Gas Tank Reserve

GAXIBM · Dec 29, 2014

Kedis82ZE8 said:
Interesting they are using a mechanical high pressure pump. It must be a negligible parasitic MPG loss from this.

Kedis The old cam driven fuel pumps had a spring that created the fuel pressure so that the cam only reset the spring for the distance that had been "used" by the spring moving the pump diaphragm that moved the fuel, thus very little loss of energy and the pressure was regulated by the strength of the spring so no bypass pressure relief fuel return was required to regulate pressure. They were low pressure pumps so the high pressure pumps may be different but could use a similar design? The high pressure pumps need more energy so it is more efficient to take mechanical energy from the cam rather than converting mechanical to electric with the alternator and then back to mechanical with an electric pump motor loosing some energy with each conversion.

Kedis82ZE8 · Dec 29, 2014

GAXIBM said:
Kedis The old cam driven fuel pumps had a spring that created the fuel pressure so that the cam only reset the spring for the distance that had been "used" by the spring moving the pump diaphragm that moved the fuel, thus very little loss of energy and the pressure was regulated by the strength of the spring so no bypass pressure relief fuel return was required to regulate pressure. They were low pressure pumps so the high pressure pumps may be different but could use a similar design? The high pressure pumps need more energy so it is more efficient to take mechanical energy from the cam rather than converting mechanical to electric with the alternator and then back to mechanical with an electric pump motor loosing some energy with each conversion.

Good point on using the energy in motion versus electric. I remember the older lower pressure cam driven ones. I helped my father replace one as a kid on an old '64 Ford Falcon station wagon with what I think was a 200 Cubic Inch inline-6 with a three on the tree. That was the simplest and easiest to work on engine I have every personally experienced.

deuce_WI · Dec 29, 2014

Not sure about all CX5s, but the GTs with tech, with their mpg / distance to empty / info display will turn orange when fuel gets low.

They'll also turn red when it's REALLY low.

Even on red you can drive another 10+ miles.

I've never seen it go red on my wife's car, but we took one for a test drive before buying that started off red at the dealership. The sales monkey didn't seem to care. We went about 10 miles and it didn't die.

I've driven about 10-15 miles with the distance remaining at zero and haven't seen ours turn red yet. Don't plan on pushing it further just to find out how empty it can get.

raystafarian · Oct 2, 2015

All right, I know I'm bumping a pretty old thread, but it's all I could find in the search. This is my first fuel up 2016FWD. So the manual says my tank is 14.8 gallons. And there are six little markers between 1/4 tank and empty. That would lead me to believe the gauge shows 24 marks for the tank less the reserve. I had 2 marks left (gas light, 30 miles range) and filled it with 11.54. Math (roughly) says the capacity between full and empty is 12.59 gallons (US). And the reserve is 2.2 gallons.

just fyi

pdlpsher · Oct 2, 2015

raystafarian said:
All right, I know I'm bumping a pretty old thread, but it's all I could find in the search. This is my first fuel up 2016FWD. So the manual says my tank is 14.8 gallons. And there are six little markers between 1/4 tank and empty. That would lead me to believe the gauge shows 24 marks for the tank less the reserve. I had 2 marks left (gas light, 30 miles range) and filled it with 11.54. Math (roughly) says the capacity between full and empty is 12.59 gallons (US). And the reserve is 2.2 gallons.

just fyi

Yes. That sounds about right. I started another thread on this topic a while back under the CX-5 lounge. When the gas range says 0 mile it would take in about 13 gallons on an AWD tank, leaving about 2.2-2.3 gallons in reserve. It's nice to know there's a decent amount of reserve left.

erhayes · Oct 3, 2015

I don't trust the repeatability of fuel tank gauges enough to risk running out of fuel. Ed

MikeM. · Oct 3, 2015

erhayes said:
I don't trust the repeatability of fuel tank gauges enough to risk running out of fuel. Ed

Interestingly, the AWD models have two fuel level senders in different parts of the tank (vs. one sender in FWD models). It's clear that this is due to the different tank shapes but what is not clear to me is whether the additional sender is designed to increase accuracy at mid-tank levels or whether it's to increase repeatability near the end of tank. My guess is the latter because that's the only thing important enough to justify the expense of two fuel level sensors.

In any case, it's apparent that Mazda put significant thought and effort into the accuracy of the fuel gauge and I have run my AWD 20-50 miles past "zero miles to empty" numerous times without issue. I figure Mazda would not have put a reserve capacity there if they didn't think anyone would use it (yes)

pdlpsher · Oct 3, 2015

I think a reserve capacity makes sense. One time I ran out of gas going up a steep him on a different car. The car ran fine before the hill. So the gas must have moved enough for the gas pickup to miss the gas. I wonder the newer cars have been redesigned to alleviate this problem.

GJ-Molestor · Sep 21, 2017

when the sensor reads completely empty you have 5-6L left.

Gas Tank Reserve

GAXIBM

Kedis82ZE8

'15 CX-5 AWD GT w/Tech Pkg

deuce_WI

Member

raystafarian

Member

pdlpsher

Member

erhayes

MikeM.

Member

pdlpsher

Member

GJ-Molestor

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