gas tank size

[waltcentral]

I assume my 1983 FLH has a 5 gal tank. The tank and tins appear to be new at some point. Tank has zero rust inside and inner fenders are clean. The service manual says:
FLH/FLHS.........TOTAL..........5 or 3.5 Gallons (US)
                       Reserve                    1.2 or 1 Gallons (US)

The bike has a Pingle petcock that goes on reserve around 2.2 Gal.
Looking inside the tank it is hard to tell how much fuel is left.  Are there any measurements I can do to the tank exterior to tell?

[david lee]

drain it in to a container with measurements on it

[Hossamania]

Bring along a measured amount of fuel, go for a ride, run it out, add the carry fuel, got to a station and fill it, and note how much fuel it takes.

[96flhpi]

The Pingel on my 5 gal tanks goes to reserve at about 4 gal.  Try to find rider view pics of bikes with 3.5 vs 5 gal tanks it's easy to see the difference.  The fillers are much closer to the rounded sides on the 3.5s.

[waltcentral]

My tanks

[Hossamania]

They look like my old 4 gallon tanks on my '95 Heritage.

[96flhpi]

Yeah Hoss they don't look as big as my 5 nor as small as the 3.5s.  You have a pic from a lower angle?  Often the fillers look a little tilted (not parallel with ground) on the 3.5, at least from the pics I've seen.

[waltcentral]

not great pics but the fill caps do not really seem to angle out like some 3.5's

[waltcentral]

other side

[One4Tone]

if you have a working speedo..you can use the mileage as a guide...

[waltcentral]

Rode today until reserve came on then 12-15 miles to the gas station. For the first time I opened both caps while bike on the jiffy stand I filled the left and capped it and then the right.
Took 4.2 gallons.
I assumed with the crossover I was good.  Good chance of 5 gal tanks.

[96flhpi]

Definitely 5s, which are rarely exactly 5 BTW.

[Hossamania]

Rode today until reserve came on then 12-15 miles to the gas station. For the first time I opened both caps while bike on the jiffy stand I filled the left and capped it and then the right.
Took 4.2 gallons.
I assumed with the crossover I was good.  Good chance of 5 gal tanks.

The reserve on those tanks was 1/2 gallon, I believe. At 40 mpg, if you went 15 miles, you almost burned out all the reserve fuel (20 miles of reserve), so the 4.2 gallons you added was almost the maximum they hold.

[96flhpi]

My experience is reserve with a Pingel on 5 gal tanks is about a gallon.  IIRC with the original petcock reserve was less than a gallon.

[Hossamania]

I forgot that it had a Pingel petcock, that will change the reserve from stock.
The only true way to know, run it completely out and fill it back up.

[fleetmechanic]

The 3.5 gallon tanks were discontinued in the mid 1970s about the time the last hand shift versions were made in 1978.  We have a 1975 with 3.5 gallon tanks and handshift.

[david lee]

The 3.5 gallon tanks were discontinued in the mid 1970s about the time the last hand shift versions were made in 1978.  We have a 1975 with 3.5 gallon tanks and handshift.

my 80 lowrider has 3.5 tanks

[96flhpi]

Fairly certain the FX models have the smaller tanks up to present.  I guess it's just tank now not tanks though.

[fleetmechanic]

I should have said the 3.5 tanks were discontinued on the FL models.  But looking in the 41-84 parts book shows 1 # for 3.5 gal. tanks from '69-'77 and another # for 1978-what was then the present.  So I dunno.

[96flhpi]

Either way, tanks are easily replaceable and the 3.5 and 5 gal tanks for a 4 speed frame are interchangeable.  OP has a new to him 35yo bike.  Good approach not to assume they're 5 gal just because they're supposed to be.

[david lee]

when i used to repair and paint harleys i used to widen tanks to take 7 gallons

[bump]

Back in the late 70s early 80s Harley put 3.5 or 5 gallon tanks on different bikes. Not all flt had 5 gallon tanks and not all fx had 3.5. The tanks are interchangeable.

[david lee]

new aftermarket tanks were crap and had to be repaired

[76shuvlinoff]

new aftermarket tanks were crap and had to be repaired

The aftermarket tanks from Mexico on my 76 FLH have been patched a couple times. Trashed the originals in an incident in 2003.

[david lee]

new aftermarket tanks were crap and had to be repaired

The aftermarket tanks from Mexico on my 76 FLH have been patched a couple times. Trashed the originals in an incident in 2003.

even used genuine are better

[crock]

mine started leaking from the tabs years ago. converted to flat sides and love them. Have an OE set  that are mix & match Bought em from an ini off wrecks. Left side from one and right from another.

[dirtymike]

I fill them full of water and weld them.