Archives

Hey CleanMPG! I'm a college kid who drives a 1997 Jeep Wrangler TJ. Now, I'm not one of those mall crawling dorks who drive SUV's that will never taste a dirt road or need low range. I'm a Coloradoan, born and raised! We live and breath the outdoors here. A Jeep is almost a practical means of transportation in this state. :D

Of course, being a college kid who loves the outdoors, MPG is a huge concern for me for multiple reasons. I've done some reading and I think I'm on the way to getting decent mileage. Currently I'm getting about 19 miles per gallon.

I drive without useing my brakes, accelerate slowly, keep my speed around 55-60, and turn my engine off at stoplights where I anticipate a wait longer then 10 seconds. I'm also working on timing stoplights but I'm not very good at it, and I'm not sure how to accelerate efficiently.

I bought this car in a private sale a few weeks ago and it wasn't taken care of that well. I've already had to repair an entire disc brake assemby. Is there anything I can do to improve mileage outside of driving style? Modifications, maintainence, etc?

Welcome to CleanMPG! You are starting off on the right foot by being concerned about getting better mileage. That is the biggest factor. Air filter, oil, tire pressure all factor in. What size tires do you have? What engine and trans?

The link in my sig will point you to some great articles here that will help you too.

Welcome. I live in Durango, CO.

Biggest physical mods will be making sure your tires roll well (don't run beefy, knobby offroad tires if you can help it, and keep the psi up) and you're as aero as possible (don't run it open-top in the highway). And by keep the psi up, I mean run the MAXIMUM pressure as stated on your tires' sidewalls. The psi listed on your door jamb placard is a MINIMUM. Ignore it! MAX psi is perfectly safe, will lengthen your tires' lifespan and will improve your vehicle's on-road handling and reduce rollover risk. If you frequently go offroading, carry an air compressor so you can air down for dirt and air back up for pavement.

Other than that it's mostly about the driving. You're already doing most of the right things. Timing lights and other DWB techniques takes years of practice - keep at it.

As far as acceleration goes, if you have a manual transmission accelerate fairly briskly in the first 2-3 gears. You don't want to drive like a grandma either because you'll spend too much time in those very inefficient low gears. But you'll also want to keep the revs mostly under 2000rpm or so as you accelerate, and be careful not to lug the engine or give it more gas than you need to accelerate at least moderately. It may take some time to strike the right balance.

If you have an automatic it gets a little trickier because you can't control the shift points, and most ATs allow the engine to rev higher than really needed under moderate acceleration. If you can get your shifts to happen by the low 2000s you should still be doing reasonably well -- try using a little more gas when the revs are in the teens and ease off a bit as you cross the 2000 threshold or so to try to encourage it to upshift. This will take some practice and some good control over your right foot.

Some of my above comments referencing specific rev thresholds may be adjusted upward or downward a bit depending on whether you have a 4cyl or 6cyl.

Air filter, oil, tire pressure all factor in. What size tires do you have? What engine and trans?

Changing the oil filter and oil is obvious, but any type of oil thats good for fuel efficiency?

31 inch BFGoodrich all-terrain tires, 5 speed manual transmission, 2.5liter 4 cylinder engine.

Point of reference, my power band seems to be 2500-4500 RPMs.

Biggest physical mods will be making sure your tires roll well (don't run beefy, knobby offroad tires if you can help it, and keep the psi up) and you're as aero as possible (don't run it open-top in the highway). And by keep the psi up, I mean run the MAXIMUM pressure as stated on your tires' sidewalls. The psi listed on your door jamb placard is a MINIMUM. Ignore it! MAX psi is perfectly safe, will lengthen your tires' lifespan and will improve your vehicle's on-road handling and reduce rollover risk. If you frequently go offroading, carry an air compressor so you can air down for dirt and air back up for pavement.

Other than that it's mostly about the driving. You're already doing most of the right things. Timing lights and other DWB techniques takes years of practice - keep at it.

Yeah, I figured having the top down is going to kill my mileage a bit. Ah well, totally worth it in my opinion. Pretty much only matters at highway speeds right?

Yeah I was thinking about airing up to the max pressure. I absolutely need to buy an air compressor anyway though. Do you offroad?



As far as acceleration goes, if you have a manual transmission accelerate fairly briskly in the first 2-3 gears. You don't want to drive like a grandma either because you'll spend too much time in those very inefficient low gears. But you'll also want to keep the revs mostly under 2000rpm or so as you accelerate, and be careful not to lug the engine or give it more gas than you need to accelerate at least moderately. It may take some time to strike the right balance.


If you have an automatic it gets a little trickier because you can't control the shift points, and most ATs allow the engine to rev higher than really needed under moderate acceleration. If you can get your shifts to happen by the low 2000s you should still be doing reasonably well -- try using a little more gas when the revs are in the teens and ease off a bit as you cross the 2000 threshold or so to try to encourage it to upshift. This will take some practice and some good control over your right foot.

