I was wondering what the mpg difference is between a 2001 5 speed and a 2001 6 speed.

 

little to no difference.

final drive ratios are very close .75 in the 5-spd .73 in the 6-spd.

6-spd has closer shift points and less rpm drop that helps with towing..

rear end ratio (3.55 or 4.10) and what you do with the truck will have more of an effect on fuel economy than what trans is in it...

 

I agree. It depends on how you drive the truck. My overhead computer shows my average at about 16.7mpgs... but I have RV275 injectors, Banks High Ram, Fleetguard AH19037 BHAF, and a larger exhaust.

If I keep my foot out of it on the flats I can see numbers between 19 and 22, and I'm running the NV5600 6 speed.

 

I get 15.5MPG at best with my mods, but when it was stock it got 17.5 at best...

4.10s are hell on fuel economy...my 35" tires aren't helping either... but they do help keep me from redlining on the freeway.

 

Originally I assumed a manual would get better fuel mileage too, but when you look at the final gear ratios, overdrive on an automatic is actually the best of all available transmissions at 0.69.

I got my info from here, which has handy diagrams of all ratios for all transmissions with all rear-end ratios:
Gear Vendors Under/Overdrive Dodge/Dodge RAM Final Drive Ratios.

 

I've got a programmer on mine and get around 21mpg with the 5spd

 

With my mods I get about 17 in the city and I've hit 24 on the highway. As long as I stay out of the skinny pedal I'm good. I'm running the nv5600 with 3:55 rears.

 

I have read this before and found it interesting. But I do not get better mpg with my auto. My thoughts are that the torque converter allows for too much power inefficiency while getting up to speed that it kills potential mpg. Stock TC has like 65% efficiency - pretty lame IMO.

My 2000 has a 5 speed with all the right mods for mpg and my calibrated scangaugeii says I get around 26-27mpg on the highway when I keep speeds at 58 in 4th gear.

 

I get 32 mpg's with mine