The difference between 2.73's and 3.73's is much more than 10-15%. You needn't guess at it - it's something you can easily calculate if you know the gear ratios. With the change, the car only travels 2.73/3.73=.73 -- 73% as far for each rotation of the ring gear. That throws the speedo off by 27% - it's going to register 27% more miles than actually driven - and 25.1 mpg becomes about 18 mpg - same % adjustment.
My last around-town tank was 200.5 miles on 8.7 gallons; I'll let you do the math. And my speedo/odo is VERY accurate -- on the Interstate the odo shows 49.9 miles for 50 miles measured by the mile markers.