Start with the exhaust leak!if its at the manifold you can ruin exhaust valves!!!! just so you know a 5.3 is about a 327 Not a 350

A 350 is 5.7

leave your stock air intake alone if you're worried about your mpg.  more air = needs more gas. 

if you REALLY want to know what your car is doing get one of these:



its called an "ultra gauge" and it costs about $60.  it plugs into your vehicles ODBII port and it will give you EVERY detail about your car.  maf, throttle position, o2 sensor readings, fuel trims, cam advance, timing, spark, etc.  you'll know exactly why you're burning excess fuel.

I pulled my boat home recently with my 08 tahoe. It gets there (270 miles) with a half a tank unloaded, but it will burn the entire tank with the boat behind it. 


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Go back to stock? Lol. If you want a lifted brodozer with fair mileage go buy a diesel, I have an 04 Tahoe bone stock with a 5.3 also with 3.73's and towing I get 16.2 avg going to and from our river house.


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If you want to calculate for a new gear ratio, use a proportion to get you back close to stock RPM at highway speeds.  Round to a lower gear ratio (numerically higher) for increased torque multiplication to help with the increased rolling/aerodynamic resistance of your lifted truck.


New Ratio         New Tire Diameter
---------------  =  ----------------------------
 Old Ratio           Old Tire Diameter

Here is how the numbers worked out for my Jeep

New Ratio         35
---------------  =  ------
      3.73              30

New Ratio = (3.73 * 35) / 30
New Ratio = 130.55 / 30
New Ratio = 4.35:1

4.33:1 would be within 1%, if it was available, except the closest ratios available for my axle are 4.10:1 and 4.56:1.  4.56:1 puts me at 4.6% of my target which will increase my RPM from 2100 @ 65MPH to 2196RPM.  However, 4.10:1 would yield ~68MPH @ 2100RPM.  Again, the added torque multiplication will help overcome the added resistance caused by the lift and bigger tires. 

Too low of an RPM will lug the engine and consume more fuel just like winging it up.

If you want to be more accurate, substitute tire roll-out for tire diameter.  Just make sure that gear ratios are on one side of the equation and tires are on the other and that new are both on either the top or the bottom with the old being on the other.

Good luck,

Joe

Big River or Needles?


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Big river.
The drive back from Needless eats up the entire tank sometimes.
Between the hills and diving into the wind... fock that drive.

-beerjet-