Why Such Varied MPGs w/ 3.4 V6?

[rideexileex]

How old is your front O2 sensor? That makes a huge difference on the stoic point if it's an old/dirty/fouled sensor. I noticed a pretty big difference on my wideband O2 sensor for the supercharger after replacing the factory O2 sensor. It ran a bit more lean, closer to the 14.7 stoic after the new sensor.

 

[AntleredRuin]

That's not entirely true. Gas engines are generally more efficient at higher throttle levels. The problem is if you're a "drive it like you stole it" kind of person that usually comes with speeding and a lot of gas-brake-gas-brake-etc... type driving. It's the brakes that are the problem, not the gas. You can accelerate briskly all you want, as long as you don't speed to the point where you have to slam on the brakes all the time.

I used to be a grandpa like driver, but I have noticed that slow acceleration doesn't really net me any improved gas mileage. Now I use as much gas as I feel like, and while I don't floor it I'm not easy on it either. The key is I get off it and coast once I hit the speed I'm looking for, which in town might be 35 - 40mph, or less. I used to annoy other drivers with how slow I accelerated, since abandoning that approach I have not noticed any decrease in mpg.

I guess this is where the other factors come in. I may be lucky with mine. I can drive i like I stole it and get around 16 or so city, but if I baby it (with the 2k RPM limit) I can get around 19 city. Babying at freeway speeds results in near 23. Driving like I stole it (while still not tailing or braking, as you mentioned) results in around 20 hwy. BTW, I have a '99 and my ass sags a little.

I understand the engine efficiency thing you mentioned, once up to speed, but it seems to me that a faster moving (higher RPM) engine uses more fuel. Just by way of how the engine works. If you can get to the same speed without having your engine work as hard, why wouldn't that result in better MPGs (provided the engine is not underpowered for the weight of the vehicle, etc)

I wonder how skid plates and running boards affect MPGs. I know they add more weight, but would they also decrease wind resistance? If air is not vacuumed up under the truck I would think that would help. I saw someone (I forget where) who used super light thin gauge sheet metal to cover the entire bottom of his 1st gen Tacoma with the 3.4. He also built a sheet metal air guard in front of his front wheels. (Think cattle/brush guard for a train, but solid). This, combined with several other 'mods' SUPPOSEDLY helped him get almost 30 HWY once.

 

[MtnT4R]

I'm seeing a lot of comments about terrain making a big difference, but that's not entirely true either. I live in the mountains at 8k feet. Every day I descend to around 7k into the valley, and then climb back to 8100 on the other side for work. So roughly 40 miles and 4k in elevation change daily. Easy acceleration, avoiding braking wherever possible, fluids, plugs, and filters in top shape- I regularly pull somewhere between 21 and 22mpg calculated on Fuelly.

I also like to mess around with coasting in neutral down certain stretches where the slope serves to maintain my speed just about perfectly, and on my last trip to Denver I set a new personal best: 15 miles coasting down from Eisenhower tunnel eastbound. calculating the next fill up netted over 23 mpg. Kinda fun to play with.

Note: This is with stock tire size and drivetrain.

 

[suppra87]

You all are making me feel pretty decent about my gas millage haha. Ive been consistently getting 15mpg mixed city/highway and 16-17mpg all highway driving @ 75-80mph. Closer to 16mph when I'm driving into the mountains. Keep in mind, mountains in my area max out around 2200ft. My truck has a small lift, 285 all-terrains, 3.4 SC w/7th injector, and auto trans. I recently replaced my PCV valve, cleaned TB, and replaced both O2 sensors. Reset the ECM afterwards and didn't notice an increase in millage but noticed more consistency. Oh and I drive with a lead foot!

 

[WallyWest]

I I understand the engine efficiency thing you mentioned, once up to speed, but it seems to me that a faster moving (higher RPM) engine uses more fuel. Just by way of how the engine works. If you can get to the same speed without having your engine work as hard, why wouldn't that result in better MPGs (provided the engine is not underpowered for the weight of the vehicle, etc)

True, but the flip side is your mpg is crap until you hit overdrive+lockup. Accelerating slowly increases the amount of time you spend in 1st - 2nd - 3rd and no matter how low you keep engine rpms you're getting horrible mileage down there. Every time you shift to the next gear you gain a good bit of mpg.

RPM doesn't really track with fuel usage either. Set your cruise at 65 and drive down a slight incline. Then drive back up. Same RPM (assuming you aren't kicking down to a lower gear, which will make it even worse), drastically different fuel consumption.

