Best Economy on Gravel
 

Hi all,

So I have a conundrum. It is apparent both from reading and first-hand experience that driving on gravel reduces mileage. All the advice I can find simply says to avoid it. Well... it's 5 miles of gravel from my home to the paved highway on my way to work. Going "around" (which still entails 1 mile of gravel) is a 20 mile detour. Obviously it uses less fuel to just do the 5 miles of gravel.

So my question is: how do I achieve peak economy on gravel?

The vehicles are a 2013 Outback 3.6R (5-speed auto) and a 2011 Chevy Silverado 2500HD (Duramax+Allison).

Yes, I know manuals are better. Trust me, the Subaru would be a stick if it were an option, but it wasn't (only the base model is available with a stick). I have a stick-shift Golf GTI, but it's a resto-mod project, so not used for the daily commute. I also have a Cagiva Gran Canyon motorcycle that I commute on when weather permits, and that saves quite a bit of fuel.

Please, no "advice" about buying/selling/trading vehicles. Not an option. The objective here is to maximize fuel econ with the vehicles I have.

Thanks!

More info:

Moving closer to work is also a non-option. My wife and I live where we do because it's free. ;) Father-in-law built this home on some acreage he owned. We live here to serve as groundskeepers. Even with the 33 mile commute, it's still a better deal than buying or renting a place close to work. Even if I could walk to work and needed no vehicle whatsoever, we'd still be spending more per month on rent or mortgage. I can flog the Subaru and still get 22 mpg, and drive the truck once a week at 16 mpg, and fuel+insurance+maintenance is still 1/3 of what we'd spend on a mortgage or rent in Hood River.

So yeah... we have the cars we have, we live where we live. I'm trying to learn how to maximize what we have.

I haven't done much testing of my current car on gravel but have some numbers for my 2001 Explorer.

Basically those numbers were all over the place depending on conditions. When the gravel was soft yet smooth it didn't seem to matter what speed I went, I would burn around 15-18 l/100km (15-13mpg) on hard roads with barely any gravel I would get almost the same as paved surfaces in the 10-12 l/100km range (19-23mpg) but on corrugated washboard I had widely varying averages from 15-32 l/100km (15-7mpg). Some of the worst averages were when I slowed down too much to "float" over the washboard and continued at that speed for science, probably contributing to blowing another set of shocks.

So my advice, drive however is the most comfortable for you on gravel. In the thick stuff there is so much rolling resistance that wind resistance is a relatively minor factor so just find a speed you are comfortable with, and in the rough stuff find the speed that works with your suspension.

Another thing to consider is how many gallons of fuel you need to save to pay for a windshield. It's OK to lose some momentum when meeting another vehicle...

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Congrats on living rent free...:thumbup:

I don't believe they're any special tips regarding driving on gravel.

If it were me, I would buy a Scan Gauge or Ultra Gauge if you do not already own one. They are much better than the factory mpg gauges if your vehicle comes with one. The reason they're better is that they give instantaneous feed back not just a average, which will allow you to find the best speed or rpm for maximum mpg. Usually just as the transmission shifts into high gear or lock up.

Find the sweet spot on the gravel and keep it there.

Other than that, just read

100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

and

65+ Vehicle modifications for better fuel economy - EcoModder.com.

:)

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Back when driving gravel and dirt roads there was a path were most drivers drove, (usual in the center) I think the best mileage would be maintained by staying in the path of other vehicles were is it more firmly packed.

Pulse and coast in neutral (engine on) to get best MPG on gravel. 30-35 peak and 15-20 reengagement. I posted this on your other thread.

regards
Mech

One of the things about driving on gravel is you should deflate the tires a bit in order to make it easier on the suspension. Otherwise, you're going to beat the daylights out of the suspension on the Subaru.

The 2500HD will take it, no doubt but you'll be missing fillings out of your teeth! I spent a good amount of time driving my dad's F350 (leaf sprung 1 ton) on gravel roads and wow! What a ride!

Any chance you'd be able to carrry an air pump and deflate the tires, then reinflate?

I'm not sure about your specific vehicle, but my focus gets the best mileage going 35MPH. If I were trying to get the most out of every gallon, I'd deflate the tires enough that I could go 35MPH comfortable (without destroying the suspension), then reinflate once I was on the highway.

patrol off road
 

i have done some remote driveing and lots of gravel in my patrol fuel use is normally around 18 mpg witch i think is pretty good for off road work :)

i have a trip planned to do the canning stock route in australia i will track my fuel use in a remote area

Canning Stock Route - Wikipedia, the free encyclopedia

Thanks for the input! I've started to use pulse-and-glide on the gravel, and that does seem to help. There is one major hill that is a continuous "pulse", but when I hit the peak (around 2000 ft), I can put it in neutral and coast the next 1-1.5 miles all the way down to our driveway, make the turn with a little momentum, then coast down the driveway and right up to the garage door. If the car had electric power steering or manual steering, I'd be able to do that with the motor off... but it has standard hydraulic power steering.

I'm really anxious to get my GTI project running again. With a stick shift and manual everything (the only "power boost" is the vacuum brake booster), it will be an ideal platform for engine-off coasting.

