The generator uses 1.75 gallons per hour or 1 3/4. 
In order to find this, we need to divide the number of gallons by the number of hours. It's easier if we first turn them into decimals. 
2 1/2 hours = 2.5 hours
4 3/8 gal = 4.375 gallons. 
Then we divide. 
4.375 gallons/ 2.5 hours = 1.75 gallons per hour.