What is your guide line for not running out....
how low do you go??

best brands....diesel?

best brands...gas?

Good question. Easy answer: fill up at 1/4 tank.

essay version:

Generally, running a tank to near empty invites problems with picking up crud sloshing around the bottom of the tank.
IF you have nice clean tanks and
IF you have large primary filters/separators and
IF you have a two tank system with a full 2nd tank, you can run the tank right down and switch tanks.

In addition to introducing extra crud, running near empty may allow air into the system. With a modern recirculating fuel system, gas engines can handle a bit of air and will purge and restart fairly easily.

Diesels, not so much. Once air gets into the injection pump and lines it may take quite some time to get it all out and the engine will not run well.

When I changed the fuel system in the 86 I had to install a higher pressure electric fuel pump to feed the fuel injection system. I removed the in tank pumps, installed a large fuel filter near each tank. New lines and tank selector valve also.

I installed a fuel pressure gauge when I still had a carburetor. It has proved very useful over the years. It now measures input fuel pressure to the fuel injection. It will show a drop when approaching empty or when filters are saturated.

With fuel gauge, good filters, a pressure gauge and a spare tank you can run pretty low... but I don't usually.

In my single tank diesel Monarch I like to fill whenever below 1/4 tank.

Agree with Steve's comments, will add:

I have a 60 gallon tank, While I do watch the fuel gauge, I usually plan fuel stops by mileage driven. I try to fill around 400 miles driven, using lowest average MPG around 9 MPG, that will usually result in a fill up of 44 gallons at 1/4 tank.

My tank is a flat bottom tank so I don't like to get below 1/4 tank. If I had a sump area in the tank, I might go a little longer. Diesels are a real problem if air gets into the fuel system, they may not recover quickly.

400 miles is also a good time to take a break!

I fill up daily. Usually when we come off the road at nite. Fill and find a place to park. I have never had the tank below 1/4.

I believe Lynn's favorite number is E and her favorite sound is the ding for low fuel warning. Gas vehicles!

Not with DBarth. I took it done to 1/8 of a tank the first couple times to make sure I got rid of most of any old fuel. I also don't like to not get below 1/4 tank. If you have ever run a diesel out of fuel you don't ever let it go low. Not a fun time purging sll the air and getting it running.

I agree with all the replies & usually don't go below 1/4 tank .
Another reason not mentioned . The generator will NOT run with less than 1/4 tank . A safety feature to keep from running out of fuel when parked . ( and running generator )
Bob

1/4 tank, for above reasons (never want to run out on a diesel).
But also a little "doomsday" prep, in case of power outage gas pumps do NOT work and you can maybe make the next station or choose the price for your fuel.

I put 47 gallons in a 50 gallon tank on ours and it had just hit a 1/8th of a tank. Was following gas buddy and the exit I wanted to get fuel at was a solid 15 cents cheaper than the previous exit. Was a little frightened it took so much so since then I don't go lower than a quarter tank or around 38 gallons.