I'll have to play with tire inflation and try to find a good compromise. If I ever get competitive about hitting absolute peak MPG numbers, I'll try the airing down and back up again. But, as you can probably imagine, that's not really a practical thing to be doing every day on my commute. That's a 5-10 minute stop in the middle of the commute... which is already 45min to a hour long (depending on weather). Taking another 10-20 min out of my day isn't high on the priority list. So... I may try that one week just to see what it does for economy, but I doubt it'll becomes a habit. Unless someone makes an aftermarket central inflator I can fit the Subaru (that doesn't weigh a ton).

On the suspension: yep, I've resigned myself to the fact I'll be replacing shocks (and fillings) more often than most folks. Granted... I already do. I've ridden in too many cars to count that have 100,000+ miles on them and never had the shocks replaced... and you could tell! I'm a lot more picky about having proper damping, so I tend to replace shocks every 50-60k miles. Now that we live out here on this road, it may becomes even more often than that. Plus the dust will tear up bushings and bearings, so I'll be replacing those more often as well. A major investment I plan on making in the next year is a 2-post vehicle lift, so I can perform all this maintenance MUCH more affordably and easily.

i would allways be carefull angel gearing on gravel over here in australia we have very loose pea rock gravel that a car can slide on easy so i would allways drive in hear on the thing of shocks i will more than likely have to replace them all after my canning stock route run as the corrigations on the track are shocking but on gravel/dirt you can never go past good after market shocks !!

Park it 2-1/2 miles from the pavement, and get your morning and afternoon exercise walking from home to car and v.v.?

^^ Actually... I am thinking (lightly) about buying the empty acreage right next to the paved highway and building a garage that could house pavement-only motor vehicles
(sports/collectible cars, sportbikes, etc). I wonder how long it'd take me to bike there... or get a really fuel-efficient dirt scoot to get there and back... something like a Ruckus, or a little 125cc 4-stroke dirt bike. If I could get it from my parents, they have an unused Honda CL175. It's 70's vintage. Gets upwards of 75 mpg. In great nick, just needs the carbs rebuilt.

Anyway...

They just regraded the road surface yesterday, then it rained, so it's slick and sloppy. Once it dries back out, I am going to be doing some speed tests in an effort to find the most efficient way to get up and down the road. It'll take time... I'm just gonna do it each day on my commute, so 4 times a week (I telecommute on Fridays). Will try 25 mph, 30, 35, 40, and 45. Anything north of that and you're starting to get into more measurable aero resistance.

The upshots of faster speeds are that you can maintain a higher gear, and you start to "skim" over the bumps. Actually, when it gets washboarded, the most comfortable speed is 50+.

Quick update: I've tested 25, 30, and 35. It seems to like 35. Haven't tried 40 yet.

Down at 25, it downshifts and revs too much on the grades... just gets bogged down and slurps down fuel at the rate of about 8mpg.

At 30, it downshifts only on the steeper grades, and drops down to 9.0-9.5 mpg according to the readout.

At 35, it will hold gear on all but one steep grade, and it only dips below 10mpg briefly on the steepest grades.

Still need to try 40.

Bear in mind, when I give a speed, that's just my "target speed"... I let it rise up to 5 mph above when coasting downhill, and let it drop 3-5 mph when climbing.

So... thus far it appears to be most efficient on gravel in the same speed range it is most efficient on pavement. There may be a small difference... maybe peak is at 35-40 on gravel where it is 40-45 on pavement. Not sure yet, but they're shaping up to be close.

what kind of rock? theres all sorts of rock roads and they change from season to season..depending on repar of the road, level of traffic, and type of traffic..

is it packed 3/4 minus with fresh layer put on once a year. a common low traffic county road set up..
other then when the fresh rock is spred the amount of added drag on packed 3/4" rock is about as good as it gets for min rolling drag..


or is it the other extreme:eek:, raw pit run driving on 4 to 7" rocks.
in that case going around will be faster...then 5/15 mph. and carrying two spares..

don't get siped tires, or pay to have the tire shop put siping on them.
just a little bit of spinning on rock can shred a tire in no time flat...

running rock roads with tires rated at 40psi with 50psi on the car is a bad idea.
there not going to last long.
if they make them, you may want to change the car over to load range C, LT rated tires versus the P rated car ones..
it adds 2 plys to the face of the tire and helps let the tire live a longer life on rock or off road in genral.
LT tires are not common on newer sube's so there may flat out not be a size that fits the auto that's LT rated.

as the truck has much stronger tires load range D or E with a max rating of 65 or 80 psi id run it no less then 45 psi unless your loaded, then up them.

On the subject of tire pressure on gravel, I would actually DECREASE your pressure for better performance and puncture protection on gravel. Lower tire pressure will help take some strain off the suspension and make the tire less prone to puncture by allowing them to flex more.

I have found LT C tires to be perfectly adequate on gravel roads in rural SK but 10 ply LT E tires are more suitable to crushed rock roads of the north where I was getting a flat a week on my 6 plys.


If you aren't getting flats though I wouldn't worry about your tire choice, especially if you are carrying a full sized spare, a good patch kit and compressor (which IMO should be compulsory equipment for anyone traveling off the black top or in rural areas).