The bikes I've had that have had reserve tanks have also been old enough to raise the disconcerting follow-on question, which is: "is the reserve gas also full of sludgey crap that's settled in the tank and hasn't been disturbed really in a year, and am i about to run that through my poor carbs?"
My friend had a truck with a reserve tank, but it was the same size as the main tank, so he would just flip the switch at every fill up to make sure they both got used.
Had this in a 70s F150. A "Main - Aux" switch on the dash, right above the 8-track player. I used to let the main tank sputter out on fumes and then triumphantly shout "Rerouting auxiliary power to engine!" while sliding the switch. Letting them empty out alternately would have been a lot smarter.
My father drove a '95 F-150 for years that had the dual tanks. Shortly after highschool I got in accident that ended up totaling my vehicle and got a couple months I was using his truck (he runs an Auto repair shop from a garage behind the house so he almost always had something available to drive) and I ended up using it to go out on a date with someone I had met at work.
I noticed on the way to pick them up that the truck was running on empty in the main tank but I checked and the aux tank was full. Then I remembered the first time my dad let the tank run down and start sputtering down the road and decided to keep going on the empty tank.
Make it to pick them up and start heading down the highway(where we were it was a good 3-4 miles to the nearest gas station) and then the truck finally started to sputter. I proceed to play along with it pretending to panic for a good 20 seconds and then I turned and saw the look on their face and couldn't help but start laughing. Switched to the aux tank and when the truck started running again I turned and and the look I was getting indicated I was being mentally murdered. Then they punched the crap outta my arm and started laughing and calling me not so nice things.
Ended up being an awesome night out with someone I'd end up being friends with for a long time. It's weird how this kind of random conversation in an unrelated internet post can drag you way back down memory lane.
This is typically used for agricultural/off-road fuel which is not priced with road taxes and as a result much cheaper. Off road fuel is dyed red in the US. If you get caught running dyed diesel on road you will be fined. Thus the switch on the dash, when you leave the highway to drive on your farm you flip over to dyed fuel to save $$.
Oh, fascinating! My first vehicle was the family's 3/4-ton Diesel '84 Chevy Pickup from the farm, and I'd forgotten it had an Aux fuel tank! This makes a lot of sense.
the two-tube design of the tank on my 1975 honda CB meant that there was about an inch and a half of tank that sat below the primary fuel port. Tank crud (steel tank, theoretically passivated, 40 years old) settles faster than I ran through a tank of gas, so the bottom layer had sediment in it fairly regularly.
I kept spare inline fuel filters in a tool roll just in case after a while.
