How much fuel left when Reserve light comes on?

[Volvocano]

Originally Posted by Clan

The only instrument which tells you how much fuel is left in the tank is the GAUGE It is a highly accurate expensive moving coil instrument and it is designed so that even when the needle is dead on the bottom mark it wont stop so just go by your gauge that's what it is for... 👍

It might be a highly expensive moving coil instrument on your clan - but the majority of cars these days that still have analogue gauges are simply servo-motor operated needles masquerading as a genuine gauge and they read what the relevant ECU tells them to read. This trend started because focus groups found that drivers are worried by gauges that actually move. So many cars with temperature gauges read reassuringly dead centre when they are in their safe operating range and only read hot when the car is bloody hot!

The fuel "gauge" on my motorbike bike reacts to how it's ridden - if I thrash it, it reads lower (sort of estimated range) and then goes up again when I'm cruising. Nothing's as it seems these days! One would hope with modern digital systems they could program the relevant ECU so the fuel gauge represents the volume of fuel left, tanks are rarely uniform in shape making a level gauge less than accurate. So when you've used say 25 litres of 50 it will read half full - which may actually be two thirds full according to tank level. My XC90 certainly doesn't appear to work that way - the first half of the tank lasts for ages and the second disappears like the wind! Useless programming as I've come to expect from Volvo.

[Clan]

Originally Posted by Volvocano

It might be a highly expensive moving coil instrument on your clan - but the majority of cars these days that still have analogue gauges are simply servo-motor operated needles masquerading as a genuine gauge and they read what the relevant ECU tells them to read. This trend started because focus groups found that drivers are worried by gauges that actually move. So many cars with temperature gauges read reassuringly dead centre when they are in their safe operating range and only read hot when the car is bloody hot!

The fuel "gauge" on my motorbike bike reacts to how it's ridden - if I thrash it, it reads lower (sort of estimated range) and then goes up again when I'm cruising. Nothing's as it seems these days! One would hope with modern digital systems they could program the relevant ECU so the fuel gauge represents the volume of fuel left, tanks are rarely uniform in shape making a level gauge less than accurate. So when you've used say 25 litres of 50 it will read half full - which may actually be two thirds full according to tank level. My XC90 certainly doesn't appear to work that way - the first half of the tank lasts for ages and the second disappears like the wind! Useless programming as I've come to expect from Volvo.

Even the 45 year old volvo 340 and 240 did that with bi metal gauges directly connected to the sender. with a top quality thermostat and oversized radiator .
you are making things up as you go along here with no proof of what you might suspect. the gauges are linear why don't you test them?

[Volvocano]

Originally Posted by Clan

Even the 45 year old volvo 340 and 240 did that with bi metal gauges directly connected to the sender. with a top quality thermostat and oversized radiator .
you are making things up as you go along here with no proof of what you might suspect. the gauges are linear why don't you test them?

I'm making nothing up - I used to code automotive ECUs for a large American automotive company. I know exactly what goes on and and it's often not about what's technically the best solution. Do you think all the VAG cheat code happened by accident? I'm sure "real" old fashion level type fuel gauges still exist and that the only difference is the damping will be in the software rather then the gauge itself - but I'd be very surprised if Volvo XC90 fuel level gauge was still a linear level meter with no interference from programmed logic.

[Clan]

Originally Posted by Volvocano

I'm making nothing up - I used to code automotive ECUs for a large American automotive company. I know exactly what goes on and and it's often not about what's technically the best solution. Do you think all the VAG cheat code happened by accident? I'm sure "real" old fashion level type fuel gauges still exist and that the only difference is the damping will be in the software rather then the gauge itself - but I'd be very surprised if Volvo XC90 fuel level gauge was still a linear level meter with no interference from programmed logic.

The XC90 has two fuel senders one each side and I have always found them Linear with the needle when moving them by hand to save you dismantling yours . There is damping of course but the gauge matches what's in the tank. Unless one of the senders is fouling an in-tank hose which can happen if you don't fit them at the correct angle .

[Mick787]

An interesting thread to see what members do re fuel.
Myself, as soon as I get any car I will fill it to the brim and reset the trip. Then drive it until the gauge is reading half full. Then fill it to the brim again. Subject to how much fuel I put in, gives me an indication of how accurate the gauge is. I can generally predict how much it is going to take to fill the tank as being the sad person I am, I will do this at other stages of use and what the gauge is reading. On my MY 2016 XC60 D5 it will do about 120 miles before I loose the first bar on the gauge. I would never run it right down, something my dad told me never to do. I don't think I've ever driven any car I've owned so far to have the low fuel light come on. I've done it with company cars though.

[Clan]

Originally Posted by Mick787

An interesting thread to see what members do re fuel.
Myself, as soon as I get any car I will fill it to the brim and reset the trip. Then drive it until the gauge is reading half full. Then fill it to the brim again. Subject to how much fuel I put in, gives me an indication of how accurate the gauge is. I can generally predict how much it is going to take to fill the tank as being the sad person I am, I will do this at other stages of use and what the gauge is reading. On my MY 2016 XC60 D5 it will do about 120 miles before I loose the first bar on the gauge. I would never run it right down, something my dad told me never to do. I don't think I've ever driven any car I've owned so far to have the low fuel light come on. I've done it with company cars though.

Have no fears every single volvo tank I have looked inside over the last 45 years what ever the age or mileage, plastic or metal is as clean as a new pin . you are carting around about 30kg of fuel unnecessarily if you treat the fuel capacity as half it's true value . Dont be afraid to use up the fuel until the needle touches the last marker too . The gauges are spot on especially at the low end. also fuel expands a great deal when warmed up so any discrepancies you might see are down to this .

[SnineT]

Originally Posted by Clan

Have no fears every single volvo tank I have looked inside over the last 45 years what ever the age or mileage, plastic or metal is as clean as a new pin . you are carting around about 30kg of fuel unnecessarily if you treat the fuel capacity as half it's true value . Dont be afraid to use up the fuel until the needle touches the last marker too . The gauges are spot on especially at the low end. also fuel expands a great deal when warmed up so any discrepancies you might see are down to this .

I normally agree with pretty well everything you advise on here because you actually appear to know what you're talking about but I have to add this about the low fuel, the pump uses the fuel as a lubricant/cooler so for that reason alone I don't recommend being in the light on reminder zone for any longer than necessary, also if you pull over and sit in the camber of a road there's a good chance the pump will be completely out of the fuel and overheating, what you say about not tugging unnecessary fuel about is very true and will lower the mpg overall, the muck in the tank I believe isn't the petrol station as such, I believe the muck comes from pump nozzle to tank filler being washed into the tank, plastic tanks sorts out the rust issue however when you said you've not found any muck in a Volvo tank did you check the pump gauze or split open a fuel filter to see what's in there? lastly the pump will work better if it's got the weight of a few gallons helping push fuel into it over it hunting for fuel in an irregular supply sort of way.