Was this in reference to my post? If so how would lower gears help fuel economy at interstate speeds? Alot of time over 70 miles per hour. Now if I can add these with a negligible change to mileage throughtout my travels I would seriously consider it since it would help with pulling my 5th wheel. I travel some accross the state and the speed limit is usually 70 everywhere but when you get out west towards Balmorhea state park at the foot of the Davis mountains the speed limit jumps to 80.

 

I probably should have quoted you, but yeah. From what I understand a 2.70 or 3.08 gear is just a interstate gear. A 3.55 is a good city/hyw gear. Its kinda in the middle, isn't not a interstate gear but not a towing gear either. It will greatly improve your mileage in the city,when towing and driving uphills.

 

Those cracked manifolds are probably the number 1 contributor to poor mileage. The O2 sensor is downstream of those air leaks. They are measuring the amount of oxygen in the exhaust so now the PCM thinks the engine is running lean so it starts dumping in more fuel to compensate.

Your larger tire/odometer error is also throwing off the calculation if you are using the odometer as the measurement of how far you have traveled. If you resolve both of those issues I bet your mileage will increase significantly.

 

Well this is not exactly an apples to apples comparison but lower gears do not always mean poorer MPG. My 2005 E250 4.6L with 4.10 gears got 18 MPG if I kept it between 65 & 70 ......18 pushing a brick through the wind! My 2008 E250 5.4L with 3.73's could only be babied to 13.5 ! My 2010 Transit Connect gets 26 but I get bullied all over the highway & people stare and make crude comments......

If you could figure the RPM your particular engine gets the optimum MPG's and the speed at which you'd travel most of the time you could gear it and get the best MPG's possible with your truck.