On my 29' Fountain with 140 gallon tank it takes 10 gallons to go from 1/4 to 3/8s. If it were a linear add or drop it should take 17.5 gallons. I have gas cans filled in the garage. Next I will see what it takes to go from 3/8s to 1/2. I hope to get this all figured out this summer. Like "The Mayor" I don't think the last 1/4 tank will take the boat far. The last 1/4 "should" be 35 gallons. I'll bet it's more like 10, maybe less...

My sender is at the back of the tank, and I also learned the hard way that 1/2 is 1/4 and at 1/4 I better be pulling into a gas dock. I also discovered that Mackinac Island doesn't have gas docks open on Sunday that same day. I also learned that day that 150' of anchor line is nothing more than a tease in the middle of Lake Huron. I check it on the trailer then figure by miles how far I can go from there.

That's the way it goes.
I had range figured out fairly well in my old 24' twin O/B Sonic until I modified the motors including adding bigger carb's.
Was about 1/2 way back across Lake Erie from the Canadian side when I noticed the fuel gauge had dropped to <3/8 when it hit me what I had done. On a trip (80 - 90 mls) that I had done dozens of times and always returning w/a 1/4 tank of gas I instantly knew there was no way I'd make it due to increased fuel use of the modded motors. Got to within about 2 mls of West Bay Inn on Kellies Island when it shut down in 4' - 5' waves!
Good news was having Boat US towing and a GPS and 100' of anchor line in 30' of water. Gave them lat/long. and they were there in 15 mins and didn't cost me a dime.
The bad was they wouldn't give me gas, would only tow me the 5 mls to my trailer. Found out later they do this so they can get reimbursed for an expensive tow bill instead of an inexpensive delivery of fuel.

502SS, thats the style of sender I bought from Livorsi and just threw in the trash.

I agree...I think I will leave it that way. One more way to F with my passengers.....

I dont even look at the gauge anymore instead i keep very vigilant track of fuel put in vs. miles run on gps vs. average mpg previously calculated through many gallons run through my boats. For an added safety net though i use 1mpg as a loose calc and keep the extra .62 and .12 for a "just in case". Seems most fuel gauges are least accurate on the bottom half (when ya need ot lol).

ScarabIII stock 365 Mags - 1.62 mpg tracked over several years
38 Donzi ZX stock 525 Merc SC's - 1.12 mpg tracked over several years

On a side note i had a friend at Tenneco several years ago telling me about a fuel sender they were working on that relied a sonic pulse in the tank to measure volume. No mater what state of agitation the fuel was in the pulse would measure just non liquid space. Pretty cool, not sure what ever happened to it though my friend went on the work at Whirlpool.

I have fuel flow sensors for both motors that will tie into my GPS and tell me MPG, GPH, Range, Fuel Burned/remaining etc once I take the time to install them and figure out how to make them all talk to each other.
One of the main reasons I bought them was for testing optimum cruise settings i.e., RPM X MPH's. I know the speed that the hull starts freeing up and running cleanly and I think I have an idea of torque peaks on these motors but so far it's all conjecture
Planned on doing that w/all the props I collected for the boat but none of them have run good enough to make the first cut.

BTW, pretty impressive #'s on your old rides based on their size. In the 7 yrs I had my old Sonic I never ran a full tank though it in conservation mode. Was always either rough (Erie) the 1st 1/2 of trip which means hammering it when you get in the lee of an island or it's nice the whole trip which means....., well you know.

Those were actual averages over a years time. I dont run 'em all that hard and i know where the optimum spot is . . . . . when i see the boost gauge move it starts suckin dead dinosaurs at an alarming rate

I have a Navman GPS with twin flo-scans on the Scarab but never thought they were very accurate. Atleast not as accurate as my old school method.

I ran mine down to where it was just touching "E" last week. 50 gallon gas tank, took 45 gallons to refill it. So I have a 5 gallons, or 10 miles, once I hit empty. I think that's pretty accurate.

I do something similar, only I use average GPH. pretty accurate.

My latest boat has Smartcraft. I can tell you within a gallon or two how much fuel I have.

Thats only if your fuel pick up goes all the way down into the very rear bottom corner of the tank


On my boats the fuel pick ups are different lengths in the tank so the port motor will run out first leaving the stbd motor with the steering to get ya in if you push it.