So im looking to get better fuel mileage out of my truck I don't pull anything with it so no need for lots of torque would changing my 47 to a 42 trans achieve that I already have 355 gears

 

Where do you think you will gain mpg? 4th and 5th year in the 42 and 47 diesel versions are the same. 1.00:1 and 0.77:1 respectively. I would not want you to spend $$ on something that is of no benefit.

Is the truck 2wd? Maybe try some taller tires that are the same width. It would effectively raise your gear ratio.

 

well at the moment top speed in 5th at 2000 rpm is 55mph but where I live most of the roads are 65mph to 80mph so my hopes are to lower rpm at the higher speeds to up my fuel economy

 

Fourth and fifth are geared identical. You will GAIN ABSOLUTELY NOTHING aside from a weaker trans with a crappier reverse ratio.

Save fuel? Drive something else.

 

Taller tires will help lower roms as mentioned. It can get tricky to find a tall and narrow tire. A wider tire will increase rolling resistance and weight.

I have 37s and 3.55 gearing in my rig. My truck cruises 55-60 mph at 2k in 4th gear. Cant remember what fifth is off top of my head. Dont drive too much on highway, but its lower obviously.

 

2000 rpm in 5th should be close about 70 mph with 3:55 gears and stock tire height.

I have this combo and mid to high teens for mpg is about what you should expect. I’ve seen 21 but that requires perfect conditions, which seldom happens.

HTH,
Bob

 

I think you will have to do driveshaft work also

 

you may want to recheck your gear ratio or evaluate your tire size. My truck has 3.55s with 285s and 2k rpms is about 68mph.

 

i have 4.10 gears and your RPMs seem to match mine at 55 MPH

I get about 19 mpg but it drops faster when going over 55

 

Might want to consider double checking the rear gear by comparing observed RPMs with calculated: Gear and Shift Point Calculator | GTSparkplugs Agree with others that something is not making sense with the stated rear gear and RPM at speed. If the actual rear gear is something other than a 3.55, then swapping to one is the most economical move if not wanting to mess around with tire sizes.


While not looking to insult, a good path to efficiency is making sure the plumbing is tight. Leaks between the compressor and turbine will lower efficiency. So, if not already done, check for boost and / or exhaust leaks and put money here first. If the truck still has the original orange intake plenum boots, probably time to yank them and replace with a new set of silicon boots. Investing in an intake leak detector will find if there are any intake leaks. Another item known to effect both power and economy is a clogged exhaust back-pressure tube / failed pressure sensor. And any signs of soot means loss of exhaust pressure.


But lets play with different trannies a bit more.

There is the gas 460 gearing which shows a slightly taller OD (0.76) than the 42 and 47's OD (0.77), but not enough to really save any money compared to the parts costs and potential shop costs.

And then there is a ZF-6 which adds one throw and has an OD of 0.73. Given the intent to never tow again, a ZF-6 seems like overkill and economics really are not there either as I am seeing where it will need some minor mods for proper fitment.

If there is a reason to swap trannies (such as it needs to come out anyway due to impending failure), then swapping trannies makes more sense. But as already noted, installing a 42 is a backward step compared to the 47's rated power capacity and does nothing for OD gearing.

Let us know how it turns out