Fuel capacity M20J

[0TreeLemur]

When calibrated after the JPI EDM900 install the reading on the right tank wasn't right in the mid-range, causing repeated "Fuel Qty. Mismatch" warnings.

Yesterday we ran the right tank dry in flight and re-calibrated.  To my surprise, starting from 0.0 usable when filled it to 32 gal. it wasn't up to the tabs!   Full to the tabs was 33.1 gallons. 

[Hank]

When my tanks were stripped and resealed (52 gal), I was billed for 52.4 gallons. Both were up to the caps when I checked. 

Some variation is to be expected, especially if the tabs are welded in during tank neck construction. (My C has no tabs.)

[Jim Peace]

Fuel density? 

[Pinecone]

Running a tank dry could bring you lower than official unusable fuel.

Unusable is based on expected attitudes, so with out pickup location, in a descent, nose low.

The proper way to calibrate is fully drain the tanks, then put in the book amount of unusable fuel.  Which is 1.5 gallons per side for my M20K 252.

[PT20J]

14 hours ago, 0TreeLemur said:

When calibrated after the JPI EDM900 install the reading on the right tank wasn't right in the mid-range, causing repeated "Fuel Qty. Mismatch" warnings.

Yesterday we ran the right tank dry in flight and re-calibrated.  To my surprise, starting from 0.0 usable when filled it to 32 gal. it wasn't up to the tabs!   Full to the tabs was 33.1 gallons. 

“Tabs” usually refers to the 25 gal indicator in the tank visible inside the tank. Did you mean you filled it to the bottom of the filler neck?

Anyway, the proper way to calibrate the tank is to drain all the fuel by removing the sump drain. Then replace the drain and add the unusable fuel which is your 0 starting point. 

Unusable fuel for a J is 1.25 gal/tank. So, the empty tank capacity is 33.25 gal. You only got 33.1 because the pickup is slightly above the lowest point of the tank to avoid picking up debris so running it dry will not completely empty the tank.