Unexpected MPG Increase

[Brian Barnhart]

According to the onboard computer in my 2006 GMC Sierra, my MPG has suddenly increased about 5% (1MPG). I drive the Sierra about 24 miles (one way) to work (mostly 55 mph rural two lane) and there have been no changes in the route or the way I drive. The increase seems to have begun a couple of weeks ago just after performing some minor maintenance. Therefore, I think it is probably related to the maintenance.

However, all I did was:

1) Run through 20 gal of Techron treated fuel before the oil change (I typically do one before an oil change).
2) Changed oil (same oil as before (5W30 Quaker State dino)
3) Different oil filter (Bosch this time vs Pure1 before)
4) Added about 2-3 oz of Auto RX (never done before)
5) Changed the front axle fluid from factory 80W90 to Mobil1 75W90.

The only other thing that might have changed somewhat (though it’s hard to be sure) is it seems we’ve had somewhat less wind lately. And I have seen wind affect the MPG on this vehicle in the past. During windy spring months, I can loose 1/2 MPG or so on average. And in the winter, the mileage will drop 1+ MPG (to about 18.3 MPG). However, the jump I’m referring to above is a sudden one occurring in the middle of summer. Driving to work (for over a year now), I’ve never averaged over 19.7 MPG in this vehicle. However, the computer now reads 20.4 and is steady to rising. In addition, I’ve noticed no difference in the engine’s running manners.

Let me provide just a little more info on the MPG “measurements”. All the measurements I’m referring to are according to the vehicle’s on-board computer. I’ve checked it several times, and it seems consistent, and reasonably accurate (though a little optimistic, “adding” perhaps 0.3 MPG consistently). When reset, the computer’s calculated MPG typically settles down to a steady value within about 2 days of my routine driving to work. I usually reset it every 2 to 3 weeks, and it’s been about two weeks since its last reset.

Given all the above, I’m thinking the sudden MPG increase must be due to the front axle fluid change. Does a 5% MPG increase seem like too much of an increase for that change? I’ve never seen a significant effect on MPG from a Techron treatment (on this or any other vehicle). And it certainly can’t be due to a different oil filter. Does a small maintenance dose of Auto RX typically increase MPG? (Perhaps I should also ask this question in the Oil Additives section).

 

[MysticGold04]

I would suspect that the M1 was the cause for the MPG jump... less friction. I noticed the same thing when I swapped out my factory fill diff fluid.

 

[Donald]

I would think the diff. fluid is not the answer. I could see it giving you a slight MPG effect but not 5%. The manufacturer would certainly want to bump up their CAFE numbers and if it only took a swap of diff. fluid to do it?

I would guess its either the Auto-Rx cleaning the ring packs providing better compression or your onboard computer is not very accurate.

How many miles on the vehicle, how long after you added the Auto-Rx did you see the MPG increase? Have you verified the MPG increase as seen on on-board computer with a full tank to full tank MPG check.

 

[salvageyardteen]

I would suspect your computer. Depending on how bad the fuel system was the Techron might help. Of course driving habit can be part of it.

 

[Steve S]

Doubt any oil will increase mpg as stated. M p g is a funny thing.

 

[unDummy]

Combination of everything.
5% is too big of a jump for any single variable.

Try a synthetic engine oil, thin ATF, synthetic PSF, synthetic rear gear oil, synthetic x-case fluid, more tire PSI, and see if you can squeeze 22mpg out of that.......

 

[ConfederateTyrant]

All of those variables can increase MPG, depending on the condition of the equipment and previous fluids. If you had worn out, or thicker Xw-90 gear oil, then a fresh fill with a synthetic 75w-90 is a major improvement. If there was any kind of junk on the gears, the Mobil 1 could have cleaned it off and increased the efficiency. The Techron could have cleaned out the fuel system and increased its efficiency. Auto-RX could have cleaned the ring packs and given better compression.

With a combination of all, it is easy to see a 5+% increase, but as I said, it could have had to do with improving the equipment condition.

 

[Bill in Utah]

Figure out the TRUE MPG over 3-4 tanks using a pen / paper with the miles driven and gas used then report back.

My scan guage is closer to the real mpg, but the computer in the car is not real close (1-2 mpg off) to old school.

I've never seen MPG improve with any oil any where. Pumping the tires up a couple of lbs over the recommended point will get a 1/2 mpg... Maybe.

Bill

 

[Titan]

I'm with Bill here. The biggest difference I see from tankful to tankful is SPEED. Second biggest difference is a tie between temperature and wind. I keep track of EVERY tank on my cars, and have put over 40,000 on one in less than a year, and another 15,000 on the other. I use the SAME oil and run the SAME routes over and over on the one I've driven over 40000 miles, and the mileage varies between 26-35 mpg. Average is just a hair under 30 mpg (which includes some city driving). Usually get 31-32 on it if I keep the speed below 75. It dropped to 26 into a cold wind in the winter, when I was going over 80 mph, carrying a passenger and our luggage. I'll run Fuel Power, or not. Kroger gasoline or Shell. Don't see any statistically significant difference related to anything other than speed, temperature, and wind direction. Tire pressure would be a variable...but, I check it before every trip and keep it constant.

