how accurate is " Distance to Empty"?

[yrwei52]

I was driving the car until it was totally running out of the fuel a couple of times before on purpose. Although there was no signs of damage to the fuel pumps, but like you, I'm not willing to test it out anymore!

How many miles past "0 miles to empty" did you get before running out?

Sorry to confuse you. I tried that test on our old Honda CR-V when it was new just to see how many actually miles I could go when the fuel gauge pointed to the empty. Of course I did carry 2.5 gallons of spare gas so that I could re-fuel it once it died. There is no trip computer on our CR-V.

 

[davefr]

The theory is that fuel is used to cool the in tank fuel pump. As the tank get's real close to empty there's simply not enough fuel to cool the pump at peak efficiency. That's why the rule of thumb is to refuel at around 1/4 tank.

 

[oldme]

The theory is that fuel is used to cool the in tank fuel pump. As the tank get's real close to empty there's simply not enough fuel to cool the pump at peak efficiency. That's why the rule of thumb is to refuel at around 1/4 tank.

interesting...I did not know that. How does the fuel cool the pump? via the pump's exterior surface (if the pump is submerged) or via interior flow? In the latter case, the cooling should be independent of fuel level. the intake tube of the fuel pump obviously needs to be submerged.

 

[davefr]

interesting...I did not know that. How does the fuel cool the pump? via the pump's exterior surface (if the pump is submerged) or via interior flow? In the latter case, the cooling should be independent of fuel level. the intake tube of the fuel pump obviously needs to be submerged.

The ones I've seen look like this. (totally submerged) I have no idea if Mazda's are the same.

 

[oldme]

gotcha, thanks! if Mazda is similar, then it make sense - 1/4 tank gas would help fuel pump cooling.

 

[MikeM.]

The theory is that fuel is used to cool the in tank fuel pump. As the tank get's real close to empty there's simply not enough fuel to cool the pump at peak efficiency. That's why the rule of thumb is to refuel at around 1/4 tank.

No, the 1/4 tank fill-up recommendation is good because you are unlikely to run out of gas that way. The in-tank fuel pump doesn't consume enough electricity to ever get hot enough to worry about - it's the fuel running through the pump that's critical (primarily for lubrication of seals) but there is some heat transfer there. The fuel pump will not overheat when not submerged. No worries there.

 

[oldme]

thanks! i am constantly amazed by the expertise shown at this site.

 

[MidwestCX5]

The distance to empty is very inaccurate on my AWD CX-5. Last week it said 1 mile to empty and the gas gauge was showing empty but I had driven less than 350 highway miles. When I filled up the 15+ gallon tank it only needed 13 gallons.

 

[MikeM.]

The distance to empty is very inaccurate on my AWD CX-5. Last week it said 1 mile to empty and the gas gauge was showing empty but I had driven less than 350 highway miles. When I filled up the 15+ gallon tank it only needed 13 gallons.

Realistically that's not "inaccuracy" so much as it's a purposefully programmed in safety buffer. Imagine how upset you would be if it said "20 miles to empty" but it ran out in 16 miles.

I agree the buffer is bigger than necessary which is why I often run mine 5-20 miles beyond the "0 miles to empty" without any worries. When I fill-up right at "0 miles to empty" it only takes about 13.2 gallons before the pump auto shut-off (15.3 gal. tank).