mods for better gas mileage

[subdoug]

My Sub collection is 95-97 outbacks and legacies. I am interested in mods to improve gas mileage, specifically converting to 2wd during non-winter months.

1. There is a 2wd fuseholder under the hood. Info on how this works from various dealers is vague and contradictory. If fuse is added (to convert to 2wd mode) will this stop the propeller shaft from turning? One dealer said this 2wd fuse should always be installed when the donut spare tire is installed on either of the back wheels. Does this mean in this mode you could run different sizes of tires on the rear?

2. If 2wd mode doesn't stop the propeller shaft from rotating, is there any way to disconnect/remove the propeller shaft so only front wheels are driving and fewer drive train parts are involved? I started to disconnect the propeller shaft on one, and realized that the short shaft from the back of the auto trans can fall out and leak trans fluid. Any way to keep that shaft from falling out?

[Manarius]

I could explain to you how the FWD fuse works, but how it works is rather irrelevant. Either way, your gas mileage will not improve if you put in that fuse and it's not meant to be used except for diagnostic purposes. If you want a car that gets better mileage, get another car or convert yours to FWD with a FWD tranny (and don't forget to remove all the rear drive parts).

[WoodsWagon]

The automatic AWD system adds mostly weight, and a bit of drag from bearings and the rear differential. Short of removing everything, It's not worth the bother to get better MPG.

 

Concentrate on the engine's efficiency. Are the O2 sensors functioning properly, is the fuel injection system clean, are the sparkplugs worn or fresh. Make sure there are no vacuum leaks, and the PCV system isn't clogged. Make sure that the right thermostat is in the car. General tune up stuff.

 

Driving style makes a big difference too. try to drive so that you don't have to use the breaks often. Slow down from 80 to 65 on the highway. If your car has a roof rack or a spoiler, you can remove them and seal the holes.

[torxxx]

yes like the guy above me said, the 2wd drive fuse, just activated a switch at the center diff that unlocks the rear clutch discs. The driveline and the axles will still be turning, and because its a EJ series car, you can't pull out your rear axles.

 

The only way you are going to keep the spindle in the back of the tranny is to zip tie or wire it in place. I've actually done this and drove for 3 months. The only thing that worried me was if my dual ranger shifter got knocked into the 4wd and I didnt notice, that spindle breaks the wire and falls out, oil leaks out then you have to worry about a blown tranny...

 

Easiest way to save MPG is change your driving habit. Dont rev past 3k rpms, keep the weight down, keep the paint clean (dirt creates a small amount of drag) run a higher PSI in your tires, change fuel filter/air filter every 3 months

 

Like 91 loyale said, the automatic adds weight. about 80 to 90 pounds of weight....

[idosubaru]

don't waste your time if you do mostly highway driving. i've driven in FWD mode and even removed the driveshaft and even a differential at one time. it doesn't make any noticeable difference in gas mileage. maybe if you do a ton of city driving it would (i don't know because i don't do that), but i doubt it based on my experience. i did it because of a bad driveshaft and bad rear diff, not to increase gas mileage but i do keep a tight eye on my mpg.

 

keep engine in tune, replace fluids.

 

get one size skinnier tires and also taller tires. will hurt handling if yo'ure into racing and taking tight turns really fast. but will improve driving in rain and driving in snow (the thinner tires will). helps a good bit with mileage. get larger rims/wheels too helps. increase rolling diameter (larger overall tire diameter) and lighter wheels helps as well for highway mileage anyway.

 

synthetic diff fluids front and rear, high tire pressures and don't carry around needless weight.

 

on an XT6 installing a non-stock muffler helps a little bit as well. the stock muffler sucks on those things. don't know about your car though.

[NorthWet]

After ensuring your car is in proper tune, the biggest factor in fuel economy is your right foot. Beyond controlling rate of acceleration, slower engine speeds mean reduced fuel consumption. Gary touched on this with taller tires. In town, I run in as high a gear as practical. On the highway, I have taken to running 3-5 MPH below the speed limit (traffic safety permitting); at the times I do my 25 mile drive this is pretty easy to do. This makes quite a difference in my Aerostar (current DD ).

 

How big a difference does reduced RPM make? I was playing with my new-to-me Legacy 5-speed (3-speed at the moment ), and holding throttle constant at a steady 55mph and depressing the clutch caused the engine RPM to rise only about 500-1000 rpm. In other words, the aomunt of energy needed just to spin the engine at an extra 1000rpm is the same as the energy needed to cruise the car at 55MPH on the level. (circa 12-20HP!)

[All_talk]

In other words, the aomunt of energy needed just to spin the engine at an extra 1000rpm is the same as the energy needed to cruise the car at 55MPH on the level. (circa 12-20HP!)

 

Thats without aerodynamic resistance... For the average car, the required road HP at highway speed (50-70mph) is around 40HP.

