On a recent 144 mile round trip travel day ,
I got 30 miles per gallon milage.

In the 9 years of doing this same trip ,I've never gotten better than 24 m.p.g.

I used the trip meter on the car.
I had Bonnie check my math.

I had the trans. Rebuilt last year, would be the only difference.
So, I give up, how does that work?

Same octane gas same gas station.

Is it possible your 4th gear never worked until you had it rebuilt?

I am pretty sure I would have noticed that as I get it up to 60 m.p.h. three times a week when going in to the big city.

To add:
80% of the time I was on the interstate at speeds of
60-70 m.p.h.

If the transmission was slipping that would definitely affect your mileage.

Slippage usually develops over time and you may not have noticed.

Do things seem firmer now? Shifts crisper, acceleration better?

More cruising, less stop and go? Less hard accelerations? Tire pressure different (materially higher than previous trips)? Different tire (and tire weight)? Any tune-up work when the tranny was rebuilt?

Improving highway miles alone by that much seems like a lot. But "city miles" (or behavior thereof) mixed in may? What's the expectation for City vs Highway mileage for the car?

Gas pump fuel meter variance (some portion of 1.2 gallons?)?

Yeah, what year car and which engine? That seems like too big of an increase in gas mileage... unless you really couldn't get into overdrive until the transmission was rebuilt.

I have long wondered at the accuracy of gas station pump meters. Yes I know they are regularly inspected, but the temptation to sell "just a little bit less" gas on every sale must be very tempting in a extremely thin margin business.

I have many times put more than 5 gallons by the meter into a 5 gal. gas can. Either the can builder was generous, or the meter was off. On the flip side, I don't ever remember getting more gas than I paid for.

quote:

Originally posted by architect:

I don't ever remember getting more gas than I paid for.

Eat at Taco Bell.

What year LeSabre? My 2nd vehicle was a '92 with the 3800 engine in it. I had to replace the PCV valve a couple of times, which did have an effect on mileage. If this were the case for you, you'd likely be feeling other effects such as hesitation, though.

quote:

Originally posted by architect:
I have long wondered at the accuracy of gas station pump meters. Yes I know they are regularly inspected, but the temptation to sell "just a little bit less" gas on every sale must be very tempting in a extremely thin margin business.

I have many times put more than 5 gallons by the meter into a 5 gal. gas can. Either the can builder was generous, or the meter was off. On the flip side, I don't ever remember getting more gas than I paid for.

There is a variance allowed, 5 gallons is never exactly 5 gallons (what they use to measure it). With temperature changes gas can expand or contract. When fuel is cold, you get a little more gasoline. The variance allowed is 0.5% or 3 ounces either way out of 5 gallons. They do test pumps on a regular basis. It is possible something was wrong with the pump.

If a lockup converter is not locking up, the loss of power to the rear wheels is about 10%.
(and that's about what you experienced)

You may not realize it in some vehicles.

So, as stated, the rebuild may have fixed it, but ask your mechanic if it was verified to be working. If it was and is working correctly, there is another reason for the increase in MPG.

FWIW, lockup converters. (in case the above results in anyone wondering "why", the statement.)
Older cars before lockup converters would realize this loss of power transmitted as a normal operation.

The converter is a fluid coupler, and they never reach 100% efficiency.

A lock up converter has an electrical/mechanical clutch that applies and locks the converter so the drivetrain is in 1:1 and transmitting all the engine's power to the rear wheels.

If the transmission (clutches) were slipping, the transmission would fail in a short time.

But you can (often) drive with a lock up converter in the "unlocked" state for the life of the transmission.

If you run ethanol, and the tank/pump delivered non-ethanol, you will get much better MPG as the fuel contains "more energy" than ethanol fuel.

Trucks have "mis-delivered" to the incorrect tank many more times that you might know.

"04" model year. With the good motor according to my mechanic.

In September I ran two, half tanks Costco higher octane premium ethanol gas through it..

I am about half tempted to do the same trip again this weekend .

The tank was full when I left and stopped and filled it up at the same station on the way home.

Sigmonkey answered your question.

One of the most impactful effects on gas mileage is the driver's style. This isn't always the same. Especially, now that the weather is turning colder, many drivers like to let their car idle for a while to warm up the interior. This can play hell with gas mileage calculations. Not to mention that idling doesn't really warm up the engine to operating temperature.

OTOH, some drivers will drive aggressively before reaching optimal operating temperature, this is not good either. Also, not to mention, the gas characteristics change because of temperature so you have more energy in a gallon on cold days than you do on hot ones.

Yes, a lot of variables make it difficult to determine why mileage changes from trip-to-trip. There are phone apps that track miles and gas fill-ups over time that at least make the data capture easier. I used one called aCar on Android in the past, I am sure it or something similar is available for iOS as well. Gave it up when I retired and stopped driving the car every day.

This car is front wheel drive

That EVAP system uses a charcoal canister to capture fumes when filling the tank. When you top off several clicks at the pump handle, instead of the first time it stops, it will saturate the canister.
My bet would be on the canister being still saturated after the 140 mile trip, even though the tank seemed full, it took less fuel to fill it.
EvAP systems have several things that work together, this is a basic explanation.

I don't keep feeding it once the handle clicks off.
I was told by numerous people here that there
Is no upside to keep feeding it.

quote:

Originally posted by bendable:
I don't keep feeding it once the handle clicks off.
I was told by numerous people here that there
Is no upside to keep feeding it.

True. Can screw up the evap.

That's a big jump. Have you verified the result by repeating the experiment on different pumps from different gas stations.

I incrased my mileage in the 2mpg range by cleaning my fuel injectors, using Shell v=power nitro gas exclusively, running a new Valvoline brand oil designed to clean piston rings, and using high end fuel system cleaners in the tank every oil change. The old avg including stop and go was 17.5, the new avg is 19.5 for the same. I can't claim this is true since I'm not a real motorhead, but this is my experience with this F150, other vairables being equal.