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Erratic / Unpredictable Gas Mileage Mystery

Ok. I’ll start at the beginning - I do know that all cars get erratic gas mileage for all sorts of reasons - but this is

weird. And, apologies - in order to ensure you all that I’ve tried to think of everything - this is somewhat long.

On a regular basis I drive the exact same route to and from work over the exact same roads - about 150 mi round trip. About 90% of the route is on an interstate with the cruise control set - I don’t touch it unless I have to. The other 10% is almost all rural highway. The route is so predictable and consistent that I can time it to the minute. Despite that consistency of driving, sometimes I get as low as 29mpg. Sometimes as high as 36mpg. I also taken into acct whether I’ve been running the compressor (A/C or defrost on). That’s not it.

The details:

Car: 1997 Ford Escort w/ 210K (2.0L, 4cyl, Auto Trans - 4 sp w/ OD).

Mileage Calc Method: fill tank, set trip OD, fill tank and do the math. I am a gas mileage junkie - I do this for almost every single tank of gas, so I’m not just looking at a one time quirk. I’ve also even checked my trip OD against mileage markers on the hwy.

The list of normal things that don’t explain it because I do regular maintenance and/or have changed/fixed:

- tire pressure and condition (I even pay attn to air temp since on cold days I’ll have less than when it is warmer)

- air filter

- plugs/wires/ignition coil pack

- alignment

- fuel filter

- O2 sensors

- dirty injectors

- fuel evap system

- clean MAF

- clean engine oil

Basically, pick your list of “top ten” get better gas mileage tips and I do them.

Here’s one kind of thing that would make sense, but I really don’t think it is it - for a while I was getting a Torque Converter Clutch lockup code that would come for a few days and go. I dropped the pan/filter a couple of times, and no more code. (And my MIL does still work). But the reason I really don’t think it is it is that the TCC seems to function perfectly normally. Going by the little RPM effect it has, it locks when I’m up and cruising and unlocks when I change speed. My hwy speed is so consistent day after day that I would know - based on RPMs if the clutch were stuck off.

But - I think I may be looking for something like this. Something that sometimes works and sometimes doesn’t. But something that would have no noticeable effect on how the car runs, since it always runs just fine.

I’ve reached my limits on figuring it out. Any takers?

After dealing with a bad electrical connector on an overdrive transmission with a lock-up torque converter, I can say unequivocally that a TCC problem will easily explain the problem. When I had this connector problem, the TCC would drop in and out, and occasionally it would drop out of OD. When it was working good, I was getting 24 MPG with the V6. When it was acting up, the gas milage dropped to as low as 16 MPG. After going around and around with transmission shops insisting on re-build before doing any diagnostics, a gear-head friend of mine came to town and helped me figure it out. We fixed it by splicing in a salvaged connector I got for $5.

You may have noticed that the Escort isn’t the most well engineered car on the road. Your car isn’t like a laboratory test car that somebody could use to test gasoline. It could be really crazy like atmospheric pressure from day to day, or even wind speeds. Throttle sticking, temperamental bearing, cat loading up, brake calipers, parking brake and you may never find the cause. I think the high gas mileage is due to a computer glitch.

Actually, you need to figure a couple of other factors into this: are you absolutely sure you are the only one driving this car. I know that sounds funny but we were all teenagers once who waited until their parnts fell asleep. Yes, I know this is a long shot. The other (More probable) factor is that you are at the mercy of the quality of gas you are buying from the gas station. Do you always put the same octane in the tank? and, you really have no control over what kind of additives the station puts into the fuel. You don’t even know what happens to the fuel between the time it leaves the refinery to when it reaches the station. I used to drive a fuel delivery truck in LA. Trust me, you really don’t know - and wouldn’t want to know what happens in transit.

To record how you are driving, you could use something called a CarChip. It will record your speeds, stops, time, and whatever group of 4 parameters you choose. This way you could see if the difference in mileage is in your driving. It will record the trip data for whoever drives the car.
The CarChip plugs into the OBD connector under the dash. You download it to your computer. Google for it: DriveRight CarChip.

