Substancial MPG improvement

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Jul 31, 2006
I have a 2002 Civic EX, automatic with ~79k miles. I've changed the oil about every 3k miles wherever I could get the best deal. I'd also buy the cheapest gas, but I would regularly use gas treatment (STP mostly). Last year, I bought a house so now I do the oil changes myself in the garage. I'd always used Fram filters and cheap Wal-Mart SuperTech oil, 5W20 grade dino. I started tracking my mileage about when gas prices were close to $3/gal in early May. I also switched to using Chevron gasoline. At the time, I was doing a lot of highway driving, and I was getting around 33-34 mpg with the A/C on about half of the time. My driving pattern changed after a couple tanks, and I was now getting about 29-30 mpg for short 10-15 miles trips, 3/4 highway, 1/4 city driving with the A/C on most of the time. I had a set of fairly new Bosch Platinum Splitfire plugs in for this period until just recently. My second to last oil change was Castrol GTX 5W20 with an OEM filter (I was smartening up a bit after reading forums like this).

About two week ago, I did a bunch of stuff to my car because I was unhappy with my mileage. First, I changed out the Castol GTX, flushed the engine with some fresh left over oil that I had laying around, and dumped in some M1 5W20 along with a can of Lubro Moly MoS2. I started it up, and it almost instantly felt smoother that ever before. My model of Civic is known for pathetic noise insulation, so I could easily hear the difference. It was especially noticeable when accelerating in first and second gear- smooth as butter. (Aside- I didn't like how the transmission was feeling at the time, so I drained and filled with Honda ATF, which made it a little snappier between gears.) I also changed out the Bosch plugs with a set of OEM plugs (very expensive!). I did not notice a diffence in how the engine ran with the new plugs, and the old ones didn't look too bad (only 7k miles on them). The last thing I did was to fill up the tank at a different station (Valero) in the morning (b/c I read that gas expands with temperature, and it's been over 100 a lot lately). It's also the cheapest gas around. That tank gave me almost 36 mpg, for a solid 20% mileage improvement over a very consistent 29-30 mpg for 3 months. I had the A/C on most of the time and my driving habits haven't changed at all.

Does anyone care to speculate what change(s)contributed the most to my mpg increase? I feel that it's mostly due to the moly.

Originally posted by dek:
Does anyone care to speculate what change(s)contributed the most to my mpg increase? I feel that it's mostly due to the moly.

Sure, I'll speculate. First off it is hard to get an accurate MPG measurement for just one tankful, especially on a small gas tank like in a civic. I would guess the accuracy of a single tank measurement to be no better than +/-5 or 10%. Also if you want to see a fuel economy increase, it is hard not to subconsciously change your driving style.

The Mobil 1 oil you installed is probably thinner than the GTX you drained out, which should result in slightly better fuel economy. Different gas station might contribute (especially if one has ethanol & other doesn't), filling in the morning might help, new plugs might help. Fresh ATF might have helped quite a bit if the old stuff was causing your tranny to shift funny or not lock up. You said you were having problems with it. I doubt the moly has anything to do with it.

I would rank the probability of contribution as:

#1. fresh ATF or different gas or both
#2. subconcious change in driving style
#3. new plugs
#4. fresh M1 oil
#5. everything else
#6. magic
#7. moly

Just my opinion.
I agree with Leo, dek.

Track your mileage bcarefully for about 4-5 subsequent tankfuls. When the average of all the tankfuls shows an improvement, you know you have something.

For other minor things you could do, run more air in your tires (but never exceed the sidewall maximums) and change your air filter (I didn't see it mentioned above).

Plugs made a big improvement in my '95 Civic DX. I wouldn't run anything in a Civic except NGK V-Powers. No platinum plugs.

You may also be due for new wires. I'd go OEM or NGK ... or possibly Accel.

With all the changes you made, narrowing down the most significant one may be impossible ... but maybe still worth discussing.

