Okay, this is related to climbing because I drove to Wyoming this summer and did a lot of climbing there...

I own a 2005 Nissan Xterra, rated for 15/21 mpg. In the city, I get 15-17. Highway, up to 22, sometimes 23. But when I was on my recent trip, I noticed that I was routinely getting 24-26, even when driving off-road (though not when using 4wd).

As I got back into the Eastern states, highway mpg went back to that 22-23 range even though the speed limits are lower there.

When I drove out to Colorado and Utah back in 2005 in the same car, I noticed the same thing. And when I rent an SUV in the West, I always get much better mpg than the rating. When I rented an Outback a few years ago, I was getting 33-35 in a model rated for 27.

So what gives? Is it something to do with the altitude? Is it something to do with the octane? In the East, the lowest you can get is 87, but you can find 85 almost anywhere in the mountain states.

I'm certainly not complaining-- I saved almost $150 from my fuel estimate even though I drove a few hundred more miles than I planned to-- but I am curious.

On freeways it seems like the quality of the road surface makes a big difference. I notice when I get on I-15 I can haul ass with barely touching the pedal. Other freeways, not so much. The roads out east definitely have to be repaved more often due to the wet and cold winters, so maybe all those patched over potholes slow down your car?

I had a somewhat opposite experience last summer driving from Indiana to Colorado. My gas mileage was the best in Indiana and Illinois and continued to drop as I headed across Missouri and was at its lowest in Kansas and then MPG seemed to improve while we were in Colorado. On the drive home I saw the same results - about 22 MPG in Indiana/Illinois and 15 MPG in Kansas.

The winds were terrible in Kansas which partially explains the drop there - but a 1/3rd drop still seemed rather severe just from wind. For a while I thought it might be related to octane but I can't confirm.

Fortuntaley I get to try this experiement again in a couple of weeks. I'll let you know if I see a difference this year.

MarkDidier wrote:I had a somewhat opposite experience last summer driving from Indiana to Colorado. My gas mileage was the best in Indiana and Illinois and continued to drop as I headed across Missouri and was at its lowest in Kansas and then MPG seemed to improve while we were in Colorado. On the drive home I saw the same results - about 22 MPG in Indiana/Illinois and 15 MPG in Kansas.

The gas in MO and KS may have had a lot of ethanol in it, and ethanol produces worse gas mileage than "pure" gas. In Iowa one day, I fell for the "Plus" advertised at 89 octane but cheaper than the 85. It was only after fueling up that I noticed it had high ethanol content. Got about 18 mpg on that tank. I didn't make the same mistake again.

When I drove through Kansas in 2005, I did get a little lower heading west across the state, but I attributed that to the more or less steady uphill grade across the state. Mileage was much better heading east across the state.

Catamount wrote:My guess is that is has to do with stop and go traffic. Even if you're not in a traffic jam in the east, you are frequently slowing down, speeding up etc. On and off the gas constantly. In the wide open west, the speed of the vehicle is likely to be far more consistent on average. Could easily account for a couple of miles per gallon difference.

I also own a 2005 X-Terra. Good vehicle.

Good points and worth considering. However, this is even true for stop-free driving on interstates and open highways over a few hundred miles. The best I've ever gotten in the East on a tank is 24. It was easy driving over mostly flat terrain in the Tidewater area.

Flat roads give better mileage than hilly roads, especially in 4-cyl cars. If you have to climb 5000' in 15 miles, you won't get the greatest mileage, and it isn't made up when you go back down (partly, if you have a hybrid). If you can hear the automatic transmission downshifting to 2nd a lot on hills, mileage will suffer. Often cruise control exacerbates the problem.

I've never gotten better than 26 mpg in my 2003 outback, which was rated for 28 highway. The old epa ratings were based on a "highway" speed averaging less than 50 mph. Above about 55 mph, the efficiency of the 4-cyl goes down from air drag.

Legally, speedometers can be up to 10% off on the overestimation side, and 1% off on the underestimation; often the odometer has the reverse error. I've seen many odometers overestimate miles driven by 10%. A friend's car got great gas mileage until she put on the correct size (larger) tires.

