I am looking into purchasing an 07 qcsb hemi that has a lift and 35 inch tires. I am not sure if the dealer had replaced the rear end gears or if they are still factory. So what I am wondering is if they still are factory what kind of gas mileage can I expect? And also would it be best to swap gears and if so to what ratio and what type of gas mileage would I get with them?

 

qcsb hemi w/ 35's = crappy milage. If i'ts got 4.10 (or higher) gears you'll at least have accelration with crappy gas milage, otherwise slow with crappy gas milage.

 

I expected crappy mileage but what exactly is 'crappy' mileage

 

You can always lift the rear and place a chaulk mark on the bottom or the drive shaft and the bottom of the tires, place the truck in neutral and turn the tire and count how many times the drive shaft spins. That will give you an idea of what gears are in it.

 

I would guess, depending on the gears and how you drive, you'll prolly get around 12-13 at best. I seriously doubt the dealer replaced anything on the truck, espcially not the gears. As far as the gear ratio goes to milage, the lower gears your running the better milage you will get around town and short trips but you will suffer on the Highway at 70 and above and visa versa. Personally, If its got 35's on it and it doesn't have at least 4.10 gears, I wouldn't touch it.

 

9-10 mpg would be my guess and lower gears you expect to give better milage on the highway can actualy give worse highway milage because the engine is lugging with those big tires and is out of it's powerband

 

Well, I have 35s and weigh in at close to 7000#s, with a full toolbox and an empty bed. If I ain't got the heaviest 1500 on the site, it's gotta be close.
I have 4.56 gears and get right at 12 mpg in everyday use which is primarily in town type driving. Some lights, stop signs, 35 mph and occasional 45-55 mph between towns. Trips up to the farm, I can squeeze 16 mpg if I hold it to 70 and usually have my ATV in the bed.
Now, that being said, I have a good bit of performance mods that also help my gas mileage when I can manage to stay off the gas pedal. CAI, Tuner, LT Headers and an e-fan all contribute to improve gas mileage. But the weight of my truck is an equalizer, my guess is you can expect about the same fuel economy I get with a mostly stock, lifted truck with 35s and 3.92 gears and MDS (pray it doesn't have 3.55s with those tires!)...

 

The voice of experience is always to be heeded. Glad ya weighed in on this, with a more informed opinion

 

I can tell you guys I filled up with 93 octane today before heading out of town. As you know I have 456s with Auburn LSD and 36s. I drove 2 hours on the interstate going anywhere from 75 to 90 and checked my mileage when I go there and found myself getting 16.5

 

Impressive