Fuel gauge and reserve?

pyoung62 · May 30

So I’ve seen this happen a couple times now. Fuel light comes on, shows 50-55 miles to go. The first time, I was on an interstate, so started looking for an exit. Went about 5 miles, then it went to 43 miles, went another mile or two and it then said LOW. Never read anything else. I found a station about 4 miles later and filled up. It took 23.2 gallons. In a 24 gallon tank, to go from 43 miles to putting in over 23 gallons in less than 10 miles was kinda’ shocking. Today, I had roughly the same experience. Went from 40 miles to go to LOW, got gas about 5 miles later and put in about 22.5 gallons. Anyone else see the same? Is there a hidden reserve over the 24 gallon tank capacity?

calgator73 · Saturday at 11:47 AM

7 hours ago, pyoung62 said:

So I’ve seen this happen a couple times now. Fuel light comes on, shows 50-55 miles to go. The first time, I was on an interstate, so started looking for an exit. Went about 5 miles, then it went to 43 miles, went another mile or two and it then said LOW. Never read anything else. I found a station about 4 miles later and filled up. It took 23.2 gallons. In a 24 gallon tank, to go from 43 miles to putting in over 23 gallons in less than 10 miles was kinda’ shocking. Today, I had roughly the same experience. Went from 40 miles to go to LOW, got gas about 5 miles later and put in about 22.5 gallons. Anyone else see the same? Is there a hidden reserve over the 24 gallon tank capacity?

Don't trust the gauge... Use a scanner with live data to get an accurate read on remaining fuel.

Also check this:

https://gmauthority.com/blog/2025/06/gm-keeps-you-guessing-on-empty-unless-you-know-this-trick/

Edited Saturday at 11:50 AM by calgator73
Added source info

pyoung62 · Saturday at 01:10 PM

So are you saying the range shown in the MyGMC app and the dash will be different?

Also, what scanner? An OBD scanner? I can’t imagine keeping that plugged in while driving around.

i guess my main question was on the capacity. When it jumped from 43 to LOW and then I put in 23.2 gallons 4 miles later, did I really have less than a gallon left? Or is there an additional reserve capacity. If I ran it dry or completely drained the tank and refilled it, would it take exactly 24 gallons and no more?

newdude · Saturday at 03:06 PM

On 5/29/2026 at 11:58 PM, pyoung62 said:

So I’ve seen this happen a couple times now. Fuel light comes on, shows 50-55 miles to go. The first time, I was on an interstate, so started looking for an exit. Went about 5 miles, then it went to 43 miles, went another mile or two and it then said LOW. Never read anything else. I found a station about 4 miles later and filled up. It took 23.2 gallons. In a 24 gallon tank, to go from 43 miles to putting in over 23 gallons in less than 10 miles was kinda’ shocking. Today, I had roughly the same experience. Went from 40 miles to go to LOW, got gas about 5 miles later and put in about 22.5 gallons. Anyone else see the same? Is there a hidden reserve over the 24 gallon tank capacity?

When the light turns on, there is approximately 2 gallons left. Light on = reserve point.

When LOW hits, you've got closer to 1 gallon left.

There IS some buffer to that. The gauge and warnings will be slightly conservative at times. Its all based on your current live MPG and rolling MPG average as to when that turns on, plus going from driving to a complete stop, the gauge will move as the float is moving and the tank settles down.

So I'd say you'd have to be looking at your MPG average when you get down that low and see where its at. If you are getting 12-14mpg, its gonna hit the LOW mark faster than if you were getting 18-20mpg.

Edited Monday at 02:13 PM by newdude

StingerZ16 · Saturday at 04:20 PM

1 hour ago, newdude said:

Its all based on your current live MPG and rolling MPG average as to when that turns on

This is the key. Average on your DIC is based on miles average. Where miles may be able to be set at low range or high range (or between).

I just keep in mid that I have 2 gals left and mentally calculate how far that will take me based on current driving.

Highway may be up to 40-50 miles. Town would be closer to 30ish. Then travel no more then half of that.

I have observed that the last 2 gallons of fuel goes alot faster then the first 2 gallons

This holds true for just about any car. My Santa Fe starts at 500 mi range and I better get gas before 390 miles.

MaverickZ71 · Sunday at 09:03 PM

Always refuel when it gets down to 1/4 tank, especially in hot weather. Anything lower than that just wears out the fuel pump faster. And that way, you won't have to worry if you can make it to the next gas station or not.

TrueBlue · Sunday at 10:00 PM

So, if they can create an app for your cellphone that can show you detailed information about the fuel use in your truck and the remainder thereof, why not just put that functionality in the truck to begin with? I think I know, but regardless of why, they really should.

Phantom 4D · Sunday at 10:04 PM

As was said earlier, tanks have a built in reserve based on tank size and vehicle model. The milage to empty is subjective, based on current speed and conditions. If you're driving 70 it's going to drop faster than driving 50. I ran my previous truck with the 5.3 empty just to check the stats. the milage to empty went down pretty much based on my speed. I put 2 gallons from a gas can in to get to the station. The pump cut off at 22.6 gallons.

diyer2 · Monday at 01:18 PM

Why run the tank this low?

Silverado4x4 · Monday at 01:32 PM

7 minutes ago, diyer2 said:

Why run the tank this low?

Yeah I dont get it either as the gas in the tank keeps the pump cool and lubricated, when it drops below 1/4 tank the pump is no longer being cooled and lubricated by the fuel which will lead the early failure.

diyer2 · Monday at 01:45 PM

Yes sir Silverado4x4. This has been discussed before but people don't know this.

KARNUT · Monday at 02:05 PM

I’m fueling at half maybe a little under on trips. One traffic jam will change your ways. Especially if the detour is off the beaten path.

Atlas · Monday at 02:41 PM

There are often two assumptions made in these kinds of threads:

1. That the meter on the gas pump is actually calibrated and the outdoor temperature was exactly 60 degrees at the time of fueling.

2. That the vehicle-in-question's fuel tank holds only exactly its described capacity, i.e. if it's a 24 gallon tank and you put an exactly measured 24 gallons in, the next drop will spill out, and that's generally not true.

Tanks don't have a reserve, the gauge has a reserve. It will indicate empty usually a gallon or two before the tank runs out. But what's in there is part of the tank's total capacity.

Gasoline tanks typically are larger than their stated capacity because they have to have room for volatility and vapor expansion. You can "stuff" the tank by continuing to run the fuel dispensing pump past several clicks, filling very slowly, or pulling the nozzle slightly out, and other tricks. IMO, don't do it, you can also stuff your EVAP lines, charcoal canister, and stuff that shouldn't be saturated with liquid fuel.

I like the solutions proposed above. Just don't run the tank down that far. If you think you'll need to, carry extra fuel.

Edited Monday at 02:43 PM by Atlas