I will add a couple of possibilities.

First you did not say, but if the differences relate to time if year, you may be getting winter gas vs. summer gas. They will give different mileages. Different temperatures will give different mileage readings.

You may be filling the tank to slightly different levels. How often to you fill it (when it is half empty nearly empty just a little towards empty.)? When do you stop filling it? When it first clicks off (this is what you should be doing)? If so it may be clicking off at different times. Don’t try topping off the tank as that can damage the vapor recovery system.

Thanks much for all of the suggestions. Unfortunately I’m trying to come to terms with the “you may never know” response. Though I will say that the glitch is the low gas mileage. I know that most are skeptical of the EPA ratings, but I’m not. As I noted, I’m an mpg junkie and always do all I can to maximize, and every vehicle I’ve ever owned meets or exceeds EPA (If it doesn’t I do all I can until it does). The EPA on this car is 34 hwy.

Luckily my kids are still 9 & 11 so they’re not joy riding and my wife hates the car. And I figure if anyone was stealing any gas once in a while it would be a lot more than the amount it takes to get it down from 35 to 29.

Beefy Norm points to the thing that I can’t possibly figure out - what’s in the gas, and maybe this caris just sensitive to that. I have gone through periods of not only the same roads, but also the same gas station and pump. But even that is no guarantee of the same brew/mixture. I do know that it isn’t a summer/winter difference based on the different mixes for different seasons. The up and down of the mpg happens winter spring summer & fall.

I also have tried to figure out whether differences in auto-shutoffs matter (partly by using the same station/pump), but I’ve played with the math on that a bit and find it unlikely. If I fill up before about 1/4 tank left, I also don’t so much trust the mpg results since those little differences can make a difference.

Anyway, thanks all.

you mention that you have ‘taken into account’ the AC, and defroster. how?

do you drive with the windows up, down, sometimes up, then down?

does it have a sun roof? open, closed?

do you have a roof rack? permanently installed, or removable as you need it?

do you have a bra on the front? or a wind bug deflector?

there are so many things to account for a mileage change.

have you changed tires recently?

since your engine has high miles, i would suspect something more on the order of age related, compression, wear, and just plain wearing out.

“Taken into account” means that if I have been driving a lot w/ the AC/defrost, windows open, city rather than hwy etc. etc. then if I fill the tank and end up on the low side of mpg, then I don’t wonder about it. I was hoping that my opening post would have indicated that I really do get all of the typical things involved in mpg.

The issue is that I can fill up - drive a tank and get 35mpg. Fill up again and drive the exact same roads in the exact same way (with the same roof rack, lack of bra/sunroof, etc.) and get 30mpg.

I have been thinking that there may be some sensor or other part that might be intermittently malfunctioning (but without noticeable impact on how it runs) that might do this. The TCC is certainly that sort of thing and I don’t rule it out, but as I noted it seems to function as it should. Other than that, there is unknown variability in fuel mix/quality - but if there is some “part” that might account for this, then I stand to save money by figuring it out.

Have you considered the effect of the wind? Especially on an economy car like an Escourt, wind on the highway can have that large an effect on mileage. At highway speeds going against the wind is a significantly higher resistance to overcome than going with the wind.

I agree with mountainbike. Wind, or lack thereof, can have a significant effect on fuel mileage, either positive or negative. I also suggest ethanol may have something to do with it.

I regularly make a 500 mile round trip across Pennsylvania on the turnpike, always at the same speed, using cruise control, just like you, and the mileage for my cars varies within a 5-6 mpg range depending upon whether I’m driving into the wind, with the wind, or with a crosswind. Driving into the wind really drags down the mileage, while a tailwind helps enormously.

As far as ethanol goes, when I fill up at a station with 10% ethanol the mileage on either of my cars (Acura with a manual transmission and Subaru with an automatic) drops by roughly 3 mpg. If I can find a station without ethanol the mileage goes back to normal. Ethanol is a scam, plain and simple, and it’s costing us all money and not helping ANYTHING.

I think the variation you are seeing is normal if you take the weather and the presence or absence of ethanol into consideration.