Thriftier is the comparative degree of thrifty.

thriftier, thriftiest

The word thrifty is short enough that the comparative and superlative forms are thriftier and thriftiest.

Boat fuel consumption is based more base upon gallons per hour. A 31' cabin cruiser with a moderate V8 gasoline engine may consume up to 3 gallons per hour at 3/4 throttle. Standard outboards are much thriftier.

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 4 words with the pattern T--I-T---. That is, nine letter words with 1st letter T and 4th letter I and 6th letter T. In alphabetical order, they are:

thriftier

thriftily

thristing

thristles