 

Just a side note to a side note.

 

And a note ON topic... Most engines have a "sweet spot" where the volumetric efficiency (VE) peaks, this is the RPM that the engine breathes best. Above this point the mechanical efficiency drops off pretty quickly and fuel mileage declines rapidly, for Subarus this point seems very pronounced. I have a long commute and have experimented with cruise RPM a bit, in my EA Subes this break point is about 3300RPM and around 2700RPM in my Turbo Legacy.

 

Gary

[subdoug]

Now I see I've opened an interesting thread with my original post. Unfortunately some have gone hopelessly off topic, behavioral mods being confused with vehicle mods.

 

I could write the book on behavioral mods and gas mileage. My number one pet peeve is the guy in the diesel truck who floors it when the light turns green, pushing a lot of unburned fuel right out the exhaust. Pet peeve number two is the guy who drives all over the parking lot, trying to find the parking spot closest to the front door. I like to pull in and grab the first spot available, even if it means I have to walk a little. I give myself bonus points if I don't have to back out of the parking spot. Triple bonus points if I can exit the lot timed so I make the next green light, without having to accelerate rapidly.

 

In addition to driving sensibly, I think the behavior mods which give the best mileage involve reductions in vehicle inertia changes, avoiding transitional behavior. Keeping the vehicle rolling at a steady pace. Enuf on that. Perhaps driving behavioral mods warrants it's own thread.

 

Back to the vehicle mods. 91Loyale, thanks for your suggestions. I keep the engine in good tune, change air cleaners often, check the pcv valves, run good platinum plugs and so on. I have been wondering about aftermarket plugs vs original subaru plugs though. I assumed if O2 sensor was malfunctioning I would get an obd code, but maybe that's a bad assumption.

 

Grossgary: I haven't tried synthetic fluids (in trans, differentials, engine) yet but I anticipate that the reduction in friction would be worth a try. Have been worried about syn in engine oil though as other vehicles I owned in past always started leaking at the seals when I tried synthetics like Mobile 1 back in the eighties. Anybody have experience running syns in their +100K miles subarus without developing leaks?

 

Also Gary, I think the tire changes are an excellent idea... reducing the rolling resistance. I have been thinking about this one for some time now, and since I drive pretty sensibly I am more than willing to sacrifice handling for better mileage. Seems to me the legacy/outback tire footprint is way too wide. I'll have to consult with a tire expert for recommendations. Consumer Reports measures rolling resistance on their dynamometer when they test all-season radials, and the best performers were Michelins and some of the Japanese mfgrs.

 

Torxxx, I took a close look underneath today, and it looks pretty easy to remove the rearmost propeller shaft, leaving the front in place which would secure the transmission spindle. Don't see any easy way to remove the rear diff though. The sticky problem is those rear drive axles. I did a quick test with the prop shaft disconnected, and there does seem to be quite a bit of friction when I rotate the rear prop shaft, making the rear wheels turn. I guess my arms just don't generate much hp!

 

I did drop an auto trans from one of my subs recently and I must say it really is heavy. Unfortunately in my family fleet (4 running and two for parts) we only have one manual trans. I'll try and avoid the auto trans in the future. I've heard the clutches don't last on these cars but I suspect that is due to driving habits. My brother has about 190K miles on his original clutch in his 97 Outback SW.

 

Enuf rambling... tnx for all the great posts. Keep the thread alive, and as I perform "mileage mods" I'll report on the progress. -Doug (aka subdoug)

[baccaruda]

you CAN run with the driveshaft pulled and without losing fluid.

 

There is a super-secret modification you can perform which prevents loss of fluid. You need:

 

An intact CV boot.

A 5-speed gearshift knob.

A large and a small hose clamp.

 

1) Cram the gearshift knob (bottom-first) into and through the small opening of the CV boot so that the knob is inside the CV boot and the threaded end is sticking out of the opening. Do not "slide" it or "stick" it or "pop" it in, it must be "crammed."

 

2) Clamp the boot around the knob. It's not likely to slip back into the boot due to the tight fit, but why take chances?

 

3) Slip the wide opening of the boot over your driveshaft-less transmission and clamp it down.

 

You now have a silly-looking although functional cap which will limit your fluid leakage to the amount of volume inside of the CV boot. Add fluid as therefore necessary.

 

I will be offering pre-made kits for this purpose for $60 shipped

[subdoug]

Here's some info I've dug up which should help with decisions re mods for better mileage (both behavioral mods and vehicle mods):

 

1st Case Scenario- Stop and go city driving vehicle energy use:

45% driveline friction

35% overcoming inertia(could be higher if stop sign @ every block)

15% tire rolling resistance

5% air drag

 

2nd Case Scenario- Steady Highway Driving energy use:

60% air drag

25% tire rolling resistance

15% driveline friction

 

Thus removing an unused roof rack might make a significant difference to the highway driver, but would never be noticed by the guy who just tools around town. The "townie" might want to consider running synthetic gear lube in both his front and rear differentials.