I believe getting to the higher gears quicker, combined with better efficiency at higher throttle, makes it a wash between accelerating slowly or quickly. But I admit I have nothing but anecdotal evidence for that.

 

[spruce]

I live in a hilly area. What goes up must come down. Putting it in neutral is dangerous and even illegal some places and really doesn't make any difference.

The biggest difference I've found on completely stock 4runners is between my old '97 with 3.91 gears and my '99 sport with 4.30 gears. I averaged 20+ every tank in my local area with the first but only 18-19 with the 4.30s. Both will do slightly better going 60mph or less on a flat highway/long distance trip.

A Scanguage will really show you where you can improve.

 

[gamefreakgc]

Highest contributing factors in my opinion (others have also shared these):

1. Tire size - has direct influence on MPG's due to the circumference of the tire. You aren't actually getting worse mileage in most cases, just your odometer and speedometer are now off. But most people don't correct their numbers.
2. Gas - Summer/winter blends will ruin MPG's at certain times of the years. What octane you put in makes a huge difference. My best tank on 91 in CA - 19.7 MPG. My best tank on 93 in OR - 21.9 MPG.
3. Maintenance (including O2 sensors) - I keep my maintenance on all items as best I can for this very reason. Can easily gain 1-2 MPG's back with routine maintenance.
4. Terrain - If you are constantly chugging up hills and then braking on the way down your MPG's will always look like the city range, even if you are on a highway.
5. Driving style - This one goes hand and hand with Terrain. Lead foots don't get good mileage but we know that anyway
6. Mods - Some engine mods gain MPG's. Some I've done is switching to synthetic fuel, aftermarket air intake, and yes, the S/C. When I can keep my foot out of it I get better MPG's since my engine breathes better. That and a straight-through muffler.
7. Tire tread - This does affect MPG's. An all-season road tire will get better MPG's than a M+S tire with an aggressive tread.

 

[Bstrom]

Wow - I'm blown away with the response this thread has received! Let me say how thankful I am for all the experiences you guys have offered. Excellent observations all...

And my questions are answered, obviously. Oversize tires, 4WD drivetrain/stock gearing, and a constantly full tank are probably the biggest culprits, in spite of maintaining current consumables. So much driving being done under 2000 rpms is a no-man's land for economy. My highway runs are only 10-12 miles at a time too.

I buy from the same convenience store which has rendered inconsistent mileage tank to tank - has me suspect of lower grades gasoline? I drive like an old man - never over 2000 rpms unless going up a hill, and then never over 3000 rpms. Easy on takeoff and turns, etc. Almost feel like I'm making love to it I'm so gentle. Lot of slower driving overall.

Guess it's the price of owning one of the best vehicles (still) on the road. I'm done fussing...

 

[Bstrom]

Just FYI, it's the forward O2 sensor that matters. The second sensor is just there to make sure the cat is working properly. The engine isn't doing any fuel trims based on that reading.

Thanks Jeremy - is there any way to determine that front O2 sensor's condition? I've gotten no codes for anything since owning this rig. I'm doubting now that it's making any trouble but with 190k on this vehicle can't help but wonder if a changeout would help.

Need to do a longer highway run and calc the mileage again to see what it can really give, first.

Also wondered if timing can be advanced slightly for better low end grunt. This is a popular mod on my Miata, but it's an OBD1 vehicle. Not sure you can do this with an OBD2 vehicle. Whaddayathink?

 

[ShooterMcGavin]

Thanks Jeremy - is there any way to determine that front O2 sensor's condition? I've gotten no codes for anything since owning this rig. I'm doubting now that it's making any trouble but with 190k on this vehicle can't help but wonder if a changeout would help.

Need to do a longer highway run and calc the mileage again to see what it can really give, first.

Also wondered if timing can be advanced slightly for better low end grunt. This is a popular mod on my Miata, but it's an OBD1 vehicle. Not sure you can do this with an OBD2 vehicle. Whaddayathink?

Nope. Ignition timing advance is all computer controlled on the 5VZ-FE.

 

[ThorInc]

Highest contributing factors in my opinion (others have also shared these):
..................................................Some I've done is switching to synthetic fuel, ...................

