If you had an auto C5, the gear ratio was 2.73 rears or if you had the performance rear it was 3.15. Now you have a 3.46 rear. Stick versus stick the rear is the same, the tranny is slightly different. That would explain a small amount of difference. I'm not sure why the GTO gets such bad mileage. My 02 Z28 with an A4 and 3.08 gears with 335 RWHP got 29-30 on the highway and 19 in town. My Z06 C5 got 18 in town and 26-27 on the highway. The GTO is getting 17 city and 22-24 highway with the same type of driving. I think the weight has something to do with it, but that can't explain all of it. The CD is good for the GTO and matches the C5, but the GTO does have more frontal area. Still it shouldn't be such a large difference IMO. Oh well, I didn't buy the car for mileage, so I can live with it I guess.