How much fuel do you really have left when that fuel light comes on

I ran out of fuel across the street from where I was fueling. I get that from my dad, running out of fuel is not the end of the world, but getting to a receiver after they stop receiving could be.

Anyway, I got 5 gallons, and it took about 20 seconds or so to get it started. Unfortunately the way our fuel cards worked at the time, I couldn't fuel all the way, so I don't know how much is really usable. 2 150 gallon tanks.

 

I ran out of fuel in the parking lot(on my way to fuel) at a pilot a few months ago. I picked up in Virginia Beach and I couldn't stop at Charlies(only place to get fuel in the area) because I was oversize/high load and the route took my all the way around to I-664 to get back to 64. I managed to make it off the exit, I think it was 211 or 205 on 64. I turned left to go to the Pilot and it started stumbling, turned left and started to make my way around and ran out about 200 ft before the pumps. 2016 KW900 but with a 99 Detroit. Put some fuel in the filter and about 5 gallons in and it fired right up. That taught me to fuel when it gets to about 1/4 or 1/8 even before the needle gets near the redline..... But previous trips tell me that when that red light comes on I only have about 50 miles...

 

Well this is slightly incorrect. Except for the air intrusion part. A 12.7 Detroit will have to have the filter pulled and filled, then use just enough ether to get the engine to kick over so the pump will prime and get fuel to the injectors. A C15 has a hand pump that allows you to prime the filter and pump. Newer engines like the ISX actually have an electric lift pump that can self prime with about 5 cycles of the key. Maxxforce 13's have a hand pump as well.

 

First figure out the capacity of your tanks (it's stamped by the fill tube), and subtract 5%. That is the maximum fuel you can fit in the tanks, accounting for the air gap at the top and the anti-siphon that makes topping "all the way" more time consuming than I am generally willing to deal with. I have 2, 100 gallons tanks so in theory I have a maximum of 190 gallons of fuel.

Subtract 15 gallons per tank, so in my case I have a maximum of 160 gallons of "usable" fuel, but I generally don't like running much past 150 burned. As others have pointed out, the fuel pick up line is at the bottom, but not underneath and not necessarily centered. Go up or down hill and you may "run out" because all the fuel pooled to one side and let air in the line. About 6 months in I was at one of our yards checking in with service listening to a driver argue with the service advisor. The driver said he couldn't have run out of fuel as he only added 150 gallons of the 200 gallon capacity so it had to be the fuel pump. The service advisor pointed out that the ramp off the interstate (where the driver ran out of fuel) is rather step and leans to one side. It was an eye opening conversation.

Fill your tanks to max. Reset the digital fuel/trip meter (depending on how your truck is spec'd).

Run until the light comes on.

Stop and fill all the way up. Compare the fill amount to how much the ecm thinks you've burned - it should be within a few gallons. Generally when my fuel light comes on I will end up adding 125-130 gallons. So I have at least 20 gallons of "usable" fuel, probably more like 30. In other words I have between 200-300 miles before I will start getting really nervous about my fuel level. This is on a 2019 p4 cascaida. On my 2012 Cascaida the light wouldn't come on until I hit 160 gallons burned - so if the light came on I got fuel right the frick then.

Right now I am running essentially local with fuel stops all around, so I don't really care about my fuel level. I fuel when it is convenient time wise. When I am OTR, I reset the digital fuel gauge every time I fill all the way up. This way I can tell exactly how much fuel I have burned and what my range is. If I do a partial I make sure I end on a 50 or 75 and just keep a running tally in my mind.

 

This is the fuel tank chart I was talking about. It’s from a Stevens Transport’s manual and probably only applies to cylinder shape fuel tanks. You also need a fuel stick to use the chart. It also helps if your tank cap is behind the truck cab so you can easily slide in a fuel stick to measure how many useable gallons you have left.

 

I have a 75 and a 120 but this is a day cab. Once I'm down to 1/4 tanks I'm fueling if I haven't already. I had the fuel light come on only once but I was heading back with a load anyways. We fuel at the yard.

I've never run out of fuel and never will. Nothing worse than fuel left at the yard and not in the tanks. I run 100 miles daily it's easy to determine my fuel needs

 

Ehh, not so much how much you have left, but how far can you go. That is impossible to calculate due to hills, load weight, and other traffic. Then worst case scenario, the pumps are down at the truck stop.

But, generally speaking, probably 20-40 gallons when the light comes on.
There's been times I was able to drive over 200 miles after that, including having the needle on E for an hour and refilled with 180gals.
But I've also had a time where the truck only took 170 gals and I had to reprime the engine before pulling off the fuel pump.

Consider this, the tanks are cylinders so the bottom 3 inches will be much less volume then the middle 3 inches. It's the same with cars, the tanks aren't perfect squares, they tend to be a completely random shape with bulges and gooves. I doubt that fuel gauges are designed to take into consideration variable volume per length/width of the set level. So a 100 gallon tank would truely be at 50 gallons with the needle at half, but above/below that it will be off. The needle showing 1/4 would be less then 25 gallons due to the volume being less. It's just a measurement of the level in the tank, not the actual gallons that remain out of capacity.

 

There are two positions of a fuel gauge needle that are accurate. Those are full and empty. The positions in between are for reference only, based on your experience with that vehicle. So...
The T680 I drive has two 120 gallon tanks. The amount of fuel left in the tanks varies when the light comes on. If running on flat ground, they take around 175 gallons. The other day, I was headed East on I80 in Iowa and took exit 284 to go into the I80 truck stop. At the top of that ramp, when you turn left, it gets a little steeper. When I did this, the fuel light turned on. Anyway, I went in and fueled and it took just under 160 gallons.
So, I always try to operate on the safe side of things. I don’t want to run out. I would lose credibility and cost unnecessary expense. Here’s what I do. I keep a small yellow pad on a clip board, I write down the date, location, gallons filled, and the miles on odometer. I know for sure, from full I have 175 useable gallons. 175 gallons times 7miles per gallon equals 1225 miles I can drive. There are variables. Extreme wind. Very heavy load. Is the truck running poorly, in need of service? Are you running in mountains? Etc.

I know it will run further than the 1225 miles, but why push it? Running out of fuel is a major PITA! Don’t try it, you won’t like it.

 

Check out my comment #36. The chart I posted can give you a rough idea of how many usable gallons you have left. So in the case of the OP, he can stop the truck once his low fuel light comes on, and use a fuel measuring stick to determine how much farther he can go. The chart compensates for unusable fuel sitting in the bottom of the tank.