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Discussion Starter #1
How come it seems that everytime I fill up I get better gas mileage? About 5-10 mpg more on the same route.

Is it just me or is the Insight that sensitive to fresh gas? I travel the same route every evening to work. I average about 80-85mpg on the way there and 65-70mpg on the way back. Today I gas up and I do 90-95 there and 75-80 on the way back. Its almost like that everytime I gas up.

It seems that as I use up my tank, my mileage decreases. Weird because I thought it would increase since the weight of the gasoline in the tank is decreasing.

Maybe I'm seeing things. I just wanted to see if anyone else experienced this.
 

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Are you zeroing when you buy gas? If so, you are starting the next segment with a warmed-up car and so will begin at a much higher mpg than you would starting from home with a cold engine.
 

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This has been brought up before so you are not alone. I have two different possible reasons based upon how much improvement and for how long the improvement last.

Short term improvement for the first 5-10 miles or so:

If you overfill the tank, some of the gasoline will end up in the evaporative control system. While you are driving the first few miles, you may see amazing fuel economy numbers because some gas is getting into the engine from engine vacuum and its flow into the engine is not being registered (metered or counted) through the electronic gauges. This will result in an overall richer mixture for some time while providing more power than normal so your foot is off the accelerator more. Then presto, you appear to be getting better fuel economy just by filling up. It has also been discussed that constant overfilling of the tank could lead to evaporative control system malfunction.

Longer term imporovement over the first 50 miles or so:

I believe many of us will reset the gauges after filling up (sort of a fresh start) so we want to start out with the best possible mileage to begin with so we try harder (in the begining). I think this has to do with the driver wanting to start off with nice numbers more than anything else. Even more so if the driver starts seeing what appears to be great mileage for the first few miles after a fillup (as suggested above).


The second suggestion is just a feeling as to why but I believe the first suggestion is true for short term improvements.

JoeCVT - Just your average CVT owner
 

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Discussion Starter #4
hmm... interesting.

Well, I don't overfill my tank. I let the nozzle click and then I'm done. And I do reset the Trip meter on each gas tank, but I did not reset my FCD.

Maybe it might be some fuel that that is not being registered from the top. Because it does seem to go back to normal after my first round trip.

Thanks for the info guys!
 

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get going early effect

I've noticed the same effect, good mileage afer fillup, less as the tank goes on. I've decided that it's due to my psychology, when I reset the trip mileage meter, which I always do, starting out it's fun to do good. Then I run into the usual stop lights, stop and go traffic and it's less interesting.

So I just stop being so careful and things go south. But, heh, with the numbers you are getting, you should be enjoying the ride.

Regards, Jim
'01 #191
 

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This happens to me also. I also dont overfill the tank. I have noticed I can maintain 150mpg at speeds around 40-45mph, where as during mid tank, I can only do maybe 100mpg at those speeds. Definitely something to do with the fuel system somewhere, or maybe the bar graph just reads incorrectly for a little while after a fillup, being very optimistic on the mpg.
 

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This has been reported before, but I'm sorry I can't remember which thread. Two things are likely to be going on:
(1) the vapors from the top of the gas tank are displaced by the incoming fuel, go into the evap system canister, and are fed from there into the intake. This reduces the amount of gas that is needed, so the system registers a lower injection volume and hence a higher mpg.
(2) In the short term, refills often happen somewhere in the middle of a trip after the engine is warmed up. So right after filling up you can register a higher average mpg because there is no warmup penalty due to higher gas consumption while the engine is cold.
 

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better mpg after fillup

I don't overfill either, but have definitely notice a difference for the first 5-20 miles. The length of this phenomenon does seem to vary greatly (due to driving conditions? due to atmospheric conditions?)

This is not in your head, it is a real phenomenon. My car will register "lean burn" mpg values (120-150mpg) while I can see that I am NOT in "lean burn" (Scanguage II). It is more difficult than normal to get into "lean burn" under this condition, but I experience amazing mpg values while also having extra power which makes maintaining great mpg values much easier whether in "lean burn" or not. It makes it easier to maintain speed, or even accelerate (slowly) while still getting well over 100mpg.


Why?

My guesses.

1. Yes, the engine being warm at fillup helps.
2. Maybe the fillup adds oxygen into the gas during mixing.
3. Maybe the engine computer takes some time to adjust to the new fuel mixture and allows the engine to run less cleanly (but more efficiently) for awhile.

"something" is definitely different (???)
 

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This has been brought up several times in the past and the concensus seems to be fuel vapors in the evap control system. The engine vacuum sucks in that additional vapor during the first few miles and it gives the "appearance" of getting great MPG since the fuel injectors do not have to supply as much fuel compared to what happens when there are no more fuel vapors entering the engine.

Some say that they never overfill but if your car is on an angle while fueling, vapors could still get into the evap control system.

This does not have anything to do with lean burn since this scenario also happens with CVT cars....

JoeCVT = Just your average CVT owner
 

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Some say that they never overfill but if your car is on an angle while fueling, vapors could still get into the evap control system.
Vapors ALWAYS go into the evap system when you fill the tank: that's it's job. When the tank is empty, it has roughly 10 gallons of an air/fuel vapor mixture in it. Rather than just release all those vapors into the atmosphere, the system passes the vapor mixture through the canister which adsorbs the fuel vapors and lets the air pass through. In this way, only relatively pure air goes out into the atmosphere. Then while you drive the vapors are purged into the intake and burned together with the fuel which is injected.
 

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My car will register "lean burn" mpg values (120-150mpg) while I can see that I am NOT in "lean burn" (Scanguage II). It is more difficult than normal to get into "lean burn" under this condition, but I experience amazing mpg values while also having extra power which makes maintaining great mpg values much easier whether in "lean burn" or not.
That is probably right. Because if I am going a steady 40mph at 150mpg, I can press the gas down to 100mpg and I can get some assist bars, and as I let off the gas, it goes right back to 150mpg. Too bad this phenomenon only lasts only a couple of miles and is just a glitch in the MPG meter bars. The car is probably using the same amount of actual fuel as before. So nothing to get excited about, we are just being lied to for the first couple of miles on a new tank.
 

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Vapors ALWAYS go into the evap system when you fill the tank: that's it's job. When the tank is empty, it has roughly 10 gallons of an air/fuel vapor mixture in it. Rather than just release all those vapors into the atmosphere, the system passes the vapor mixture through the canister which adsorbs the fuel vapors and lets the air pass through. In this way, only relatively pure air goes out into the atmosphere. Then while you drive the vapors are purged into the intake and burned together with the fuel which is injected.
When I was talking about people that say they never "overfill" the tank, I should have said gas (instead of vapor) can get into the evap control system if parked at a angle. I am just suggesting a possible reason why people that don't "overfill" can still have this happen to them.

This 150 MPG scenario seems to happen more when people try to put as much gas into the tank. Some people claimed that they put in like 11-13 gallons. The gas pumps are not that much off in calibration.

This is not really a glitch in the MPG meter coding. It would be like me attaching a tank of some sort to supply fuel vapor to one of the engine vacuum hoses. Now the car can accellerate and maintain a much higher MPG based on what is going on with the fuel injectors. However, the MPG is deceptive because there is another unexpected source of fuel being put into the engine.

JoeCVT = Just your average CVT owner
 
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