69thunderbird

make sure your computer is calibrated to 5.13 and whatever size tire you are running.

when i had my gears installed they left calibration at 3.73.

i'm getting up to 19 MPG using my cruise control in an 4 door on the highway with Mudd Tires. averaging 18 when i don't step into it. average 15 MPG when driving normal.

compare your RPM's and MPH on following chart to see how close you are.

http://www.angelfire.com/fl/procrastination/rear.html

jkkat

With RubyDoo's 6 speed and her 35's and 5.13's i can get 19 dueing 60 and 65 gets about 18 then city is down to about 15 or slightly less depending on how hard i hit the gas.

brent_f70

6 speed, 35's and .13's on a 2 door and i average 16.5-17 mostly in town driving and I rarely go over 3000 RPM.

cgrace12

I've noticed that exact opposite. When it is very hot 90+ my MPG shoot up big time! I can get close to 17mpg when it's hot, and when it cools down it drops back into the 15's. During the winter I have trouble keeping it above 14. With the colder denser air the computer will add more fuel. That's the point of a CAI - more fuel = more power.

mikemil

4dr auto, 37s with 5:13 get about 16.5mpg mixed driving (speedo and calculator method)

RgnNxn08

I have 5.13 and 37's and am averaging around 15mpg

tambil68

Maybe, with the heat in Florida I also get very moist air, which might be why my mpg gets worse.
Your cold weather probably brings all the moisture into play for you.
Just a thought.

Littlejon

I wish that was the case in Minnesota, it is like 150% humidity all summer long

JPop

I agree, and the best formula for success with an automatic transmission is under gear and over convert. Unfortunately I'm not aware of any after market torque converters yet. Making the torque converter do some of the heavy lifting aids in low rpm traction and improves drivability.

Target rpms are also important as there is a huge jump from OD to 1:1. With the underpowered engine plant hitting that slight grade at 70mph will slam you into the 3500 rpm range, well above the 2500 rpms where the torque curve flattens and reaches a point of diminishing returns.

The real call on gear ratios is how your vehicle is driven. Daily Driven, highway cruises on a regular basis suggests opting for a gear not so tall as what would work for lots of 0-35 blasts through the city or on the trail isn't the same.

JPop

Agreed, the biggest factor is always the right foot.