For first 1,000kms it is advisable to not keep engine rpm constant for longer periods. Engine will break in better if it is run on all engine rpms for some time. Give engine some time to warm up after starting in the morning. Do not floor the paddle or run the new engine near maximum rpm for first few hundred kms.
Keeping a lighter foot on accelerator will give you good mileage. Also driving at a speed which requires minimum breaking will give you good average. Engine rpm's within the range of 2~3k normally is a better compromise between good engine life and efficiency.