There will be no difference between a 1500 and a 2500 really. What will affect gas mileage is your rear end gearing, and over all what configuration the truck is. 4door vs 2 door ect.
I dont get why people think just because it is a 1500 that its better on gas. Still the same engine. My 04 2500 hemi will actually get better gas mileage then my cousins 05 1500 4.7.

Aaron

 

Love it or hate it; the 1500 has MDS and the 2500 doesn't.
2500 is about 500lbs heavier and is higher.

Gas mileage (for me) is close (1-2Mpg different) but not the same.

 

my 04 QCSB 4X2 Hemi gets 13-15 in town and 18-21out on the highway, it just depends on what ya need it for, and if ya really need the extra capacity of a 3/4 ton truck, if not stay with the 1/2 ton, with that said i love the 2500 and would doubt i would go back to a 1500, thats all from the peanut gallery, Brian Lee

 

I forgot about the MDS stuff, but like i said configuration will affect the gas mileage and rear end will the most. I can almost pull 19 mpg with my 04 2500.

 

i get 11.8 to 12 mpg while towing a 9000lb horse trailer through coastal hills.

 

That's weird that you ignore the weight difference between a 1500 and 2500 (heavier frame, rear, solid front-end ,etc) but you bring up truck configuration...for the weight i'm guessing. You can't have both man...you either have to acknowledge weight matters when it comes to mpg/speed or it doesn't.

Yes it's still the same engine but more engine load is required to get all that mass moving and maintaining speed. Gearing does help like you said but only so much....the higher rpms on the highway will hurt MPG.

 

I just took a trip out to Austin Texas this weekend and averaged a whopping 13.5 mpg running 75-80 on I-10. Granted mine is a 1500 megacab 4x4, but you get the idea. I wouldn't buy a 2500 with a hemi if your even modestly concerned with fuel mileage. In addition the cruise has a heck of a time maintaining 75 mph even on the smallish hills west of tallahassee. The truck would lose about 4-5 miles per hour before it decides to drop down 2 gears and then it goes like hell back up to 75. Makes for an entertaining ride in hilly country. I have to agree with the others who said a Hemi is a bit under powered in a 4x4 2500 (which is basically what a 1500 mega cab is).

 

Did you hint to the truck that you were in hilly conditions by using tow/haul? Cruise control can't anticipate hills, it can only respond to them. By the time it responds, you have already lost your speed.

 

I tried it once but it seemed like it wanted to stay in 4th gear all the time with the converter unlocked...I was thinking it would make the mileage even worse as the fuel mileage readout dropped 3-4 tenths over 20 miles. But, it did keep it from dropping all the way down to third gear and about 3500 rpm when it wasn't in tow/haul mode. Maybe they should have called it the tow/haul/hilly button.

 

- repeating from other threads / speaking from experience - I've pulled 7,000lbs of trailer plus payload on long, steep grades in 2nd gear running over 4,000 RPM to keep 55-60MPH and my trans temperature tops out about 190F. The engine lets you know it's pushing some power, but coolant temp, trans temp, and oil pressure all stayed happy. I don't know if the 545RFE will lockup in 2nd, but I'd expect that even if it wasn't, I'm so far beyond stall speed that it's effectively locked. If not, I don't think my trans temp would have stayed under control (can anybody answer the lockup question?).

FWIW, O/D is specifically for flat/cruising and fuel economy. Every owner's manual I've seen says to not use it in hilly conditions or under load. "If the truck seems like it's straining, or can't keep speed, you need to take it out of O/D". Also, you weren't dropping mileage because of not using O/D, but because you needed more power for the hills. Not being a smart-alec, but the relationship and effects of load, RPM, speed. gearing and power-band are not always obvious. Your mileage would have dropped more if you let the engine continue to struggle rather than letting it produce the power it needed.