Just be careful reading too much in to a couple of tankfuls.

Oh, my computer usually overstates the manually-done mpg by an average of 1.6 mpg. Although occasionally, it is almost right on. Just too many variables....consider the variable of the accuracy of the fuel delivery pump at any given station, and you'll see it's very hard to be accurate.

 

[mechtech2]

You performed helpful maintenance, and saw an improvement.
For instance, if one injector was partially plugged, the others would be richer, as the O2 would read the average. Now, things are even and stable - all cyls working the same.

 

[bmwtechguy]

When I've had ARX in the crankcase, I have seen consistent, measurable increase in fuel economy, that went back to normal after drained out. I think that stuff somehow reduces friction in the engine while doing its cleaning thing...sort of a multi-functional additive.

 

[INDYMAC]

It's the wind (especially on a truck). I've noticed this to affect MPG up to 20% on my annual pilgrimmages from Texas to South Dakota each fall. I always hope for a good tail wind, but always seem to get a stiff crosswind or head wind.

 

[Spike T]

I think you need to study this a bit more before coming to a conclusion. Many tankfuls over some time, with care being taken to observe weather conditions, drive similar speeds as much as possible, and manually computing miles driven/fuel used. Preferably, using the same pump for fuel if possible. I compute all my mileage manually, and I can get exact with fuel fill, as my Ram/Cummins allows me to fill to the same spot in the filler neck all the time.

 

[InvalidUserID]

After driving the same route everyday the for the past seven years in my 3800II engine and calculating after every tank, I found that the biggest factor in my fuel economy was speed. Oil changes (different brands & types), fuel cleaners, ARX and others made very little difference compared with driving 65 all week, 70 all week, and mixed.

This is over a course of seven years and over 100,000 miles.

 

[Titan]

Quote:

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After driving the same route everyday the for the past seven years in my 3800II engine and calculating after every tank, I found that the biggest factor in my fuel economy was speed. Oil changes (different brands & types), fuel cleaners, ARX and others made very little difference compared with driving 65 all week, 70 all week, and mixed.

This is over a course of seven years and over 100,000 miles.

---

THAT's what I'm talking about! I agree 100% with your assessment. There's just too many variables to "know" after a couple of tanks full, precisely what, if any, is your resultant change in MPG.

 

[Brian Barnhart]

One poster said it probably wasn’t the change to synthetic on front differential lube because GM would use synthetic if they could boost the CAFÉ numbers. Do the front wheels of a 4X4 rotate when the MPG is measured for CAFÉ? If not, there would be no gain on CAFÉ with synthetic fluid.

I’ve never once achieved over 19.7 MPG in the last year of driving the same route at the same speed. Yet for the last 4 weeks, the computer has settled at 20.3. I will continue to monitor and check, but assuming the computer, fuel sensor, and/or odo haven’t gone haywire, I’m thinking one of the “changes” had an impact since the sudden MPG increase coincides with that service. It seems too much of a coincidence to never see over 19.7 for a year and suddenly go to 20.3 after the service to claim the increase is a result of wind or weather. Since the truck only has 13k on it, I’m thinking it was either the M1 gear lube or the Auto-RX, or possibly a combination of both.

 

[Titan]

Quote:

---

One poster said it probably wasn’t the change to synthetic on front differential lube because GM would use synthetic if they could boost the CAFÉ numbers. Do the front wheels of a 4X4 rotate when the MPG is measured for CAFÉ? If not, there would be no gain on CAFÉ with synthetic fluid.

I’ve never once achieved over 19.7 MPG in the last year of driving the same route at the same speed. Yet for the last 4 weeks, the computer has settled at 20.3. I will continue to monitor and check, but assuming the computer, fuel sensor, and/or odo haven’t gone haywire, I’m thinking one of the “changes” had an impact since the sudden MPG increase coincides with that service. It seems too much of a coincidence to never see over 19.7 for a year and suddenly go to 20.3 after the service to claim the increase is a result of wind or weather. Since the truck only has 13k on it, I’m thinking it was either the M1 gear lube or the Auto-RX, or possibly a combination of both.

---

Or, since it has only 13K on it, maybe it is finally "loosening up" enough to have less internal friction?

 

[Loobed]

I got a big increase in "highway" MPG when I switched from 80W-90 to Amsoil 75W-90 synthetic gear oil in the front differential of my 2001 GMC sierra Z71. I think it improved by 2 mpg. But that was at night when the air was cold, so it may have helped a little. I was also using 0W-30 Castrol at the time. City milage didn't improve a noticable amount.

 

[Loobed]

another "funny" possibility is that when your tires wear they get smaller, so it looks like you are gaining milage. A 1/2 inch off the diameter could make a little difference in the speedo/odometer