almost 800km to a tank !

i set my trip meter before i went away on saturday and allmost got 800 km to a tank of fuel. i drove 110 kph..but i somthing that has been bugging me is i find my truck doesnt like to stay up to temp....it usually doesnt sit right underneath 90-100 degrees. it runs around 80-84 going down the high way unless you hi a big hill and put a bit of load on the motor... but in this case the motor not being up to full temp would that would cause it to burn extra fuel.... right ?