You're not accounting for drag. Sure, your RPMs are lower, but it's still taking more fuel to overcome the extra drag that is being placed on the car. It's why most major highways are posted at 60-65 mph. They don't just post speed limits for safety, they account for fuel mileage and time too. Now, granted weight, tire pressure, rear gears, octane rating and condition of fluids will all play a role in mpg too, but that is beside the point.

 

one thing i forgot to include was animals if they`re troublesome if im cruisin those highways in the night?

 

Is 5-10 more mph of drag that much stronger on the car?
It just sounds like my engine's working hard going 63 in 5th, like I need to down shift. I'm convinced 6th is better until I see numbers lol

 

Yes the white tails can be a problem at night. I have logged a LOT of miles on 81. I have never had an issue with running 75 ish. I try to stay just under. Here in PA the tickets go by Miles over the limit. The Step on is anything over the limit the second step is 11 MPH over and the fine jumps up pretty quick. I think a lot of the state cops are looking for over 11 tickets as they generate more money for the same work. You may want to slow it down a bit around Harrisburg.

 

You can at 65 in 6th can't you? I mean there might not be all the power but you don't need that if you're on flat road and not passing. But with my A4 if I go above 70 in OD you can tell the car wants to shift again (but can't) once you let off the gas and feel how much work it's doing. If I keep it around 60-65 it's fine. You can also can tell a major difference in gas mileage. That may change with the M6 since you have double overdrive.

 

thanx alot for the tips! normally when im on 87/90 i set cruise 74 without issues even if theres one with a radar gun sitting ahead limit theres 65 i believe..

does anyone know normal temps for the cats? I borrowed a infared thermometer and saw at operating temp the cat at 320F, does this sound normal? i know its one of the hottest parts on the car, but with the way how the car has been running as of lately when hot, its like a ten year old poorly maintained 4 cylinder.

 

Yes, 65 is about the lowest I could probably shift into 6th. cruising AT 65 in 6th is probably the best possible mileage. And yeah, when im at 60-65 in 5th if i let off the gas ill get rear ended ;D