99 Montero Gen 2.5 Fuel Efficiency Issue

[TheMole]

What i said earlier was all in 2wd. Didn't leave the pavement during the last trip I took

Ah ok. Have you checked the pressure in your tires lately? Also, not sure if the gas refineries have changed over to the winter-blend, but it usually yields less mpg. For reference, I got 14.9 mpg driving about 60% city on the first tank that I've logged on fuelly.com. The car was originally rated at 16/19 so don't think it's too bad.

 

[BayMonty]

Bumping this up.

I’m having the same concerns with my 97’ 3.5L. When I first bought it completely stock at the end of February, I filled it up immediately after purchasing and drove 98 miles home in the SF Bay Area...evening, no traffic, probably averaged 75mph. The gas gauge was a hair above the quarter tank when I got home. I new I could get just a tad over 100 miles before it was directly on the quarter mark.

If we have 24.3gal tanks, that’s about 17mpg.

Over the next few days, the fuel economy stayed true. I commute 26 miles round trip on the freeway. About two weeks into ownership I do all the major maintenance items (timing belt, water pump, spark plugs and wires, etc.). Prior to maintenance the Montero had no guts above 3000 rpm. After maintenance it didn’t get much better. No CEL’s, but the Brake light has been on since purchase. Hoping to get that sorted today. About a week ago I ran over a metal object on the freeway and it sliced the ABS sensor wire on the right rear causing the ABS light to come on. New sensor on its way. Other than these things, it’s running smooth.

A few days after all the maintenance (mid-March) I put on 31x15 BFG KM2’s. I notice fuel economy drops to about 15-16mpg.

Not worried too much.

Two weeks ago I put on 32” M/T’s. Now I’m getting about 75 miles per quarter tank. Which is about 12-13mpg. Tire pressure is good, running smooth, temp is good, no major issues popping up. Just need to put on the new abs sensor at right rear when it comes in and either clean the brake sensor contact at the master cylinder or replace entire unit. Nothing that should be dropping my fuel numbers like that.

I was running the 31’s for only about a week before I put the 32’s.

Thoughts?

 

[BayMonty]

Are you calculating around the tire size?

If your tires are 9% larger than stock, then you've actually covered 9% more miles than your odometer indicates (if you haven't otherwise compensated with gears and such) - if your odometer says you went 100 miles and you used exactly 6 gallons of fuel that's going to look like 16.6mpg but really you went 109 miles and got 18.1mpg..

Going by positions on the fuel gauge is kind of meaningless too because the profile of your fuel tank isn't continuous and even if the float or gauge is built to somewhat compensate for this the key word is "somewhat"

If you want to calculate real MPG you'll need to use GPS and fill-to-fill fuel volume - or at least zero out the calculations for your tires and gears vs. stock and then use fill-to-fill fuel volume.

These vehicles aren't particularly aerodynamic but it certainly gets worse when they have crap loaded on the roof, accessories bolted all over the outside, lifts and tires that expose more of the non-smooth bits to the wind. Tires themselves are not at all aerodynamic and they're heavy, and it's rotating weight... tires can damage fuel consumption in a huge way even after calculating out the difference in circumference.

To ensure the vehicle is running as efficiently as possible, properly adjust your TV cable (this makes a significant difference if it's too slack or too tight, both turn gasoline into heat needlessly) and consider everything in the emissions system a maintenance item - that includes oxygen sensors if they're two decades old they're dead, CEL or not.

Just completed a half tank test. Last week I brought it down to touching the Empty mark. With a 24 gallon tank, I filled it half way (12 gallons). Interestingly, the needle stopped at a hair under 3/4 full after putting in 12 gallons.

I am back down to touching the Empty mark and I’ve driven 187 miles. It’s right where I should be as far as fuel economy.

 

[Salonika]

Just completed a half tank test. Last week I brought it down to touching the Empty mark. With a 24 gallon tank, I filled it half way (12 gallons). Interestingly, the needle stopped at a hair under 3/4 full after putting in 12 gallons.

I am back down to touching the Empty mark and I’ve driven 187 miles. It’s right where I should be as far as fuel economy.

When I do this type of test I do it based on a full tank because I trust the pump shutoff more than my gas gauge. Fill it up until the trigger pops once. Zero out the trip odometer, drive as far as you want, then fill it up again to the first trigger pop. Odometer reading / gallons = mpg. I think there is less error this way than trying to base it on needle position.

 

[BayMonty]

When I do this type of test I do it based on a full tank because I trust the pump shutoff more than my gas gauge. Fill it up until the trigger pops once. Zero out the trip odometer, drive as far as you want, then fill it up again to the first trigger pop. Odometer reading / gallons = mpg. I think there is less error this way than trying to base it on needle position.

That’s a great tip! I’ll definitely try that next.