 

I would welcome anyone who wants to contribute any suggested vehicle mods given these scenarios. Given the above, my original idea about converting the 95 legacy to 2wd would only be of benefit if I drove around town alot, which I don't.

[keltik]

I switched to one size thinner/taller tires on my wagon. I dont have any exact mpg figures yet but i have seen the rpm required for 100kph cruise drop from 2900 to 2800ish. Ive also recorded a 3mpg gain from driving at 85-90kph instead of 100kph. My car only needs 2400rpm at the reduced speed.

[robm]

Your highway scenario is interesting. Do you know what speed that was measured at? Highway fuel consumption is very speed dependent. Even 5 mph slower can show improvements. 10 mph is huge.

 

Many years ago, Car and Driver magazine used to publish their measurements of horepower requirements for air and rolling drag for the cars they tested. Small econoboxes were pretty typically 15 HP @ 50 mph, and half was air drag and half was tires. They calculated this with coast-down measurements, or something like this, so they couldn't estimate drivetrain losses. But since power required is proportional to the cube of the speed of air resistance, and the square of the speed for rolling resistance, it gives a good place to start guessing from.

[mtsmiths]

Highway fuel consumption is very speed dependent. Even 5 mph slower can show improvements. 10 mph is huge.

 

OH LORDY! I can see it now, with $4.00/gal gas on the horizon, can the return of the double-nickle be far behind?

 

[robm]

You have the choice right now. 55 isn't that bad, as long as you keep it up steady. Not having to stop as often for gas helps cut down the time on long road trips. I find driving a bit slower even makes it less stressful, so I don't have to stop for a break as often, and can keep going longer.

 

For example, I drove from Vancouver, BC, to Terrace. That is about 1400 km, and I took about 18.5 hours to do it, with the cruise control at a steady 93 km/h. I stopped once for fuel, twice to shop, and once to chat with a friend for an hour, so time on the road was less than 16 hours.

 

Between Cache Creek and just north of Quesnel, I was passed 3 times by the same SUV! He was doing 110-120 km/h, and didn't even have to speed up to pass me. Because he stopped so often, he maybe beat me to Prince George by 15 minutes, and spent a LOT more on gasoline to do it. That is about a 5 hour trip.

 

My fuel consumption for that part of the trip was over 15 km/liter, or about 36 mpg US. That is the best I have ever achieved, probably due to new tires of the correct size (165/80-13) and fresh alignment. The 4 hour trip through the Fraser Canyon is good and twisty, with lots of hills, so that kind of mileage is amazing at any speed.

 

Pretty good for an old Loyale wagon, full of stuff!

[mtsmiths]

In Montana, 55 on state highways or interstates can get you killed ... even in the slow lane.

[robm]

Yes, I realize I am lucky. There just aren't that many people up here.

[subdoug]

Your highway scenario is interesting. Do you know what speed that was measured at? Highway fuel consumption is very speed dependent.

 

...But since power required is proportional to the cube of the speed of air resistance, and the square of the speed for rolling resistance, it gives a good place to start guessing from.

Sorry, I wish I knew the hwy speed that was referenced. The generalizations came from tirerack.com in a section that addressed tire rolling resistance.

 

Re the pwr req'd statement above, I believe the equation is F (force) = M (mass) X V

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(velocity squared). I think what you are trying to say is that the force acting upon the vehicle (as wind resistance) increases with the square of the vehicle velocity. In layman's terms, double your speed, and you'll have four times the wind resistance. This is why the cartalk guys Click and Clack say to roll your windows up and turn on the AC (if needed) if you are driving above 40mph.

 

The obvious behavioral mod for increased mileage is to slow down, but this is not always practical. The obvious structural mods for hwy driving would be do whatever is practical to reduce turbulence and induce laminar flow around the body of the vehicle.

 

Robm, I'm impressed with your mileage figures. The most I've every been able to squeeze out of my EJ22 95 legacy is 33 mpg, and that was the trip I took where I put the automatic in neutral and coasted down some hills that were several miles long. I've often wondered if that coasting could damage the transmission, so I asked a guy at a transmission shop once. He advised against it at high speeds...said the trans wouldn't get enough lubrication. Was he right?

 

Can anyone tell me how to get html code turned on so I can type in a simple velocity squared??? Html is listed under vB code, but doesn't seem to work.

[robm]

Yep, and power required is force times velocity, so power required to overcome wind resistance is velocity cubed. Rolling resistance is linear with speed, so power to overcome it is the square of the speed.

