MPG with late Gen 2 Montero + larger tires?

[All-Terrain]

I am currently running 31x10.5x15" BFGoodrich AT KO's on my '97 SR --- this has the later SOHC 3.5, lower rear end gear ratio, etc.

What sort of MPG would you expect me to get on this setup?

I am currently getting in the region of 15mpg on the highway, using 93 Octane and cruise control as much as traffic will allow, running around 75mph, relatively flat roads.

I ask because I had thought about going slightly taller on the tire size, but I do need to retain some level of practical fuel consumption, because I drive this truck at least 60 miles a day. I feel like I am filling up quite often as it is.

Previously, I had a '99 4Runner 3.4 V6 5-speed, with 265/75/16's (32's), and it ran right around 18.5 mpg, same usage, with 87 Octane.

 

[eurosonic]

Im getting 14-15 avg with 35s and 87octane

 

[SoCalMonty]

I'm OCD when it comes to calculating my fuel mileage.

I did 4 top-offs on the DV trip. With each fill and odometer reset, I got the following:

19.19 mpg (filled before I left, had 1/4 tank remaining from a big bear trip)

19.22 mpg (new best...topped off in Trona, roughly 200 hwy miles at 55-60mph)

11.4 mpg (all dirt, off road)

17.65 mpg (filled after the drive back to OC)

...in-town stop and go driving is usually mid-14's mpg.

'95 SR on 33's with a tire and fuel cans on the roof. 89 octane. I rarely exceed 60-65mph.

 

[Toasty]

I get 15-16mpg in town on 35x13.5's full armor, manual hubs and 4.90's, best i've gotten on the highway was 18mpg flat ground no wind.

 

[All-Terrain]

Thanks for the info. From the sound of it, you are all getting similar MPG to me, but with less factory trucks. So my MPG is probably 1-3 MPG less than it should be...?

Where do I start to address that? My truck has been dealer-serviced since it was new, I have records going all the way back. Nothing significant in there that would point to an issue.

Maybe I should start with replacing oxygen sensors? Other older cars I have had benefited from new oxygen sensors, and I believe mine are old.

Also, I don't know if the previous (first) owner used 87 Octane out in California. Maybe I should be? I would think the ECU would recognize the higher octane fuel, especially since it is recommended for our trucks.

 

[jeep-N-montero]

Thanks for the info. From the sound of it, you are all getting similar MPG to me, but with less factory trucks. So my MPG is probably 1-3 MPG less than it should be...?

Where do I start to address that? My truck has been dealer-serviced since it was new, I have records going all the way back. Nothing significant in there that would point to an issue.

Maybe I should start with replacing oxygen sensors? Other older cars I have had benefited from new oxygen sensors, and I believe mine are old.

Also, I don't know if the previous (first) owner used 87 Octane out in California. Maybe I should be? I would think the ECU would recognize the higher octane fuel, especially since it is recommended for our trucks.

You are experiencing the same issue as my 98 that's only seeing about 320-330 miles out of a full tank, I do plan to replace all of my o2 sensors when I upgrade the exhaust next month. As for the ECU recognizing the higher grade fuel, you can either wait awhile for it to go through more drive cycles or simply disconnect the ECU so it can start fresh.

 

[All-Terrain]

You are experiencing the same issue as my 98 that's only seeing about 320-330 miles out of a full tank, I do plan to replace all of my o2 sensors when I upgrade the exhaust next month. As for the ECU recognizing the higher grade fuel, you can either wait awhile for it to go through more drive cycles or simply disconnect the ECU so it can start fresh.

Gotcha.

Which oxygen sensor(s) have the most "priority" within the loop? In other words, will you see more benefit from replacing one particular oxygen sensor versus another? My truck, as a CA emissions truck, has four sensors. So while I will plan to replace all four, I will probably do one at a time every couple weeks.

Also, with regard to the ECU... do I have to disconnect the ECU itself, or can I just disconnect the battery for an hour or two, and then re-connect?

 

[jeep-N-montero]

Gotcha.

Which oxygen sensor(s) have the most "priority" within the loop? In other words, will you see more benefit from replacing one particular oxygen sensor versus another? My truck, as a CA emissions truck, has four sensors. So while I will plan to replace all four, I will probably do one at a time every couple weeks.