Some of my above comments referencing specific rev thresholds may be adjusted upward or downward a bit depending on whether you have a 4cyl or 6cyl.

I see. The annoying little arrow thing on my dash tells me to shift up at 2000rpms consistently, so I've been shifting up around 1850 just to keep it from flashing at me. I don't feel the engine lug and I can get adequate amount of acceleration from that on the flats. But, hills, oh god. Hills. 31 inch tires and 4 angry little squirrels = no acceleration unless I'm in the center of my power band (like 3200 rpms).

That's another thing. How should I handle going up and down hills? I do my best to maintain speed going up and shift down when I need to. On descents I've been putting it in neutral and trying to gain as much speed as possible, only applying engine power once I dip back down to the speed limit. For steeper hills, I engine brake. Proper technique or no?

Manual tranny, 4cyl, as stated earlier.

That's another thing. How should I handle going up and down hills? I do my best to maintain speed going up and shift down when I need to. On descents I've been putting it in neutral and trying to gain as much speed as possible, only applying engine power once I dip back down to the speed limit. For steeper hills, I engine brake. Proper technique or no?

You've got the right idea here, just might do better if you allow speed to drop more. I admit, that can take some getting used to. On the uphill stretches, you'll probably do better if you set the throttle to drop to some minimum target speed at the crest of the hill rather than keeping speed constant. I like to avoid gearshifts while going up hill if possible, but that won't be possible with every hill (or every vehicle). If you have a really steep uphill section, you may want to park the engine close to it's peak torque speed or a bit lower for minimum BSFC. On the downhill side you had it basically right. For "shallow" hills, coast in neutral as long as you can. If you go too fast, engine brake. Use the highest gear that will keep fuel from being burned. If you follow a lot of the same routes, it can be fun to learn the hills and learn what target speeds you want at different points to make the rest a roller coaster.

You've got the right idea here, just might do better if you allow speed to drop more. I admit, that can take some getting used to. On the uphill stretches, you'll probably do better if you set the throttle to drop to some minimum target speed at the crest of the hill rather than keeping speed constant. I like to avoid gearshifts while going up hill if possible, but that won't be possible with every hill (or every vehicle). If you have a really steep uphill section, you may want to park the engine close to it's peak torque speed or a bit lower for minimum BSFC. On the downhill side you had it basically right. For "shallow" hills, coast in neutral as long as you can. If you go too fast, engine brake. Use the highest gear that will keep fuel from being burned. If you follow a lot of the same routes, it can be fun to learn the hills and learn what target speeds you want at different points to make the rest a roller coaster.

BSFC?

How can I tell when a gear is keeping fuel from being burned?

BSFC= Brake Specific Fuel Consumption

If you have a ScanGauge you can see it, but basically your vehicle should cut out the fuel injectors when engine braking if the RPM is above some threshold. For cars that threshold is often about 1100 RPM. For trucks it can be higher (sometimes much higher :() You can feel it "let go" when it gets down to the threshold because the engine begins idling and you lose the engine braking you felt down to that point. To prolong it down to lower speeds, just downshift to keep the RPM high enough.

With those giant tires, you've effectively got good, low rpm, gear ratios. Keep it as low as you can for the power needed, but without having to completely floor the pedal. That's the ideal case, and you can downshift when you need to.

Some cars (1996 civic :rolleyes:) have such gear ratios that they're essentially downshifted already, all the time.

Changing the oil filter and oil is obvious, but any type of oil thats good for fuel efficiency? Any good synthetic will do. I've heard of people running a lighter weight oil than spec (at least in cooler weather) but I don't think it's going to make a significant difference.

Yeah, I figured having the top down is going to kill my mileage a bit. Ah well, totally worth it in my opinion. Pretty much only matters at highway speeds right?Won't matter much below 40mph or so. Around town, enjoy the open air.

Yeah I was thinking about airing up to the max pressure. I absolutely need to buy an air compressor anyway though. Do you offroad?No, but I mountain bike, which is what makes me aware of optimal tire pressures: highest on pavement, fairly high for rim & tube protection in fast rocky terrain, lower for maximum traction, REALLY low for flotation on sand or snow.

But, hills, oh god. Hills. 31 inch tires and 4 angry little squirrels = no acceleration unless I'm in the center of my power band (like 3200 rpms). Hills are hills, and sometimes you've gotta juice it to get up. Just make sure not to rev any higher than you need to, and it will help if you allow your speed to sag a bit as you climb. Traffic will of course limit how much you can do that.

And at least you've got tall gearing. Those of us with sub/compact cars and undergeared MTs would kill to have your gearing.

