My 1994 Honda Civic DX 2door Coupe automatic (177,561 miles) is having a HUGE drop in MPG between the first half of the gas tank and the second half. It is a loss of well over 20-25 MPG, with the second half barely worth 6-9 MPG! While the first half is the normal, expected 30+ mpg.

Test Civic, running perfect:

11.9 Gallon Fuel Tank Capacity
Online Statistics: Civic DX 1.5L AT (Honda Civic) | Fuelly
[ 21.8 Lowest MPG, 31.4 Average MPG , 43.6 MPG Best ]

[Math:]

11.9 Gallons * 21.8 MPG = 259.42 Total Miles Per Tank [Worst]
11.9 Gallons * 31.4 MPG = 373.66 Total Miles Per Tank [Average]
11.9 Gallons * 43.6 MPG = 518.84 Total Miles Per Tank [Max Potential]


The Lowest MPG, 21.8 MPG was achieved while driving 90% City.
The Average MPG is the cumulative average of all MPG tests ran (138)
The Highest MPG, 43.6 MPG was achieved driving 100% Hwy.

This test civic was mechanically checked for optimum running conditions for testing purposes, and is stock 94 civic dx coupe 1.4L.

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MY CAR...
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Tested my car today. Its not in that great of shape, it needs work but i dont know what to target, so thats why im here.

The test results of todays test were completely in-line with my week-to-week experiences, and was done 98% on hwy. I generally drive 60% city/country , and 40% hwy, so those results are even worse.

Ran car nearly empty.
Purchased 12 Gallons regular gas, Shell Gas station.
Filled civic.
Started civic. Watched gauge go up to "F"
Drove on Highway until nearly empty.

STATS
11.9 Gallon Fuel Tank Capacity ( 5.95 Gallons per Half)
Drove a constant 70 MPH, economically, all HWY.
Drove 51 Miles before needle dropped below "F" even a little.

Drove 194.3 Miles at Half Way (5.95 Gallons consumed)
194.3 Miles / 5.95 Gallons = 32.65 MPG [Hwy]

Assuming this rate, I should have achieved a total of
194.3 Miles * 2 = 388.6 Miles / 32.65 MPG

My actual odometer read 253.4 Miles when my gas gauge reached Empty.

Math
194.3 Miles (Divided by) 5.95 Gallons = 32.65 MPG {First Half}59.1 Miles (Divided by) 5.95 Gallons = 9.93 MPG {Second Half}

Which comes to be a loss of 135.21 Miles Driven, and a loss of 22.72 MPG between the first 5.95 gallons and the second 5.95 gallons...

which is insane!

Just for Fun.. 253.4 Miles (Divided by) 11.90 Gallons = 21.29 MPG


253.4 Miles / 21.29 MPG is a far cry from 518 Miles / 43 MPG.....


21.29 MPG, full tank, HIGHWAY miles, 70mph constant rate.

# My real-world day-to-day, week-to-week results are even more shocking, I average about 30+ MPG on the first half tank, and 5-6 MPG on the second half. when driving normal, about a 60/40 city/hwy split. #


Results
11.90 Gallon Max Capacity
First Half (5.95 Gallons) = 194.3 Miles (32.65 MPG)
Second Half (5.95 Gallons) = 59.1 Miles (9.93 MPG)

Total MPG per Tank, pure HWY miles = 21.29 MPG, with a loss of 22.72 MPG between the first half and second half of my gas tank.


I am currently dealing with this problem by never letting my civic go below half a tank, putting in 20$ at a time. Utilizing this method i do get near the expected "400 miles per tank" mark, but it is extremely annoying and stressful to maintain this method.

Anyone have ANY idea what the hell is going on with my car?

Its completely stock, 1.4L engine, auto trans, no mods, only thing is a 2-3" lower on ebay coilovers.

Tires are all same size, but are in varying condition and all generally poor condition so to speak.

random notes:
Recent oil change, New trans fluid, no visible leaks, idles very rough, in Park it revs up and down over and over. It never overheats, always stays under half.\
Car "whines" a little while accelerating through the gear. Tranny maybe? Driving normally its not bad at all, but pushing the gas pedal down hard causes it to sound almost like a turbo.....but...it uh, doesnt have a turbo! haha...=/

large amount of White smoke out of tail pipe in morning when cold, sat overnight but clears up very fast once driving..

Does not pass emissions, registered in county that doesnt need it. It failed because of the Catalytic Converter.

Once driving past the half-way gas gauge mark and the horrible MPG comes into play, there is a very apparent odor present. Fuel smell, not sure what. Also a rather noticeable loss of acceleration is present, and very obvious whilst going uphill.



Only things i can think of...
Air filter, fuel filter, spark plugs...? im so lost how to repair this civic!

 

I'll be honest.... you are reading wayyyy too far into this. the gas gauge isnt the most of accurate of things. What you are seeing is called the "Honda half tank". Primarily affecting 92-00. The gas gauge wont move off of "F" for about 60 or so miles. Once you hit the half mark, the needle just starts dropping.

I really didn't read the entire post since it seems extremely long. lol.

Fill up the tank 100%, drive the car a few hundred miles, then fill up the tank again. Take the mileage driven and divide it by the amount you just put in to fill it up the second time. Boom, overall mpgs.

You never use the entire 11.9 gallons too, so you are working against yourself using that a number to divide by.

As for other culprits for lost mpgs, do you have any check engine lights on?