 

I was impressed, too. As I said, I attribute it to the new, narrower tires and alignment. I used to run 185/70-13's, and managed to find 165/80's. All they can set is the toe-out, but they did set it, and there was a noticeable difference in the steering wheel (it now points straight ahead, from 15 deg. left) and also in the yaw stability. (A sudden hack at the wheel damps itself out quicker than it used to.) There was noticeable wear on the outsides of the old tires, which is why I had the alignment done, so I hope that is gone too. Worth the $50, and worth considering for your car, if there are any indications at all that there is something not quite right in the steering.

 

Your car weighs more than mine, has a larger engine, and probably more frontal area. The automatic will really kill your mileage.

 

I coast on hills, as well. A 6% grade will allow the car to maintain speed in neutral, greater than that will allow it to increase. Not a problem with a standard trannie, not recommended with the auto. The fuel injection does cut back a lot when going down hill, so we probably don't save much gas on that part. It is coasting up the other side that saves fuel. Hey, if I know the weight of the car, that 6% grade will tell me how much drag the car really has, won't it!

 

Body mods are expensive. The best bet for a Loyale would be to lower it, but then I would lose the rough road ability, and compromise the great access for service. Even at $1.15/liter for gas, it would be hard to pay back the investment in new suspension parts and labour. A front air dam would help too, but again, rough road ability would be compromised, and the cost probably wouldn't match the benefit.

[starkiller]

mentioned this in another thread but i took off the cross bars on my 86 wgn and gained btwn 3-6mpg...every tank...keep them stored in the back though just in case...a nice free mod

[subdoug]

mentioned this in another thread but i took off the cross bars on my 86 wgn and gained btwn 3-6mpg...every tank...keep them stored in the back though just in case...a nice free mod

Great no cost/low cost idea! If you saved 4mpg and drove 20K per year, at $3 gas you might have saved about $235. If you improved by 3-6mpg, may I ask what kind of driving do you mostly do with the 86 wagon? Is it mostly highway?

 

A low cost/no cost mod I always do is remove the AC belt for the winter months, which is quite a few months here in Colo. But since you are supposed to occasionally circulate the freon in the winter, when I do winter maintenance on the cars I put the belt back on temporarily and run the AC for a bit. Re mpg I have no idea if this saves anything, and would be suspicious of any calcs I do because gasoline density is higher in the winter.

[robm]

Gas density higher in winter? Not so, I am afraid.

 

Winter gas tends to be LOWER BTU content, as they put more volatile components in it. Probably the density will be reduced slightly, as well.

 

All reasonably modern gas pumps (less than 20 years old) adjust the gallons shown to give you gallons at 15 deg. C, no matter what the temperature of the fluid. (I am not sure what they adjust it to in the US, 60 deg F maybe?) I am not sure what density they base the measurement on.

 

Winter driving conditions (wet roads,snow on the road, snow tires, cold engines and bearing grease, higher air density) all combine to reduce mileage considerably.

 

So it will be hard to compare summer fuel consumption, with AC hooked up, and winter, without.

 

My guess is the difference will be minimal. If the compressor isn't compressing, the extra drag due to the bearings will be very small.

[subdoug]

Gas density higher in winter? Not so, I am afraid.

 

Winter gas tends to be LOWER BTU content, as they put more volatile components in it. Probably the density will be reduced slightly, as well.

 

All reasonably modern gas pumps (less than 20 years old) adjust the gallons shown to give you gallons at 15 deg. C, no matter what the temperature of the fluid. (I am not sure what they adjust it to in the US, 60 deg F maybe?) I am not sure what density they base the measurement on.

Then I guess the advice I keep seeing about purchasing your gas early in the day when it is coldest so you get a little more for your money is incorrect.

[idosubaru]

Then I guess the advice I keep seeing about purchasing your gas early in the day when it is coldest so you get a little more for your money

this is a valid statement. at higher temps more gas is lost to fumes/evaporation. filling up in the heat of the day will result in more gas being lost in fumes and less going into your tank. probably a small percentage, but it helps with pollution/smog as well. this is a slightly different conversation than they are having about BTU's and winter density...blah blah blah. i don't know the specifics of that, but people debate gas properties every time a gas mileage thread comes up.

[robm]

It is hard to not talk about gas properties when discussing miles per gallon, and the gallons keep changing.

 

It would be great if we could buy it by the heat content, instead of by the gallon. Then we would be talking about kilometers per kilojoule, or miles per MBTU. Seasonal and regional variations in fuel content wouldn't make any difference, then.

[keltik]

It would be great if we could buy it by the heat content, instead of by the gallon. Then we would be talking about kilometers per kilojoule, or miles per MBTU.

 

Then we can buy chocolate by the calorie.

But seriously ive often wondered just how much of my gas never makes it into the tank, on a good hot day theres a stream of thick wavy fumes coming out the filler. Over a 60 liter fill its gotta amount to something?

As for a fuel saving mod - is LPG a viable option in the US?