That's another thing. How should I handle going up and down hills? I do my best to maintain speed going up and shift down when I need to. On descents I've been putting it in neutral and trying to gain as much speed as possible, only applying engine power once I dip back down to the speed limit. For steeper hills, I engine brake. Proper technique or no?On moderate, rolling hills you're best off doing DWL -- allow your speed to droop on the climbs and increase on the downhills. Please consider (traffic permitting, of course) your speed to drop BELOW the speed limit on the uphills, and not having it go as far above the limit on the downhills. This will save additional gas.

In any event, if the hills are steep enough to allow for coasting, then you'll definitely see a benefit by putting it in neutral and picking up speed on the downhills and roll it out as far as you can before putting it in gear and hitting the gas again.

The rule about coasting in neutral vs. in gear is simple: If you're trying to coast for maximum distance (the usual highway sistuation) then definitely do it in neutral. If, on the other hand, you need to burn off speed anyway (stop signs, curves or curves ahead, or your speed is otherwise getting too high) then put it in gear and take advantage of DFCO.

The reason coasting in gear with DFCO is more efficient than coasting in neutral if you need to burn off speed anyway is that in neutral, you still need to feed your engine somewhere in the neighborhood of 0.3-0.4gph to keep it idling. But when you're coasting in gear (if fuel cut is engaged) it takes ZERO gas to turn your engine because your momentum is doing it for you. DFCO exists for emissions reasons (combustion is usually incomplete as you decelerate in gear), rather than fuel economy reasons, but you can definitely use it to your advantage. As mentioned above it may take some time to figure out where DFCO comes in: on my diesel it will engage at nearly any RPM; but my gasser will only hold DFCO down to about 1300rpm and usually needs to be around 1600rpm to initially engage it. Be aware that the threshold may vary too; mine seems to change considerably depending on how warm the engine is and how long DFCO has been engaged.

And at least you've got tall gearing. Those of us with sub/compact cars and undergeared MTs would kill to have your gearing.

With those giant tires, you've effectively got good, low rpm, gear ratios. Keep it as low as you can for the power needed, but without having to completely floor the pedal. That's the ideal case, and you can downshift when you need to.

Some cars (1996 civic :rolleyes:) have such gear ratios that they're essentially downshifted already, all the time.

Even with 4.10 gears, with 31 inch tires (something like 10% more diameter and weight then stock) and my tiny little 130-something HP engine, I have zero power on the highway. Lowrange offroad is another matter entirely and completely unnoticable, but for daily driving in hilly terrain at highway speeds, its a pain.

Why are tall gear ratios an advantage? I would think high RPM's at speed is a bad thing for hypermiling... For reference, I'm at about 2250RPM at 55 mph, due to my tires.

tall gear ratios = low rpm, good for mileage.
short ratios = high rpm, good for rock-crawling.

You're right, low rpm is better for mileage.

Also, zero power on the highway = a good thing for mileage.

If you really need to accelerate you SHOULD downshift (and your 4th gear is probably as tall or taller than many cars' 5th gears). Then upshift back to a more optimal cruising gear when you don't need the power.

tall gear ratios = low rpm, good for mileage.
short ratios = high rpm, good for rock-crawling.

You're right, low rpm is better for mileage.

But...

I have a 4.10:1 gear ratio, which is sustantially bigger then the 3.07:1 gear ratio on the 6 cylinder Jeeps, so, given the same size tire and same transmission gear, my RPM's are higher at x speed. Jeep did this because those 4 angry squirrels have zero power. This is offset a little by my larger tires, unfortunately. I know cats with heavy 37 inch tires and the 4cyl and they needed to install a 5.12:1 gear ratio just to drive around town comfortably.

Also, zero power on the highway = a good thing for mileage.

If you really need to accelerate you SHOULD downshift (and your 4th gear is probably as tall or taller than many cars' 5th gears). Then upshift back to a more optimal cruising gear when you don't need the power.

Hmm, I see. So, if I need to go up a hill, its better to maintain the same throttle position and downshift, rather then pressing the throttle?

Depends. If you're starting to lug the engine (i.e., "it has no power") then you should probably downshift.

Hi Pallando:

___Those 31’s may be helping your ratios but are killing the I4’s capability to accelerate your Jeep? For off-road, you may need the ground clearance consider a set of std. all-season tires and smaller rims when you are not off-road. There are plenty of smaller cars and trucks driving around you and they get by. Carrying those 31’s back and forth to work is probably like carrying 1,500 pounds to and from the same and that can hurt when accelerating. The lowering effect and smaller frontal area of all-season radials will help improve your aero dynamics immensely as well.

___When you say no power, you have a stick, right? Use it when you need to accelerate, do not just step into the pedal and hope for the best.

___Finally, 2,500 to 4,000 R’s is far too high. You should be shifting quickly just above lug in the next higher gear.

___I hope that helps?

___Good Luck

___Wayne