I have an aftermarket mileage computer (Scangauge) that lets me see what MPG I'm getting and have noticed the same thing when driving in the mountains. It took a while but I figured it out, for a standard transmission anyway. Going uphill obviously you get worse mileage, maybe 15 mpg vs a normal flatland 25 mpg for me. But going downhill I was generally not even using the gas, on steep descents in particular I was either coasting or engine braking. That led to phenomenal numbers! On one short trip down from the hills behind Salida I hit 200 mpg! The engine was doing nothing but running accessories and providing some braking. So if you average the numbers, assuming no net altitude gain by the time the trip is through, and you've spent half your time getting slightly worse mileage by climbing uphill and half your time getting several times better mileage from coasting downhill, it's very easy to skew the average toward the more efficient side of things. Now this is just what happened to me with my underpowered 4 banger and tendency to drive conservatively on steep stuff, but that's what I've noticed happening when I've been able to really watch the mileage. I easily record 40 or 50 mpg days in the hills when normally at home I'd be happy just to tag the upper 20's.

Bob Sihler wrote:

MarkDidier wrote:I had a somewhat opposite experience last summer driving from Indiana to Colorado. My gas mileage was the best in Indiana and Illinois and continued to drop as I headed across Missouri and was at its lowest in Kansas and then MPG seemed to improve while we were in Colorado. On the drive home I saw the same results - about 22 MPG in Indiana/Illinois and 15 MPG in Kansas.

The gas in MO and KS may have had a lot of ethanol in it, and ethanol produces worse gas mileage than "pure" gas. In Iowa one day, I fell for the "Plus" advertised at 89 octane but cheaper than the 85. It was only after fueling up that I noticed it had high ethanol content. Got about 18 mpg on that tank. I didn't make the same mistake again.

When I drove through Kansas in 2005, I did get a little lower heading west across the state, but I attributed that to the more or less steady uphill grade across the state. Mileage was much better heading east across the state.

Here in Missouri we don't have ethanol unless advertised as such. (E85, etc.) I think the same goes for KS, but Iowa I think does mix it in. One thing that might be making a slight difference is humidity levels. Midwest air can contain a lot of water. Could be the computer is just really leaning it out when the air gets thinner and you're experiencing a decrease in horsepower but just not noticing any difference in performance other than the reduced fuel use. The tires would be at a higher relative pressure in a thinner atmosphere. I don't know, could be a lot of things. When I've noticed mine it's been during days with steep ups and downs, haven't really considered what could be doing it on longer more flat days, but I have noticed the difference fuel wise.

robzilla wrote:... So if you average the numbers, assuming no net altitude gain by the time the trip is through, and you've spent half your time getting slightly worse mileage by climbing uphill and half your time getting several times better mileage from coasting downhill, it's very easy to skew the average toward the more efficient side of things. Now this is just what happened to me with my underpowered 4 banger and tendency to drive conservatively on steep stuff, but that's what I've noticed happening when I've been able to really watch the mileage. I easily record 40 or 50 mpg days in the hills when normally at home I'd be happy just to tag the upper 20's.

Time averaging versus distance averaging!

Everything is better in the West.

MoapaPk, what you say is true, but all of that suggests I should be getting worse mileage when I have my car out there, not better. Higher speeds, more hills and steeper ones, etc. I can't speak for all the variables FortMental listed, but for some of them, it is indeed the same or about the same in both locations. This has happened twice now, years apart, so it's not a coincidence. Maybe my car is just magic.

robzilla-- interesting points, and something I've wondered about as well. Thanks for sharing that. And yes, I have a manual transmission, too.

