we purchased 2005 STS with 4.2 V8 just took trip from Wisconsin to California and back varied speeds on highway from 70 to 80 mph. best mileage (via computer)was 21.8 mpg. is this normal? my 5.3 V8 four wheel drive Chevrolet Avalanche does this good or better.when we checked on Cadillac it was rated as 25 mpg. on highway what gives?