Also, with regard to the ECU... do I have to disconnect the ECU itself, or can I just disconnect the battery for an hour or two, and then re-connect?

You can disconnect either, I know the previous owner of my 98 ran regular gas and when I reset my ECU and ran premium I felt a difference, just make sure you fill up with premium before the reset. As for the sensors, one at a time sounds the logical way to do it on a budget because they aren't cheap. Best way to do it is look them up on Rock auto and get the part number, then plug the part number in on ebay to see who is cheaper. I saved a ton of money doing it this way.

 

[All-Terrain]

You can disconnect either, I know the previous owner of my 98 ran regular gas and when I reset my ECU and ran premium I felt a difference, just make sure you fill up with premium before the reset. As for the sensors, one at a time sounds the logical way to do it on a budget because they aren't cheap. Best way to do it is look them up on Rock auto and get the part number, then plug the part number in on ebay to see who is cheaper. I saved a ton of money doing it this way.

Cheers, thanks!

 

[SoCalMonty]

All T,

It really doesn't sound like your O2 sensors are the culprit. The pre-cat sensors are the primaries. Post-cat are for minor trim. You can also check their voltage output/cycling with an OBDII reader which will give you an idea as to whether or not they are working properly.

That said, it sounds like driving style is the main reason for your (almost idendical to EPA) fuel mileage. At 75mph on the freeway, I'll pull about 15.5-16 mpg on 33's...which is on-par with 15mpg on your 31's. If your fill up average is 15+ mpg, you're right in line with what the vehicle is supposed to be getting...unless, you live on a freeway onramp and 100% of your mileage is highway, no stops or lights, and always at 60-65mph. If those were your conditions, I'd expect closer to 18.

 

[nwoods]

33x10.5, no lift, stock gearing, 18-20mpg. Quite amazing really. My turbo Volvo gets only 21 on the freeway.

 

[All-Terrain]

33x10.5, no lift, stock gearing, 18-20mpg. Quite amazing really. My turbo Volvo gets only 21 on the freeway.

I am definitely not getting 18, and 20 would be a true miracle. I would love to get 18.

 

[mapper]

Have you tried a new air filter? Or any fuel injector cleaner? Are you nearing the time for new plugs?

A clean air filter is a cheap no-brainer in my opinion. Your dealer servicing may not hit the filter frequently enough. I live in the dusty west and change my air filters every 10k or so. Also, I always run a bottle or two of Techron through my cars every year or so (old habit from the days of Bosch mechanical fuel injection). In any case, I almost always see a slight bump in fuel economy after I run through the treated tank. Unfortunately plugs are kind of job on these trucks but not terrible, I tend to change plugs in my cars when I see some reduction in fuel economy.

I have also had good results from changing pre-cat O2 sesors. On this vintage vehicle I don't think the rear does anything other than monitor the changes from pre-cat levels, it merely tests the efficiency of the cat and throws an p0420 if cat is degrading. If you truck is starting to burn any oil from age (valve guide seals) the O2s may be starting to suffer.

edit: By the way everything I've noted above is experience with other cars. I haven't had my montero long enough to draw any fuel economy or maintenance conclusions specific to it.

 

[All-Terrain]

Yep, new OEM air filter and plugs.

I will run a dose of Marvel Mystery Oil through it every 5k miles.

I was thinking about replacing the fuel filter as well, since I believe it is original - not 100% sure on how old it is, though.

My experience is the same as yours, regarding Montero's and other cars. I have had a number of cars, I do my own maintenance as much as possible, so I absolutely get what you're saying.

 

[jeep-N-montero]

Yep, new OEM air filter and plugs.

I will run a dose of Marvel Mystery Oil through it every 5k miles.

I was thinking about replacing the fuel filter as well, since I believe it is original - not 100% sure on how old it is, though.

My experience is the same as yours, regarding Montero's and other cars. I have had a number of cars, I do my own maintenance as much as possible, so I absolutely get what you're saying.

Marvel is not as good for fuel injected vehicles as you might think, Lucas or Techron are far better.