MPG difference in opposite directions
 

We travel from Cleveland to Columbus and back every week or two for the past couple of months in our TCH. Distance covered is around 150 miles each way. Our mpg for Cleveland to Columbus is around 33 to 34 mpg. The return trip gets about 38 to 39 mpg. The trip to Columbus is during early Saturday afternoon. The return trip is at night. Average speed is 70 mph each way. Does anyone have an explanation why there is such a significant difference?



I do not think it is altitude. Columbus elevation is 902 feet. Our home town elevation is 935 feet. I do not see much difference in traveling between night and day. Temperatures are slightly cooler at night but do not seem that large. We have the AC on in both directions.


The only idea that we can come up with is that the trip north has a tail wind. I do not notice any.


Have you experienced any big differences in mileage going in opposite directions like this?

Re: MPG difference in opposite directions
 

Wind and hills make a HUGE difference. Every time speed doubles, wind resistance quadruples. And you can see the immediated difference between a flat road and an uphill grade on your dashboard mpg meter.

Re: MPG difference in opposite directions
 

I made multiple trips to a bordering city (say, around 70 km round-trip) during one weekend. It was mixed terrain and it was mostly freeway driving with some level sections, but the route was the same. The live fuel efficiency tended to be better in the same one direction.

Re: MPG difference in opposite directions
 

neohoopsfan — I have consistently found very significant differences travelling between Waterloo and Toronto, Ontario (~125 km each way, approximately east-west). Even allowing for the elevation change due to the Niagara escarpment (~100 m) doesn't explain this difference. Surprisingly (to me), the explanation turned out to be the prevailing winds, usually from the west, and not particularly fast. On a couple of Waterloo-Toronto trips I carefully measured the fuel consumption each way. I then constructed a mathematical model (including engine/transmission efficiency, tire rolling resistance, and air resistance) from which I could deduce the wind speed/direction necessary to account for this difference. I then accessed the Environment Canada Web site and checked the actual wind speed/direction during each half of each trip. What I found was quite good agreement between the estimated and actual wind speeds. The moral of the story is that even a relatively small but consistent wind can result in a significant effect on fuel consumption because its small additional force on the car acts for the whole duration of your trip each way (i.e., for a very long time).

Stan

Re: MPG difference in opposite directions
 

At least in small airplanes, power required to go faster increases by the *cube* of the speed increase. (Tho' you get some of that back, since you don't have to put out the higher power for as long.) I'm not sure it's directly comparable to cars, since there are other effects involved, but it does give you pause.

But I have to agree, I made a round-trip up the Willamette valley a couple weeks back and noted a couple m's pg difference between the directions. Next day wif repeated the trip and commented on the diff. Since we both got the better mileage headed south, which is nominally up, I put it down to a light north-to-south wind drift.

Benton 7jul08

Re: MPG difference in opposite directions
 

Its always better when you leave Columbus... :)

Slow down a bit - to say 60mph and both legs will be 40mpg or higher....

Re: MPG difference in opposite directions
 

There is a very large difference in my daily commute into Philadelphia from the western suburbs.

Topologically, Philadelphia is downhill from where I live. It is also in the same general direction as the prevailing winds. Although it is a 35 mile drive, it isn't unusual for the drive IN to work to deliver gas mileage in the high sixties. The return trip will get only in the low fifties or even the high to mid forties (in winter).

I like to think of my commute into Philly as being essentially downhill; topologically, morally, spiritually.

Re: MPG difference in opposite directions
 

Yes, yes, yes..... Check out what the wind loading (sq ft of exposure) your vehicle presents to the air. Typical cars are around 5-10 sq ft, then get a piece of plywood with same dimentions and get buddy to drive you in his/her pickup truck bed while hoding the plywood facing front. Start out going 5mph and feel the pressure your car feels at 5mph and then at 70mph....just kidding of course. you can try this with a small piece of cardboard held out your window.......myth busters where are you?????;) ;)

Re: MPG difference in opposite directions
 

Scale the plywood according to the vehicle's Cd (aerodynamic drag coefficient) -- 0.34 for the '08 Highlander or 0.35 for the RX. It's not just the frontal area, but the shape and detail of the body that follows as well. The Cd maps the overall body to the equivalent 'flat plate drag' -- your plywood.

I cringe when I see the folks that leave their ski rack, cargo pod etc on their cars whether they're in use or not. They're both increasing the frontal area and screwing up their Cd. I remember Scientific American ran an 'Amateur Scientist' column years ago that walked the experimenter through a procedure for determining a vehicle's rolling and aero drag. The areo drag part used coast-down measurements: find a level section of freeway and a calm day. Accelerate to, say, 70 and shift to neutral, then time how long it takes to decelerate from 65 to 55. Even without going through the exercise of reducing the numbers to drag figures, it might be eye-opening to run the test with and w/o all the extra hardware, just to see what it does to the coast-down times.

Benton 9jul08

Re: MPG difference in opposite directions
 

I would bet on the prevailing wind . Maybe the colder denser air over Lake Erie blows in-to the south - when the air over the inland areas heats up. I would guess you don't get any reverse flow -like over a warmer body of water because Erie is probably always colder than the land in summer.


Charlie