Winter fuels are usually good for a 1-3 mpg loss, a 34% loss in mileage is a significant loss. Did you have adrastic drop in temperature?, if not, I would start looking into it, you may have a bad plug, injector or sticking valve, etc. Even letting iot warm up for 5-10 minutes wont consume that much gas to cause a drop like that in mileage.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed