93 DX fuel consumption

93 Corolla DX, with 242K miles on it. Original owner.

I have a strange fuel consumption issue with this vehicle, and wanted to ask before starting to change anything out on it. Average commute is < 20 miles round trip each day. Used for mostly around town driving - no long trips, and in fact, seldom anything longer than 50 miles. Driving style is not significantly changed within the last 5 years.

I'm not using any super-accurate mileage calculation, but having driven the car for 23 years & putting at least $20,000 worth of fuel through it, I know what "normal" mileage should be. On a trip/commute (>= 25 miles one way), it has gotten at or slightly over 30 mpg throughout it's life. Around town mileage has been historically been around 27 mpg. The fuel capacity says it's 13.2 gallons, but when I fill up (from E on the fuel gauge), I have generally put between 11 - 11.5 gallons. When I fill up from the fuel gauge indicating a half-tank, it takes about 5.5-6 gallons max. Putting 5.5 - 6 gallons from E will put it at a half-tank. So, the fuel gauge seems to be generally accurate (or close enough).

Here's the consumption issue.

1. The car sips gas throughout the first half tank after a fill up. Routinely, I get 180 miles out of the first 5.5 - 6 gallons, around 30-32 mpg. If I fill up again when the gauge shows a half tank, it takes between 5.5 - 6 gallons, and it will average around the same mpg.
2. On the other half tank, I can get 100 miles from it, maybe up to about 125 miles, an average of 20-22 mpg. Filling to a half tank will repeat similar mileage. Anything over that amount (between 1/2 & 3/4 tank) will bump the mpg up slightly.

To a tank (between 11 - 11.5 gal), it will average around 24 mpg. On the first half of a tank, the low is about 27 mpg & the high is around 31 mpg. On the second half, the low is 18 mpg and the high is around 22 mpg. Not sure how long I'd say it's been happening, but I do not put a ton of miles on it each year (approx 2000 in the last 12 months).

It's due to have the timing belt replaced after the first of the year, and I'd like to figure out what would cause the odd consumption pattern I've noticed. Some info:

• The car uses a little bit of oil (maybe 1/2 quart over 3000 miles) & has been changed at 3,000-3,500 mile intervals its entire life.
• It idles fine and runs good & it starts every time, and has never had any issue like this.
• There is only a tiny puff of smoke when it cranks first thing after sitting overnight - it does not do it otherwise.
• Plugs have been changed since last timing belt (at 180,000 miles), but not recently. Best guess is 40,000 miles +/- on them. I cannot recall when/if I have changed plug wires, but it cannot be more than one time.
• The thermostat works fine. All belts and hoses get changed at the same time the timing belt (60,000 mile intervals).
• No known electrical issues.
• I think the O2 sensor has been changed, but cannot remember at what mileage. Best guess is between 120,000 and 150,000 miles.
• Decent (Hankook) all season tires, with approximately 30,000 miles on them, no unusual wear on them, rotated regularly and uniformly inflated.
• Fuel tank has the locking door on it & it works fine.
• There is no known gas leak anywhere, and no smell of gas.
• There is little to no change in the driving pattern as a tank of gas is consumed. Most often driven in the 30-45 mph range.
• Having put virtually every mile on it, and filled it thousands of times, I know the "normal" use mileage has been between 350-385 miles on a full tank (11 - 11.5 gal), with approximately 165-190 miles being a half-tank.

Even though I'm only approximating the mileage I get, I do notice that I travel considerably fewer miles once the tank is half-empty.

Any ideas as to what might be causing this unusual consumption?

Has your overall mileage changed? Also, have you thought about keeping specific records for a few weeks to see if there's a pattern?

I'm wondering if it might not be the fuel gauge that is starting to get a little off. I have a 93 DX as well with 238K, and ever since I got it a year and a half ago, I've noticed that a half tank by the gauge is not actually a half tank by capacity.

Yes, James - the overall mileage has changed. Like clockwork, I used to get between 350-375 miles to a tank. Now it's about 100 miles less - overall.

That's what struck me as strange. Over the first half-tank, it still projects to that (approximate) mileage. And, when the gauge says it's a half tank, I can put almost 6 gallons in & fill it up. Get another 170-180 miles out of that also. But, once it hits the half-tank mark, the mileage goes to about 10 mpg less than the first half.

If I had a leak, it seems like it would be the opposite (terrible mileage on the first half-tank), or the mileage drop would be overall (i.e. relatively constant drop over the entire tank). It's backwards to that.

That is strange. I would still suggest keeping specific records when you gas up for the next couple of months. It's easy to do on paper, you basically need just your mileage (regular odometer, not tripometer), exactly how much you put in, and the date. Other things to watch might be the station you gas up at, the time of day, and/or the price. There's a couple of really nice phone apps available as well if you want to keep track that way (biggest advantage, they do the math for you!).

You might also try checking the forums over at Toyota Nation. They have a lot more activity over there.

My kinda car! I had my '93 DX for twenty years before replacing it with a 2012, which turned out to be the worst car I've ever owned, but that's another issue.

A for the fuel gauge, I've never seen a Corolla gauge that doesn't fit the description of your description of its behavior. In fact, that's pretty typical of gauges in general, and nothing to worry about. They're not precision instruments, and really aren't much use for tracking mileage.

The only method I've found that gives even a modicum of accuracy is to keep a faithful record of every fuel stop, filling the tank until it won't accept any more fuel, and then recording the amount. This will give you the fuel consumption since the last stop, and it's a simple matter of dividing the mileage since the last refueling by the amount of fuel replaced.

Perhaps too obsessive a procedure for some, but over the long term, it works quite well, and can reveal anomalies that might indicate a problem.

You should be filling the tank when it is on E and then record. From full to half and half to E will not serve the purpose. If still you feel there is issue in the mileage then try seafoam.

I'm impressed with how detailed your concern is! Nice!
Something I can tell u is similar to what Neil mentioned previously when your gas says half that doesn't mean exactly half your gas tank is not a perfect square or rectangle or any shape like that as where half a tank would be half.
Your tank is actually semi-round rectangle with an intrusion down the middle bottom for the exhaust. This means that your top half of your tank carries more fuel than the bottom half which would explain your half fuel tank concerns. Also similar to what Muhammad mentions previously as well if you want to do fuel consumption test it's most accurate way is to let your tank get near empty and check results as a whole not with halves as this will lead to incorrect results.
A tip I can provide is making sure your spark plugs are OEM I had NGK aftermarket plugs in my Corolla and changed to OEM iridium plugs and saw a 2mpg increase. Also a vehicle with the mileage your vechile does have wear and tear and you may not see the fuel mileage you once saw when the vehicle had lesser miles. There are many factors that contribute to fuel economy some are weather, how long you let your vehicle warm up, tires, driving habits, a/c operations or windows rolled down and vehicle wear. An example on vehicle wear could be something as simple as power steering fluid. More viscous fluid will require your pump to have to work harder to keep fluid moving which in tell is making your engine work just that much harder to keep things moving kind of like having the a/c on. All these little things may take away 3 mpg or as little as a 1/4 mpg but if there's more than one contributor than all these things add up.