dfw

**Dominic Toretto** · 02-14-2014, 01:37 PM

Just curious about how much one can expect fuel economy to suffer. I've got 3:15s. Any ideas how much of a change it would be going to 3:55s or 3:73s?

-Alex

**DirtyD** · 02-14-2014, 02:08 PM

RPMs will go up obviously.

These cars have a sweet spot for hwy MPG around 1800-2000 RPM.

For example:

285/35/19 rear tire on your car currently turns 1904 RPM @ 70 MPH in 6th gear on the 6R80.

If you upped your rear gears to 3.55, you would turn 2146 RPM, theorectically, at the same MPH.

3.73s would be a big jump and your hwy MPG would suffer, as you are now over 2255 RPM. You would probably lose 1-1.5 MPG on the highway, but city may go up a little bit.

**Dominic Toretto** · 02-14-2014, 04:57 PM

Thanks Derek.

-Alex

**KurvStr8nr** · 02-16-2014, 04:00 PM

I have 3.73's and get around 20mpg in town - if I can keep my foot out of it and 22-23 on the hwy at 75-80mph.

**Hddcrasher** · 02-16-2014, 05:22 PM

How the heck do you guys get that kind of mileage in city? I have the auto and only get about 14mpg

**KurvStr8nr** · 02-16-2014, 06:23 PM

Part of my commute is feeder roads - higher speed. If I'm driving inner city, I'm probably closer to 18mpg.

Try changing your info display to show fuel economy. You can learn what you're doing that eats gas. As mentioned above, sweet spot for mileage is 1800-2000rpm, so I shift 2500 max when I'm trying to be efficient. When I coast up to stop lights, I put manual trans in neutral - not as convenient to do with auto.

**Hddcrasher** · 02-16-2014, 07:01 PM

I just can't lay off the throttle!

**Hddcrasher** · 02-16-2014, 07:01 PM

Hey D what app or spreadsheet did you use for those calcs? Thanks

**DirtyD** · 02-16-2014, 07:42 PM

Originally Posted by Hddcrasher

Hey D what app or spreadsheet did you use for those calcs? Thanks

Spreadsheet I made on my own. I'll PM you a link to it tomorrow

Perry · 02-16-2014, 08:38 PM

I have 3:55 gears in mine and on the highway if I drive it right I can get 24-25 MPG on my weekly trip to Dallas from Winnsboro out of my boss intake, JPC tuned, stalled auto.

Perry

**jng2985** · 02-16-2014, 08:45 PM

Not sure how the fuck y'all with 3.73 get 20+? I stay around 16city without resetting every time I fill up, so the avg has more miles/some negative to work against.

txfiremn · 02-16-2014, 11:21 PM

I was just thinking about this today. I took the kids to the Fort Worth Zoo today from Waco, I never got above 20.8 mpg. I was traveling, on average, at 75-80 mph with an A6 an stock 5.0.

**NAVAG** · 02-16-2014, 11:53 PM

Mine has 3:31 Gears or what Ford calls "Rear Axle Promo Pack"

With Bama performance tune on the Highway I get 27-28 MPG...when I go the speed limit and I set Cruise Control. Above 2K RPM and the MPG drops like a ton of bricks.

txfiremn · 02-17-2014, 12:16 AM

Guess I'll test the under 2k theory out. I have a 80 mile commute to work. I always fill up befor going, so fuel weight or amount shouldn't be a factor. I'll see what I come up with when I set my cc at 2k instead of 75 mph.

**NAVAG** · 02-17-2014, 12:44 AM

Originally Posted by txfiremn

Guess I'll test the under 2k theory out. I have a 80 mile commute to work. I always fill up befor going, so fuel weight or amount shouldn't be a factor. I'll see what I come up with when I set my cc at 2k instead of 75 mph.

I have about the same commute. 65-70mph gets me the best MPG. Much below 60 and the engine has to work harder in 6th gear. I usually reset my MPG each time I leave the house to make the 60 mile drive to work and IF I don't stomp on it much I get around 27-28. I got 29 on one trip but I drove 65 and slower the whole way. It's really hard to do. lol.

Not sure how accurate the MPG gauge is in the Premium but since I started driving slow I can make 2 1/2 trips to work and back before I fill it up again. Still too many miles and I'm going to buy a beater for DD asap.