--- Bror Jace

Originally posted by Bror Jace:

I wouldn't run anything in a Civic except NGK V-Powers. No platinum plugs.

You may also be due for new wires. I'd go OEM or NGK ... or possibly Accel.

--- Bror Jace

Aren't Iridium better than platinum for heat transfer and longevity? That's what Honda's run these days from the factory.
These Civic's don't use wires; each cylinder has it's own ignition coil attached directly to the plug.
Wow, thanks everyone for the thoughtful comments.

The plugs are Denso platinum. They're about $15 each shipped, and they're supposed to last 110,000 miles (I didn't know this before I changed the originals out). They look exactly like the NGK platinums for whatever that's worth. If I feel ambitious, I might try the NGK V powers, but the one's I have are specific to my model (VTEC). I think that the temperature rating might be higher (?). The wires are different than what I'm used to dealing with, because the coil is at the plug end. Do these wires still need to be changed?

The transmission works fine. There was a slight slipping between gears, and I attribute this to a quart or so of cheap Dextron III ATF that was in there. About 7k miles ago I flushed it with 5 quarts of it before cycling in Honda ATF for a few drain and fills. Now there's about 0.4 quarts (out of 6.2 total) of Dextron III in there. The drain plug magnet had a thin coating of very fine metal particles. I saw the same thing the last time I changed it. Is this normal?

The tires (Michelin Harmonies) are kept at 36 psi, and I check them every week in the morning. I think they're rated for 44 psi max. Should I go higher than 36 psi?

The air filter is OEM and about 4 months old(seemed to be soaked in oil). The oil filter is is M1, because I got the deal at AutoZone. I forgot to mention that I just changed my PCV valve at 79k miles. The old one was a little clogged. It was the most expensive PCV valve that I ever bought- $23 at the dealer, and it wasn't in stock either. What a racket. To be absolutely complete about my modifications, I replaced the old oil drain plug with a magnetic drain one with the switch to M1.

I will run a 2-3 more tanks of the same gas and post an average with standard deviation. I have many samples from the old setup to make a good estimate for the Chevron gas. I'll switch back to Chevron for a few tanks to see what it looks like for the new setup. I don't put many miles on her, so it will take about 3 months for the final verdict on the gas. It really want to try FP60, high MW PIB, and or acetone at some point too.

One last thing. When the car is idling, I notice that the tachometer jerks a little randomly and the idle is just a wee-bit rough. I put Chevron injector cleaner in there a few tanks ago (when I was getting 29-30 mpg, with no improvement in the idle or mileage). I might have a slightly clogged injector, or maybe one of the O2 sensors is going. It's so minor that most people would ignore it until getting about 3x worse. Suggestions are greatly appreciated. It runs just fine otherwise. I don't do this very often, but I ran her into the VTEC RPM range after dumping in M1, new ATF and plugs, and it felt good.
Hot weather also helps. Hot air is lighter, so it's easier for the car to push it out of the way. High altitude driving helps even more.
A rough idle might be a dirty Idle Air Control (IAC) valve. But it's probably just crummy gas. I've been using Chevron 89 with FP60 since the beginning of the year, and sometimes there's a slight stumble in the idle. But overall I think it's gotten better with long term use of FP60. I don't think it's realistic to expect any one shot of in-tank FI cleaner to work: it's continuous use that prevents any build-up.
I've never heard anything good about using Bosch plugs in a Honda product, so stay with stock.
But after all is said and done, I think your biggest improvement in MPGs (on a normally running car) comes from a light foot.
Now there's a good point. The temperature for my last tank of gas was quite high, over 100 for 11 days in a row, including a couple record highs. I'm close to sea level, so the temperature probably makes a big difference in air resistance.
Yes, that would be true. At sea level, the air is thicker, so changes in temperature would make a bigger difference than at high altitude. Humid air is lighter than dry air. Speed would be a bigger facator at sea level, too.
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