I had a twin tank like yours in my 1975.
The whole tank thing has been spoke about many times!! Not sure we ever got the right answer?
Some say to just hook up both tanks and run it, but when you do that one always seems to run out before the other?
Some say you need equal leanth of hose on both sides to have them drain at the same time, Nope, not on mine?
Same with the vents, have to all be the same, suppose to have equal pressure?? Yep you guessed it, no worked for me?
What I did at first was Add a three way switch, Now I have right switch for right tank, middle off, and left for left tank, Then drive were I was going then swap tanks. This way I always had gas,
After I swapped my 165 I/O to a out board by way of a bracket, I used the space were the motor was to add a 25 gal tank. This worked great as most all my trips never used more then 20 gal, problem solved.
After you figure out how many miles you can go on how much gas, you wil be set, My gas gauge never worked and I never ran out of gas, except like you, one tank empty and one full.
20 gal gas in a I/O is like 3 mpg but I always thought 2 mpg. 2 x 20 is 40 miles per tank, 80 with both tanks.
The rule of thumb is 1/3 to your spot, 1/3 to get home, 1/3 for reserve.
I now have 1 tank in the Sea Ox with 110 gal gas! No more worry's!
Now when this happens we will talk about it, when the tanks get full, and you are splashing gas out the vents as you drive the boat down the road.
