Anyone else notice considerably smoother shifting when gas tank is full compared to low.

[JohnSilver]

Maybe it's just me, maybe I'm crazy, but I've been semi testing this with low gas levels and a full tank. The vehicle seems to shift much smoother when gas is full or above half. When it's low it definitely shifts jerkier. If this is a common thing "auto gurus" know I apologize. Can't say I've experienced it before.

I'm aware running low gas levels is not great on an engine, guess I've just never noticed this much of a difference before. If anything it'll motivate me to keep gas topped off regularly

I'm also running 245/65/17 K02s that have been on lower end tire pressure lately, along with the RC suspension kit. Going to bump up pressure tomorrow in prep for a little road trip, 6ish hours of highway driving.

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[Meanderthal]

It’s a pretty common myth and google has this to say:

“No, cars do not perform better with a full tank. Gasoline weighs about 6 pounds per gallon. A car with a 15 gallon tank is carrying 90 pounds of fuel. For a 3,000 pound car that is 3% higher mass to accelerate and stop.”

bronco sport is heavier than 3,000lbs so an even lower percentage is gasoline weight.

Okay but he is asking about transmission shifting differently, not engine performance.

 

[Dude]

Okay but he is asking about transmission shifting differently, not engine performance.

Oops. Got it. Deleted my response. Thanks!

 

[Jake10080Guy]

I actually was in a 2012 Ford Fusion, it did the same exact thing, sort of weird, when you got low on fuel, it would pause and jerk a bit hard shifts, more noticeable on bigger engine load

 

[JohnSilver]

I actually was in a 2012 Ford Fusion, it did the same exact thing, sort of weird, when you got low on fuel, it would pause and jerk a bit hard shifts, more noticeable on bigger engine load

Yes basically this.

 

[CactusBS]

Maybe it's just me, maybe I'm crazy, but I've been semi testing this with low gas levels and a full tank. The vehicle seems to shift much smoother when gas is full or above half. When it's low it definitely shifts jerkier. If this is a common thing "auto gurus" know I apologize. Can't say I've experienced it before.

I'm aware running low gas levels is not great on an engine, guess I've just never noticed this much of a difference before. If anything it'll motivate me to keep gas topped off regularly

I'm also running 245/65/17 K02s that have been on lower end tire pressure lately, along with the RC suspension kit. Going to bump up pressure tomorrow in prep for a little road trip, 6ish hours of highway driving.

I have noticed this as well on my Big Bend (pretty much stock except for Wildpeak A/t Trails). I thought it was all in my head at first, but it is definitely smoother overall with more gas in the tank.

 

[JohnSilver]

I have noticed this as well on my Big Bend (pretty much stock except for Wildpeak A/t Trails). I thought it was all in my head at first, but it is definitely smoother overall with more gas in the tank.

Yeah, like I said just motivation to keep it filled lol

 

[Dude]

I did read there can be reasons for the smoother drive for example potentially higher fuel pump pressure (with more fuel in the tank) and other fuel related possible reasons however on that the article it did say if the fuel delivery system was nominal then amount of gas remaining in the tank would not be a fuel delivery effect. The article also listed a handful of other (non fuel) possible explanations for what you experience.

 

[Mark S.]

Okay but he is asking about transmission shifting differently, not engine performance.

Engine performance can affect how the transmission shifts. The computer expects a given torque value for a given throttle position and RPM. If the engine is not producing expected torque the computer may get "confused" about how and when to shift.

The fuel tank incorporates a vent system to equalize air pressure in the tank as you burn fuel. If the vent system is malfunctioning you can get a vacuum in the tank which could affect fuel delivery to the engine. That seems unlikely, however. I would expect to see an ETC if the tank vent system isn't working properly.

 

[JohnSilver]

I was waiting for the smart engine guys to chime in, appreciate the info.

 

[Flash]

I never go under a half tank so no, I haven't experienced this.

 

[Meanderthal]

Engine performance can affect how the transmission shifts. The computer expects a given torque value for a given throttle position and RPM. If the engine is not producing expected torque the computer may get "confused" about how and when to shift.

The fuel tank incorporates a vent system to equalize air pressure in the tank as you burn fuel. If the vent system is malfunctioning you can get a vacuum in the tank which could affect fuel delivery to the engine. That seems unlikely, however. I would expect to see an ETC if the tank vent system isn't working properly.

I don't think the BS can pull a vacuum in the fuel tank. The way the filler is, any vacuum would be relieved by that flapper door just opening enough to relieve the pressure.

Yes, an engine with less power would lead to a transmission shifter later but I don't think that is what is being described by the OP.

 

[JohnSilver]

To be fair I don't notice it as being a problem, and it could be entirely caused by the tires and air pressure being lower, mostly city driving. Going to air up this evening and report back.

 

[Mark S.]

I don't think the BS can pull a vacuum in the fuel tank. The way the filler is, any vacuum would be relieved by that flapper door just opening enough to relieve the pressure.

There would be SOME vacuum until the spring tension in the poppet valve is overcome. Would it be enough to cause an issue? I don't know.

Yes, an engine with less power would lead to a transmission shifter later but I don't think that is what is being described by the OP.

I don't think so either. I was mainly pointing out that transmission operation and engine power are interconnected.

 

[Meanderthal]

There would be SOME vacuum until the spring tension in the poppet valve is overcome. Would it be enough to cause an issue? I don't know.

Yes, there could be a very small amount of vacuum. Thanks for being persnickety!

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