 

I started to read the post & thought what is this person talking about. I agree with all you said. To check gas mileage I set the mileage meter to zero fill the tank. Drive our 90 Honda LX auto, then when I go to fill it again I take the meter reading & divide it by the gallons to fill & there it is my gas mileage for that run. Our 90 Civic gets between 29-35 mpg. I have driven our Honda 100 miles & the gauge still says full.

 

...94 civics are rated from 21 mpg city up to 43 mpg hwy, and 35 is a good average estimate of mpg.

I let my car go completely empty, to the point it was running on fumes. Reset the odometer to 0 miles at the gas pump, filled it up with my debit card, and hopped on the highway.

Considering i Filled from dead empty..., it was holding the max capacity of 11.9 Gallons in the vehicle at the beginning of the test, at the same time I reset my Odo back to zero. I started the car, gas gauge went to Full. I pulled out, got on the highway, and drove back and forth for hours until the gas gauge dropped to the E. When the gas gauge was at the E, my odometer (which i reset) read 253.4 miles driven.

Which is where i get the MPG equation..

253.4 Miles Driven (Divided By) 11.9 Gallons [Filled the tank up from empty to full and reset odo]

253.4 / 11.9 = 21.29 MPG

So, considering the worst MPG i could find for a 94 civic dx is about 20, and conducted in 90% city driving, 10% hwy driving, and the best 94 civic dx MPG is rated at 43.6 MPG, and conducted 100% hwy... and the average MPG result from 138 civic tests was 35 MPG..., can anyone then tell me, from full to empty, why my 94 civic dx goes only 253.4 miles (divide by 11.9 to get 21.29 MPG), 100% highway miles, virtually no city driving, for that test?

Why am i less than half of the best mpg also conducted 100% hwy, nearly equal to the worst mpg which was conducted driving 90% city, and 15 mpg less than the average?

138 tests + my test
11.9 Gallons is a "Full tank"

MPG * Max Capacity (11.9) = Estimated Miles achieved on a "Full Tank"
Highway(hwy) MPG is generally always superior to city MPG.

Worst MPG: 20 something. 20 * 11.9 = 238. [90% city, 10% hwy]
Average MPG: 35 something. 35 * 11.9 = 416.5 [40% city, 60% hwy]
Best MPG: 43 something. 43 * 11. 9 = 511.7 [100% hwy]

 

Wait...stop..stop!

You said you drove back and forth for hours till the gas was on E(empty).

Did you have anyone in the car with you?
Do you have anything heavy in the back other than your spare?
Are your tires properly inflated according to an air gauge(not by your eyes)?
What gear do you have it in most of the time while on the highway?

Also..what type of Fuel are you using?...87? 89?..91?

If you said : No, No, Yes, 4th and 87...then you might wanna try a fuel cleaner like STP..but the best is Lucas, then try your test again....that is if you got hours of driving to spear :P

 

First of all, you car doesn't run on fumes. It either has gas or it doesn't.
You have to base calculations on the amount put in on the second fill-up, not the first fill-up or what your fuel gauge says.

No need to start at empty. Fill the car. Run the trip of 250 miles or whenever the gauge reads empty. Go back to the same station, same pump, fill it again and use the miles driven divided by the amount put in. 11.9 gallons has nothing to do with it unless you actually put in 11.9 gallons when you refilled.
The amount you put in when you start is meaningless except for the fact of filling the car. And unless you top off to the point where it is spilling out, you will never get exactly the same full amount.

 

No, Just me
No, Trunk empty. Car empty.
Probably not. Theyre all random brands, random tread left.. Same size though on all 4. My tires suck..But losing 15-18 mpg on tires?...hmm
Drive 4 (D4) (4th)

Have used a different brand fuel cleaner each fill up
have tried

STP Gas Treatment
STP Fuel System Cleaner
GUMOUT Fuel System Cleaner
PRESTONE Fuel System Cleaner

and i'm going to use LUCAS on the next try, but honestly i have noticed negligible results using any of the above products compared to using nothing at all.

 

I used to freak about my mileage drop too until i learned this issue is normal. My needle would stay steady and not drop barely at all for the first 100+ miles, then suddenly drop like a rock within 20 or so miles.


To reiterate what cvccr99 stated, the way to check your mileage is to fill the car till the gas stations pump shuts off. Set your odometer to zero, drive like normal and when you get near the end of the tank go fill up again. No need to drive to empty, but get it pretty close ( maybe just above the 'E' )
The important part is that when you fill up, write down the odometer reading and get the receipt. Divide the miles traveled by the gallons used and this will give you your average MPG reading. ( Reset the odometer to zero at each fill to track the miles traveled )

Dividing the miles traveled by the 11.9 gallons that is in the manual skews the numbers, since there will be some fuel left in the tank, so use the actual number of gallons shown on your receipt. Example : if your odometer shows 387.4 miles traveled when you fuel up and you use 10.98 gallons, then 387.4 divided by 10.98 = 35.28 MPG.

There are some fuel economy meters that are available such as the Scanguage
ScanGauge - Trip Computer + Digitial Gauges + ScanTools, but our cars are too old to run it ( pre ODB II ) so one option is to use this device :MPGuino - EcoModder
With it you can see instant mileage as well as trip mileage etc.

hope that helps.

 

ya i agree with everyone. i think what your saying is when you get on e fill up it might say 10 gallons not 11.9 then divide the miles you went by ten and not 11.9 right?