[greenhorn]

I'm not sure if my mind is playing tricks on me, but Recently I've noticed that my pickup improves drastically after I tank up. From what I read, it might be because my pump is acting up.

But i got my pump serviced just a few months ago.

What else could be at fault?

[sumeethaldankar]

I notice i drive fast when i have a full tank and slower when i am reaching the empty mark.For me seems more like a physcological thing that having more fuel in the car makes me seem to want to drive zippy without worrying about running out of fuel.

Do you feel the same when you have half tank of gas in the car ? , i mean if your fuel pump is acting up , even a half tank will do.

[prince_pervez]

On first thought the pick up should decrease as a result of more load at the back. I don't know about the others but my cars have returned exceptional mileage on full tanks. I have never noticed an increase in the pick up.

[Jaggu]

Originally Posted by greenhorn

I'm not sure if my mind is playing tricks on me, but Recently I've noticed that my pickup improves drastically after I tank up. From what I read, it might be because my pump is acting up.

But i got my pump serviced just a few months ago.

What else could be at fault?

LOL unless you have a heavily clogged fuel filter or a pump which is really weak/poorly calibrated, this shouldn't happen except in your mind.

Originally Posted by prince_pervez

On first thought the pick up should decrease as a result of more load at the back. I don't know about the others but my cars have returned exceptional mileage on full tanks. I have never noticed an increase in the pick up.

Better mileage can happen, especially in summer coz vapor loss will be minimal when the tank is close to full.

[ghodlur]

Even I have observed after I do tank full, the engine becomes a lot more smoother, the gear shifts are slick and smooth, the DTE variations are very minimal. As the fuel reaches half tank capacity, I can feel a noticeable diff in gear shifts, the engine a bit groaning and DTE going haywire.
I am unable to know the logic behind this or is this just a figment of my imagination??

[akash_m]

+1 to that. My swift behaves much better with tank full of petrol. With petrol near the 'E' mark (with still 5-8 litres remaining), the pickup and power seems to drop.

Is this technically justified or just an illusion?

[aargee]

Was watching this thread for sometime before I could say something which I thought would be considered funny, but now I'm convinced that my observations are inline with most of you people. With a full tank, here're my observations...
1. The needle comes to half mark after a very long drive, but from half to empty its faster. Jaggu - would appreciate if you can help me understand the vapour loss concept.
2. The car is butter smooth with gears shifts are slick smooth & its not this smooth when the fuel is at E. Even the engine noise can be hardly heard when the tank is full.
3. Pickup - I haven't really noticed, but will try to notice during my next full tank
4. Big lesson learned is, never fill up the tank 1 or 2 days before a long drive even though the car is parked under shade. Just fill the tank the night before the long drive & the vapour loss will be less.
5. Long drive mileage increases if the tank is loaded completely rather than starting at half & topping up the tank in between.

[Exhaust_Note]

Since a long time (years) I have been noticing the same but dismissing it as "in-the-mind". Not only cars, my old bajaj M-80, current gladdy and wagon-R all display a enhanced refinement and smoothness after a tank up.

Could it be because when the fuel level drops, the physical impurities/contamination are more (as in ppm/ density) and hence the roughness? But when fresh fuel in added there is a churn and the fuel reaching the engine is actually better?

@aargee - +1. Keeping the car parked for long time with full tank have been known to cause fuel pump issues. Not 100% sure about it though.

[k36]

I have the same experience with my Swift Petrol. Full tank gives better pickup, smoother drive and better gearshift than a less than tank close to empty.

I've been thinking about it on and off but just brushed the thought aside as an illusion. It is a surprise now that so many of us are getting the same experience!

[Gansan]

I have noticed this for more than two decades in my bikes, and more recently in my car as well. In the bikes it could be partly explained as the fuel flows by gravity and/or by the pressure the fuel remaining in the tank exerts. But in cars?

I always fill a full tank and will go for a refill when the fuel level comes to reserve in bike and near the E mark in car. This takes around a month. May be the fuel partly loses it's octane rating towards the end of this cycle and regains when I top up the tank, leading to the smoothness and better pick-up?

[Jaggu]

Originally Posted by aargee

1. The needle comes to half mark after a very long drive, but from half to empty its faster. Jaggu - would appreciate if you can help me understand the vapour loss concept.
===========
4. Big lesson learned is, never fill up the tank 1 or 2 days before a long drive even though the car is parked under shade. Just fill the tank the night before the long drive & the vapour loss will be less.

1- Fuel gauges never work in a linear fashion. So unless we have a digital gauge units, don't think it will ever give you accurate level reading based on consumption. I think you already know the vapor loss concept, refer point 4??

Simple terms, with more vacant space in tank, fuel has more space to vaporize as it heats up and hence chances of loss. With new age sealed fuel system this is restricted to a minimum. Thanks to news emission norms, which include fuel and lubricant emissions if am not mistaken

[Joe-Ker]

This is great and strange stuff.

I almost never drive with tankfull of petrol so cant comment. I live form pump to pump. Basically its because vehicles get borrowed a lot and ppl dont fill petrol if its more than half full.

But if what you guys are saying is true i think its time i changed my filing habits.
Any theories on why this happens?

I can understand the better mileage part but slicker shifts? how is that happening

[jkdas]

Might for old carb-walla ones ; gravity plays

Like sumeeth said, should be physcological. You cant fool the electronic stuff these days by default.

And it should be slower coz of extra weight.

Emm, another thought, maybe its like how Pete's box works; what did you fill in? air? So the ECU sees more air and hence pushes more fuel ...

This was fun.

[Gansan]

Originally Posted by jkdas

Might for old carb-walla ones ; gravity plays

Even for cars fitted with carburettors, how can gravity play a part? It holds good only for bikes, whether carb or FI!

[aargee]

Originally Posted by Gansan

Even for cars fitted with carburettors, how can gravity play a part? It holds good only for bikes, whether carb or FI!

+1.

Originally Posted by jkdas

Might for old carb-walla ones ; gravity plays

Even I didn't understand this point. From my understanding, all the cars have the fuel tank in the rear & the fuel pump helps to flow the fuel from the rear end to the front, so how does gravity play a role? Appreciate if you could help us understand.

Originally Posted by Jaggu

Simple terms, with more vacant space in tank, fuel has more space to vaporize as it heats up and hence chances of loss. With new age sealed fuel system this is restricted to a minimum. Thanks to news emission norms, which include fuel and lubricant emissions if am not mistaken

Thanks I got the point. But have one more question. Vapourization happens due to heat, how does empty space matter? As long as the heat is sufficient to vaporize the fuel, the fuel will evapurate no matter there's empty space on the gas tank right? Pls guide me, if I'm incorrect here.