SpiderSavage-- you are pretty much right with only a very few exceptions!

robzilla wrote:I have an aftermarket mileage computer (Scangauge) that lets me see what MPG I'm getting and have noticed the same thing when driving in the mountains. It took a while but I figured it out, for a standard transmission anyway. Going uphill obviously you get worse mileage, maybe 15 mpg vs a normal flatland 25 mpg for me. But going downhill I was generally not even using the gas, on steep descents in particular I was either coasting or engine braking. That led to phenomenal numbers! On one short trip down from the hills behind Salida I hit 200 mpg! The engine was doing nothing but running accessories and providing some braking. So if you average the numbers, assuming no net altitude gain by the time the trip is through, and you've spent half your time getting slightly worse mileage by climbing uphill and half your time getting several times better mileage from coasting downhill, it's very easy to skew the average toward the more efficient side of things. Now this is just what happened to me with my underpowered 4 banger and tendency to drive conservatively on steep stuff, but that's what I've noticed happening when I've been able to really watch the mileage. I easily record 40 or 50 mpg days in the hills when normally at home I'd be happy just to tag the upper 20's.

that's interesting...drag increases by velocity^2 so high velocities just suck up energy that isn't returned, and we see this when driving flat & fast. going uphill, its more energy, but all that energy is "stored" in potential energy to be used on the downhill since speeds are reduced and less is lost to drag.

in theory, going downhill should yield "infinite" mpg as it should cost 0 fuel, even accessories can be powered from gravity. one reason it isn't is that the engine is still idling - shift to neutral if you can, this is essentially what hybrids do.

as long as the torque / rpm profile of your car runs well in the uphill climbs, the mountains could essentially acts as speed limiters which reduces drag and improves mpg...

Bob Sihler wrote:MoapaPk, what you say is true, but all of that suggests I should be getting worse mileage when I have my car out there, not better. Higher speeds, more hills and steeper ones, etc. I can't speak for all the variables FortMental listed, but for some of them, it is indeed the same or about the same in both locations. This has happened twice now, years apart, so it's not a coincidence. Maybe my car is just magic.

robzilla-- interesting points, and something I've wondered about as well. Thanks for sharing that. And yes, I have a manual transmission, too.

SpiderSavage-- you are pretty much right with only a very few exceptions!

I guess we have lived in different areas in the east. I used to travel windy, hilly roads where I was constantly braking as I "accelerated" through corners (even when speed was constant) and the trans was downshifting.

In cold weather, there is a real hit on the mileage of a car every time it is started.

Did you rentals out west have cruise control? Did the miles reported on the odometer match what you inferred from maps and road signs?

Ze wrote:that's interesting...drag increases by velocity^2 so high velocities just suck up energy that isn't returned, and we see this when driving flat & fast. going uphill, its more energy, but all that energy is "stored" in potential energy to be used on the downhill since speeds are reduced and less is lost to drag.

in theory, going downhill should yield "infinite" mpg as it should cost 0 fuel, even accessories can be powered from gravity. one reason it isn't is that the engine is still idling - shift to neutral if you can, this is essentially what hybrids do.

As usual, your post inspired some thought. So here are a few ideas I have had regarding this topic:

The only way the accessories would be "powered from gravity" is if gravity is the sole driving force on the engine during the descent. But this is obviously not the case if the transmission is in neutral. And even in gear and using engine braking, fuel is being fed to the engine, albeit a small amount.

Anyway, a hundred watts or two of accessories is small compared to the several thousand watts used to overcome wind drag at freeway speeds, so for rough calculations, the accessory load can be ignored.

For stored potential energy to be fully returned on the downhill, no engine or other braking can take place. Any braking is energy being lost. So hills that have descents with steep grades are not as efficient as those with grades shallow enough to allow coasting in neutral.

By the way, the combination of downhill and neutral is illegal in some states (not that it ever stopped me from doing it).

I have the same experience with Midwest to WY/MT/CO each year. There are some additional variables:

Best mileage is eastbound in NE or KS. This is subtly downhill (about 5000 feet in a day's driving).

We've loaded up the family car with junk which we leave in the motel or campground when driving around during the day in the mountains. This weight has to affect mileage.

The ethanol thing.

Winds must have an effect; I lost a few MPG driving back from the Dakotas with 35mph crosswinds; a few more when driving head on into the wind.

Driving style. As the crowds go down I can leave the cruise control on for longer distances, improving mileage.

I'd love to know if the thinner air at altitude has enough of an effect on drag to affect mileage. Any engineers?