit. Someone else insists the longer distance requires more fuel.

Anyone know the answer?

any time running the engine and going nowhere is a total waste of gas. the 'go' of stop and go will consume the most gas. cruising at a steady speed, assuming your car has efficient gearing, is the lowest consumer of all. granted, it's more engine revolution per minute then idling at a stop light, but then you're covering distance and reducing total run-time to get to your destination. not all cars are equal. i have 2008 chevy malibu and even though it is a very large car that barely fits in my garade it gets over 30mpg average, thanks to its tall gearing and the absence of traffic lights in my town.

your 10minute 6.5 mile run will get better gas milage then 5 mile 20 minute run. measure it, and if it doesn't, I'll eat my pizza.

Better get in fast before PhysicsGirl or EngineerGirl beats me to it.

Your assumption that running half the time uses less fuel is not necessarily correct. The fuel burn rate will vary greatly with engine speed and load; it is not proportional to either time driven or miles driven, so neither "simple" answer is complete.

A very crude first approximation would be to take your average speed (15 mph on freeway, 39 mph on surface streets) and use the fuel burn rate for that speed. However, since one trip has a lot of idle time (0 mph) and therefore a lot of acceleration, that approximation is likely to be very poor.

Without any numbers, the more start/stopping you do, meaning the more accelerating, the less efficient you are. So, whichever route is smoothest, with the least amount of starting, will be better. It sounds like that's the longer route.

Empirically you could test this by driving one way only on a full tank of gas, and check your mileage, then drive the other way on the next tank and check it again. If all else is equal (side trips, other driving) then you'll be able to see which is better.

let's assume you have a mid-sized American car. City driving gives you about 70% of the gas milage as freeway.

half your highway commute is freeway, the other half is street, so if X=gallons/mile, you use [(0.5)(0.7)+(0.5)(1.0)](5)(X) or 4.25X.

the other commute, (0.7)(X)(6.5) or 4.55X

ergo, it seems like the freeway would be the most fuel efficeint.

However, since it takes longer on the freeway, obviously, you can't treat the freeway as open road, ergo, you're not getting the optimum gas milage whilst driving. Rather, it appears the average miles/gallon on the freeway commute is less than that of the other commute. Therefore, the surface street commute is more fuel efficient.

Note, that if you have a hybred, the opposite is true, as the real gas savings from a hybred comes in stop and go traffic.

YTH

Which route leaves you happier and less stressed when you get home? I can't imagine that there's even a dollar's difference in actual gas consumption between them, so go for the mental health calculus instead.

Me? I'd sooner drive on city streets than contend with the morons reading books, painting their nails and eating with both hands on the freeway. The difference in my stress level is worth the money.

Of course, that's only the girl way of thinking about it. ;-)

I'd agree with you that less time with engine running should translate into more fuel savings.

Hey, you could test it. Fill up your tank and take only one of the routes until you have a quarter left, then repeat with the other route. Count how many times you made it each way.

MessyONE:

Which route leaves you happier and less stressed when you get home? I can't imagine that there's even a dollar's difference in actual gas consumption between them, so go for the mental health calculus instead. Me? I'd sooner drive on city streets than contend with the morons reading books, painting their nails and eating with both hands on the freeway. The difference in my stress level is worth the money. Of course, that's only the girl way of thinking about it. ;-)

There is that. Especially since the freeway route (which incidentally is only 1/4-1/3 of the total distance) takes me through the intersection where hubby became physically incapacitated. I generally go the other way.

I was going to say the same thing. The second route sounds nicer. That's the way I'd go! :)

Physical automotive fact: 0 MPH = 0 MPG.

Having said that, I would take the route that passes the liquor store, or a strip club, or a Trader Joe's.

Your experience may vary. Cheers!

This depends greatly on the fuel consumption of your car. (For instance, my prius turns off whenever it is idling and uses the electric motor for <30 mph so city driving is far more fuel efficient).

I did poke around google some, and found one site that quoted an average idling rate of 0.5 gal/hr. So given the length of your trip and the times you quoted, it appears that idling time is irrelevant to your problem. So the biggest factor is the gas mileage your car gets in the two scenerios.

If your car gets greater than 30% gas mileage at the speeds you take for the longer route, that one would be better. However, this seems unlikely to me, so the freeway would be better from a fuel consumption standpoint.

That being said, I agree with Messy. You should also factor in your personal happiness into the decision. Also, if one route greatly increases the probability of getting into an accident that should be taken into account. Ignoring the human aspect of accidents, fixing and/or replacing a car can be fairly costly to the environment.

Anyone know the answer?

Because snakes shed their skin?

The most fuel efficient trip would be to get a bicycle and get rid of the car totally. Or just use your two feet and walk/run/jog to and from work.

Your gas mileage is highly dependant on several things. Like how fast do you accelerate when leaving a stop sign/stop light? Do you hit 60 before the next block, or only 25? Your mileage would be far higher with a lower acceleration. With a good sports car though, you can have a lot more fun with the higher acceleration, which in turn lowers your stress levels.

Checking your mileage over only one week is not enough though. You may accelerate more in one week than in another, or you may hit all greens for 10 days in a row followed by all reds for 10 days (less acceleration needed for all green lights). You will need to average this out over several months.

Also, make sure that your car is tuned up so that it is as efficient as possible.

at idle time the engine is consuming 1000rpm and covering 0 distance. at 2000rpm your engine is covering 40mph in high gear. since total distance is only 6.5 miles, we're talking fractions of a gallon of gas anyway, so your googled piece of half a gallon an hour is a considerable in proportion.

the average speed of the 6.5 miles in 10 minutes is 39mph.

the average speed of the 5miles in 20 minutes is 15mph.

obviously the 30% longer trip is a 120% faster. idle time and subsquent acceleration through the gears will consumer a lot more gas to cover the same distance.

dumb_blonde:

Because snakes shed their skin?

Good answer! Very transcendental. If an answer is going to be irrelevant, it should be completely so, or not even try.

SomebodyElse:

The most fuel efficient trip would be to get a bicycle and get rid of the car totally. Or just use your two feet and walk/run/jog to and from work.

Not an option. I want to live to see my next birthday. Even a Vespa would be this side of deadly.

Additionally, this is the hottest June on record here, based on number of 100°+ days. We saw a jogger nearly collapse the other day. Idiot had no water on him.