Over the hill, or around?


#1

A caller named Alice (I think?) on this morning’s show was in dispute with her engineer husband on whether to visit her daughter across town by taking the short route over the hill between them, or the longer route around the hill. Her husband claimed the shorter route saved time, while she thought the route over the hill used more gas and was a greater strain on the car.



If her husband opined that the any gas used to climb the hill would be compensated for by the coast down the other side, he must have not been a very good engineer. Because the trip down would be made riding the brakes to keep the speed under control. So, in effect the extra gas used to climb the hill would be eventually used to heat up the brakes and thereby contribute to general global warming, rather than doing anything useful.


#2

Either could be right or wrong, depending on how long the detour was and how steep the hill!!! And how many trafic light there are on either route.

In the Midle Ages they argued about how many angels could dance on the head of a pin. Both husband and wife seem to have nothing better to do than clutter up the airwaves with this kind of trivia.


#3

This was addreess in another post entitled “Help our hosts settle a marital dispute”.

Basically, it depends on the vehicle. See the other thread for my explanation.


#4

Hey, this has nothing to do with right or wrong. This has to do with how many angels can dance on the head of a pin. It’s an academic question whose answer is of interest primarily for its own sake. :slight_smile:

And for the sake of academic interest, Alice also give the information that the hill was about 160 feet high (by my imperfect memory), that the distance over the hill was 1.5 miles, while the distance around was 2 miles, and that there was one traffic light on the level route (implying no lights over the hill). I’d say that there would be more energy expended in raising the car 160 feet than in accelerating it from rest to local traffic speeds one time.

And the other question depends upon the size of the pin, and the size of an angel, for which I have insufficient data to draw a conclusion.


#5

Thanks for the additional info. The key information is the steepness of the hill, which would determine how much braking is needed (and energy wasted)to get down. Also, trhe amount of traffic on either route. Many variables, as you can see. The power train of the vehicle also has a bearing on the results.

Generally, engineers prefer to design gentle curves with minimal steepness to smooth out traffic flow if they have a choice. My own preference is a slightly longer distance if I don’t have to use the brakes heavily to slow down or stop. But I don’t hazard to “say” which is exactly more economical. That would have to be calculated from actual multiple field tests.


#6

Actually, the steepness of the hill is irrelevant, assuming one comes to a stop at the bottom. The amount of energy used in heating the brakes will be the weight of the car times the height of the hill, whether it’s steep or shallow. If it’s real shallow, some of that energy will be dissipated in heating the air and the tires instead of the brakes. But the amount of energy that was used to climb the hill will be expended by producing heat somewhere, and will be identical in all cases.


#7

The steepness of the hill is very relevant; if the up slope is gentle, the car will not shift down into the next lower gear, and avoid CONSUMING EXTRA FUEL. If the decent is gentle, only engine breaking and no brakes will control the speed. The fuel supply will be shut off. This situation would make going over the hill more economical that going around it, all other things being equal, of course.

You should talk to individuals who have done economy runs to get some pointers on maximizing fuel mileage.


#8

Many people here point out how having your ASE certifications does not mean you are a good mechanic. I point out that Mr. Agranoff’s demonstrated analytical skills does not mean he could perform even the simplest maintiance or automotive repair job, fair is fair.

Sorry to use you as am example Agranoff, but you fit the bill perfectly, book smart but no practical ability implied.


#9

While you may get better gas mileage going around the hill, you will probably use less gas going over the hill.

Don’t assume that mountain driving is always bad for gas mileage. The best gas mileage I ever got out of a Honda Element AWD/automatic was 30.8 mpg achieved while touring the mountain roads of New Mexico’s Enchanted Circle.


#10

Same here; this was during a long weekend with very slow traffic and coming back from one of the National Mountain parks. We maintained a steady 40 mph or so, had few stops and the highways out of the park had few steep slopes.

As any trucker will tell you, very gradual slopes are OK, and don’t affect the mileage much. Downshifting gobbles up gas, as does stop and go driving.


#11

We just got back from there, enjoying a vacation from 100 degree Texas Summer heat staying at the Sipapu Ski resort. We drove to Taos several times and scoped out Taos Ski Valley and Red River, then going to Eagle Lake to Angelfire back to Taos.

I mostly just obeyed the speed limits and kept it in drive the whole time. On some steep downgrades, I hit the OD lockout button for more engine braking, if that wasn’t enough, I would turn on the air conditioner for more braking, and if that wasn’t enough, I would just use the brakes. I really never did have to ride the brakes much though.
Most of those mountain roads were built along rivers for a reason. Water naturally finds the lowest passes through the mountains.


#12

Well, OldSchool, I’m no Tom or Ray, but I have successfully swapped engines, rebuilt heads, done ring jobs, rebuilt clutches, done brakes, timing belts, and various other medium heavy automotive jobs. Don’t jump to conclusions. Book learning neither implies nor precludes practical experience.

If you want to continue this discussion of personal attributes, it would probably be better done offline, rather than on this public forum. You can reach me at Mike@MikeAgranoff.com.


#13

I agree that the steepness of the hill afffects which way consumes more fuel but the single most important factor is what kind of car is doing the driving. If it is a hybrid with regenerative brakes then going over the hill is more fuel efficient because most of the energy that would be lost in standard braking is recovered via electric charging of the batteries.
I don’t know how to objectively measure the relative wear & tear on the car for 2 routes that are so short, but my subjective guess is that going up & down a hill shouldn’t put significantly more wear & tear on any car unless the hill is very steep or winds sharply back & forth.


#14

Simplistic measure of gas millage is to bike along the route with a bike computer that can measure your power output. The comparison of the total amounts of energy you expend going one way rather than the other should give you the answer and factor in most of the variables (excluding the vehicle of course).
Better yet, bike both ways and see which one makes you more tired.


#15

I believe the original OP post assumed a normal non-hybrid car. The power train design still affects the exact amount of fuel consumed, but the best mileage is always obtained by having the car in the hgighest gear without stalling.

You are quite right that the measurement of wear and tear would be difficult. The question was mainly regarding the fuel consumption.

Residents of San Francisco will tell you that this city is very hard on brakes, and gas mileage there is not great either…


#16

Driving up hill may use more fuel than on the level…
But the down hill use less fuel than on the level.

If the distance is the same up as down the usage counters itself out.
Thereby the two routes could be…

could be…

the same.


#17

Too may variables to give a certain answer. In the real world, the difference is going to be too small to worry about.

Let's look at one.  Going over the hill will use more fuel because the extra energy going up the hill will be lost when going down.  In many modern cars, going down in gear will cause the engine to stop using any fuel at all on the down hill as long as the engine is being turned over faster than idle speed. Which will almost eliminate the extra fuel used to go up the hill.  

Clearly this is one where there are a number of variables so a real world test is the only way to get an accurate answer, and it would require the use of your car, your hill and your (or her) driving style over different wind conditions, temperature, etc. So if you have a an extra half a million dollars handy I believe I can answer your question.