I'm new here and haven't found anybody with the fuel mileage this low. I just purchased an 03 1500 with the 5.7 Hemi. Standard cab and bed. Automatic. I'm only getting 5.6 miles per gallon. WTH??!!
I knew it would be less due to the 35" tires with the 5" lift. However, the only running improvements the previous owner had told me about was a "cold air" intake and cat back exhaust system. It does have an 02 sensor code. But even if the 02s were bad, I can't see the fuel mileage being THAT poor. The vehicle runs outright just fine. At idle, it has a slight surge at times. Any ideas what would cause such crappy mileage? I'm pretty sure whatever it is is causing the 02 sensor codes. Not the 02 causing the mileage issue. HELP!!!!!!!!!!!

 

Probably a combination of issues. . Figure out the O2 issue. Then new plugs, copper OE. And clean the throttle body. Then retest your mileage. When you calculate mpg, , are you accounting for the mileage skew the tire diameter is causing? You are going further than your odometer is presenting.

 

What gears in the axles to go with those 35 inch tires?

 

Yeah - ah, good idea... go to one of HeyYou's favorite sites with your VIN and see what size tire and gear ratio your truck shipped with. Then verify that the spedometer is off by the percentage difference (use a gps). If you don't see a correlation, empirically check your gear ratio. (rotate wheel 10x and count driveshaft turns).

https://fcacommunity.force.com/RAM/s/equipment-listing

Assuming the truck was probably designed to run within certain rpms for best efficiency at a range of normal speeds. Changing the tires moves the rpm down to attain the same speed, and the engine is likely producing less HP/Torque at those rpm - likely causing it to work harder (less mpg).

 

Two things I would do first are one, get up to speed on the highway. A nice level stretch. At speed, pop it into neutral and see how far you can coast. If it slows immediately, your brakes are dragging. If it rolls okay and only slows normally, check your engine temperature. The old cars you could put a lower temperature thermostat in to get a denser charge and make more power. That does NOT work with computers. With fuel injection you don't have a choke. The computer goes into "open loop" mode and injects more fuel to warm the engine up. When it stays there, you suck fuel like a pig. If you're not warming up, pull the thermostat. You may have a 180 or even 160 thermostat and that is bad mojo. You might even have a failsafe unit that failed and it's just open. Worse mojo.

 

After you make sure the basics are good to go, you should consider a scan tool and understanding how to use it. Lots of explanatory videos out there to see how to make the most of it.

Here is an example video that leverages fuel trim, and tells you more about o2 sensors output. What o2 sensor code are you getting?

 

I am accounting for that. Yes. The previous owner said he replaced plugs within 2ooo miles ago

 

Not sure currently. Whatever factory put in. I have to check. However I can ride at 70 mph and turn about 2700 rpm

 

The after market has quite a racket going. What folks do not realize is that, short of turbo or superchargers, just about every modification done under the hood deteriorates performance instead of improving it. Do anything you want cat-back; behind all the sensors. Leave all of the up front stuff alone. Mopar already had/has the hemi dialled in.
Based on your description and significant fuel problem here is what I would be doing to the truck.
- Rip out that pos intake system and reinstall the stock air inlet and stock sensors for mass air, temperature, map.
- Rip out the thermostat and put the hemi spec 203 deg thermostat
- Change the spark plugs. Do not put anything fancy or expensive. No platinum or iridium. The hemi loves simple plugs. Put in NGK V-Power LZTR4A-11, 5306. There are 16 plugs in that baby. Replace the plugs at regular intervals anywhere between 40k to 60k, whenever is convenient.
- After all of the above are complete. Get an OBD scanner that reads all codes and can live display fuel trim and O2 sensor readings. Get some driving and mileage under it and see what you get. Be prepared to replace O2 sensors. If called for, replace with NGK or Denso only.

Next up would be ensuring a smooth free rolling chassis.
- ensure tires are balanced and properly inflated, pumped up
- ball joints in good condition
- wheel alignment done
- u joints greased or replaced
- diff oils changed
- brakes serviced

Do those things and the overall performance of your truck will improve significantly. If it doesn't, there there is something wrong internally to the engine or the weight of your right foot.