From VM1000 manual
2.5.1 INITIAL SET UP OF THE FUEL COMPUTER \_-' Your fuel computer features the capability of 'remembering'the SIZE of your fuel tank. This is setup once after you receive your system, or if for some reason the fuel computer memory has been lost. (NOTE: A lithium battery module is located inside the DPU and will require replacing about every three to five years.) Setting up this level is easily done by following these steps: Refer to 'VM1000 QUICK REFERENCE' in lllustrations section.
STEP 1: With power OFF, press and hold 'BUTTON 4', then turn power ON to the VM1000. Wait for the display test patterns to go off, then for the 'HRS', 'REM', 'BRN', 'ADD' indicators to activate, indicating that fuel tank setup mode has been activated
STEP 2: You will program the tank size in whole units. Press 'BUTTON 3'fortens ('10.0') and 'BUTTON 5' for ones ('01.0') . lf you accidentally went too high, then just repeat pressing 'BUTTON 3' until the display resets back to '00.0' allowing you to start over.
STEP 3: lf you like the value displayed, then press 'BUTTON 4'to accept the new maximum tank size, otherwise just leave everything alone and after 20 seconds the setup mode will automatically be canceled, leaving your previous tank size unchanged
STEP 4: ADD fuel to your computefs memory according to 'Adding Fuel to the Computer.
Give that a try.
Dan
P.S. 40 gals should be fine. Anything over max tank qty will reset to value you set using above steps.