A friend and I bought trucks at the same time. Both 05 QC SRW 4X4 Big Horn short beds. He bought a 2500 with the heavy duty 6 speed and 4.10 axles. I got a 3500 with a regular 6 speed manual and 3.73. We both had 265/70 R17 tires. I avg 20.5 at 60. He avg 19 at 60. He put on 315/70 R17 BFG AT and cut his exhaust off including the cat. He now gets about 18.5 mpg at 60. I still don't think putting larger tires on will help. As far as replacing the axles that will be expensive.

 

There is no difference between a HD and regular 6spd manual trans with the CTD and Dodge truck. There is only one manual transmission you can get with the diesel. You either have an NV5600 (reverse is to the right and up) or you have a G56 (reverse is to the left and down). Both are HD transmissions. NV5600 on early 2005's and G56 after that.

You used to be able to get a NV4500 with a standard output engine (250hp or less), but you can't anymore as of 2004.

I too believe bigger tire help on the highway. You can go too big and get worse though on the highway due to increased ride height and more rolling resistance. Too big off a tire also hurst around town where a heavier tire takes more power to get going.

 

As stated above the 6sp, like I have...At 60mph is around 2200rpm's...With 315's on, I dropped that down to around 2k rpm's now...My Overhead say's 16 still...But I can't seem to keep my foot out it...LOL...So it should do better, once I can keep off the pedal...I also only have around 2600miles on it now...So will see...

 

Reflash for MPG was at beginning of 2004 for the first few thousand 600's produced.

I don't think it matters if you switch gears or the whole axle.

Not sure on your speed. There are gear ratio speed calculators on the net for free, but I'll guess around 70 mph at 2000 with 4.10's and 315's. I'll also guess 74ish with 3.73's and 315's. I have 3.73's with 285's and I'm 70ish at 2000rpm.

camshaft change isn't easy (have to take off the front end to slide out the cam, plust the accessory drive system, gear housing cover). I would tell you +4mpg if I was trying to sell aftermarket camshafts, anything to get you to buy one. I am doubtful of +3mpg on a piers camshaft.

 

"I too believe bigger tire help on the highway. You can go too big and get worse though on the highway due to increased ride height and more rolling resistance. Too big off a tire also hurst around town where a heavier tire takes more power to get going."

There is also the air resistance to think about when going to that bigger tire. a 4" bigger tire adds 2" to the height of your truck. Doesn't sound like much, but if your truck is 6'6" to start (78"), 2" adds 2.5% to the air it is pushing. Not only that, but the truck probably got lifted 4" to fit the tires, so now we are talking about 6" or 7.5% more aerodynamic drag.

The best truck tires for rolling resistance are 235/85R16s pumped up to about 75PSI. A big off roading tire with low pressures (40PSI) would be terrible for rolling resistance.

Semi tires have better rolling resistance for the weight than car tires because of the relative narrowness relative to the weight and the high inflation pressures.

 

I too only get 16-17 average, 15 around town. With yours having more power then mine, I wouldn't say your too far off. In persective, your truck has the rating of what your average kodiak/top kick had a few years ago or has now. 15 mpg for a 4*4 8k lb. truck is not that bad when you look at the big picture, 18-19 for that truck I wouldn't think is reasonable for city driving. A big block would only get 8 or so.

 

I have been getting around 18 for the last 5 tanks. Had one tank slide into the 20 range. (20.04) Don't think that it will happen again. I have the 2 degree timing mod but everything else is stock. I also keep the tires aired to max. A little rough but not bad. Even with the high fuel prices I think that I'll keep this one.

 

could I run the ring mod with my tst box or would that be to much timeing also do the 05 and 06 get better mileage then the 04.5 models?

 

I lost 1 city mpg with my 315s. Hwy mpg has stayed about the same... I could actually tell a difference in power too.

 

My it is. mileage isnt bad, i wont complain. But im suprised how touchy as at 55 i get 21 ( verified) 65 its about 17-18.5, and at 75-85 its 14.5-15. makes ya drive slow haha

 

Just back from a 2400 mile jaunt from pa. to SD. All empty driving, no tailgate, 99% interstate hwy, w/mmo-ps mix every tankfull.
1900 rpm (70 mph) got 19.6 mpg at best. 2100 rpm (76mph) ran about18mpg, 2300 rpm(81mph) dropped to 17.3
I ran each setting about 500mi. so I could get a good read.
Got home and found my new Rokk sensor had arrived! Will install on Monday and have a 600 mi. run coming this week so I will get some good readings on this mod.
44k on the engine now so I guess its broke in.

 

I see the same with mine. It is very touchy about different speeds. It is amazing to me to lose close to 2 mpg by driving 5 mph faster.

 

My 04.5 600 with BOMBs and 315s at 33,000 miles is just now starting to get reasonable mileage. Yesterday Juice on 40hp setting, 400 mile Interstate round trip with cruise on 74mph(1900rpm) got 18.3mpg. I am sure if I dropped down to 65mph would be in the 19+mpg range. That is probably 3mpg better than it was a year ago stock. I can live with that mpg considering tires and BOMBs.