Needs to get me some of that! :lmao:

 

[St Runner]

Bstrom, If you have more than 150k miles just replace it, you will probably never get a code it will just get less accurate over time. Your MAF and the O2 sensor are the two most important sensors for getting proper fuel mixture. And spruce is right, you don't rely save much gas when you coast in neutral. On my ultraguage as soon as engine braking exceeds 1500 rpm it goes out of closed loop and the engine load drops from 14% to 7% Essentially using the same amount of fuel than at idle. You will be using more fuel if you let the rpms up higher, but if they get that high you will be thankful to have it in gear

 

[MyHigh]

I think the terrain you drive and how hard you drive are the biggest factors. That is the biggest reason the ect gets better milage for some but not others. The ect is a non factor on highway driving and effective in steep terrain. I feel the 3.4 runs best in the 2-2.5k rpm range (the 2.7 about 500 rpm higher) and when I had the auto in, even with the ect on it would upshift at a lower rpm than what I wanted. Thankfully with the 5 speed I can tell my 4runner when to shift and not rely on a 20 year old computer that was not designed for the roads I drive. OP I would replace the front AF/O2 sensor if you have more than 150k miles, it is the only thing left that can really effect your mpg

I would have to agree, although my roads are most likely better en those in V.I.
My runner rarely see RPM's higher then 3-3,500 RPM's. Whether that's good or bad I'm not sure, but I have a feeling the PO was a timid driver and so I don't like to push it. I'm really surprised about oil, I still do 5w-30 and see my mpg drop when the oil gets old beyond my normal 3,000 change. It drops a couple mpg but once changed, it's right back up.

 

[7echo]

I get 15.5 on avg in city driving.

Checked it many times, usually use the same pump, same station.

Changed the O2 sensor, no difference.

Ran 6 tanks using ECT as a test and showed no difference.

Changed plugs, air filter, fuel filter, cleaned MAF, re-set ECU, no difference.

I have not used the T4R for a road trip so can't comment on that mpg yet.

 

[jeremyc74]

Thanks Jeremy - is there any way to determine that front O2 sensor's condition? I've gotten no codes for anything since owning this rig. I'm doubting now that it's making any trouble but with 190k on this vehicle can't help but wonder if a changeout would help.

Need to do a longer highway run and calc the mileage again to see what it can really give, first.

Also wondered if timing can be advanced slightly for better low end grunt. This is a popular mod on my Miata, but it's an OBD1 vehicle. Not sure you can do this with an OBD2 vehicle. Whaddayathink?

I'm not sure if there's a good way to test the O2 sensor without installing a wideband to verify the reading. I'd probably just go ahead and replace it. My front sensor was dead when I bought my truck around 170k, and the rear sensor died around 190k. That seems to be pretty typical from what I've seen here, so you probably don't have much life left in it anyway.

If you do them, make sure and get the Denso sensors. They're the OEM supplier and are a direct fit, but you can buy them from Rock Auto or Amazon for about 1/3 what the dealership wants for them. People have had some issues with other brands. You want to be careful about the part numbers, as they vary depending on year, 2/4WD, and whether it has CA emissions or not. If you have a CA emissions truck you'll have two converters under there, and it will say so on the tag on the underside of the hood too I think.

 

[gamefreakgc]

Needs to get me some of that! :lmao:

Forget the biofuel movement!

I was thinking faster than I could type.

 

[TXLX1]

MPG varies wildly for me in 1997 4runner, lifted. The variables are weight on-board, speed, wind, and terrain. I've gotten 14-23 mpg at the extremes (doesn't include trail riding and offroad traveling).

Running 60 mph with the truck empty, flat land, and no wind... I'd expect around 20mpg.

Going 70-90 mph with 600lbs of gear and 2 people... I got about 14mpg last weekend.... Which isn't all that bad considering that's a good day for my 100 series running empty.

 

[gio-geno]

fuel economy

Hello
has any of you tested what is fuel consumption, when driving downhill without accelerating or braking, on 3.4 V6 A/T ?

 

[St Runner]

I run my ultraguage with one window on engine load. Normal idle is around 14% When going down a hill using the engine for braking, as soon as the rpm's climb over 1400 the engine goes out of closed loop and the engine load drops to 7%. This is the same for both the 5 speed I have in it now and the auto I had in it originally. The open loop for the auto was slightly higher at 9%. There is no way to factor the mpg for it because it is a pretty quick thing

 

[DaMonkey]

Hello
has any of you tested what is fuel consumption, when driving downhill without accelerating or braking, on 3.4 V6 A/T ?

I have not tested this personally, but just using intuition, it depends how steep the hill is.

First of all, in an AT truck, you will need to keep the motor running so your torque converter will not lock up. That is what is going to hurt your mileage the most, but on a steep hill you will go further.

To really improve things, put your transfer case in neutral and unlock the steering so you can just shut the motor off and point the truck to where you’re going.