Truck is 99 2wd 3500 QC 5spd with 90k on the clock.
Mods are as follows:
MBRP 4" kit w/ muffler
Air Dog fuel kit w/ big lines and stand pipe in the tank
K&N FIPK
Quad XZT+
RV 275 injectors
3.73 rear gears
19.5" wheels and tires

I tow a 28' enclosed racecar trailer that weighs about 10-11k fully loaded. I thought with switching the gears to 3.73's from 4.10's and the 19.5" wheels would help due to killing the gear ratio. The truck gets a little over 20mpg empty, but my trailer absolutely kills it! I'm only getting around 12mpg towing.
I tow with the XZT box on timing only.

I'm a 3rd gen guy originally and my 04.5 got better mileage towing twice the weight! And it had every mod under the sun! I thought the 2nd gens did better than this???

 

You should have stayed with the 4.10 gears. Going with the bigger tires is what's killing you.
What kinda mileage are you expecting towing all this weight anyways?
12 isn't too bad with that kinda weight.

 

with that setup u should be well into the 20s, what are you at? i saw 23 with my truck how it is plus a XZT+ and icebox intake. ya the rims arent helping but i dont thing that could be everything.

 

Tire and wheel size could be hurting you. What type of transmission does it have?

The thing that kills mileage is wind resistance. You can get worse mileage with a really lightweight trailer that has a large frontal area than with a super heavy low profile trailer. It is usually hard to change the aerodynamics but if you really want mileage, it might be what you have to do. Lowering the truck will help as will building a much longer airdam. If you can build a v-nose or something for the trailer, that will help too.

Assuming that everything is in working order(brakes, bearings etc) on your truck, the only other thing to do would be to change wheels and tires unless you want to get involved in aerodynamics mods.

 

Ok, well I was wondering more along the lines of if the RV275's are known for bad mileage or something of that sort. The trailer is an enclosed with a huge flat frontal area, so that's probably the issue. The trailer I pulled with my 3rd gen was similar, but was a gooseneck and the aero was probably much better. I was curious if more engine mods would help, like a Mach 1.6 injector instead of the RV275's or maybe a different fueling box with more timing? I'd rather spend the $$$ on mod's than at the pump!

 

I tow about the same weight as you on a regular basis and I think you are doing well at 12mpg loaded. I have 3:55's and I'm going to 4:10's soon to help me off the line. Again, at 12 I think you are doing good. If you ask people that give you an honest hand calc'd number while towing I think 12 is at the upper end. I have a few friends with Powerjokes and they only get about 7 towing. As far as switching injectors, I personally haven't tried the 1.6's but can't imagine the gain would be worth the money spent over the RV275's you already have. Eric

 

For heavy towing, 4.10s are the way to go. You shouldn't be going fast enough with a heavy trailer to where the higher RPM are a problem.

Towing with 4.10s at 2K rpm will put you right at 62mph-- perfect for towing on the hwy. With bigger tires, rpm will be lower.

For good towing mpg, you need to have those 19.5s aired up to near max, which will cause uneven wear and brutal ride.

There's a reason these trucks don't come with 19.5s from the factory, and it isn't just cheapness. The factory 265s are rated to over 3400# per tire (SRW)-- that's 6800# on just one axle.

I don't know about you, but my truck typically weighs 2800# RAW-- leaving about 4K for tongue weight if you are ignoring the GVWR. 4K is a lot of tongue wt for a pickup truck.

For towing heavy with an auto trans, 4.10s are DEFINITELY the way to go, as the taller OD of the auto trans (.69 vs .73 for 6speed) helps to offset the shorter 4.10s while giving you the better grunt of the shorter gears.

jmo

 

I like to tow a little fast, and after a trip towing to Houston, TX and back running about 75mph the entire trip (2800mi round trip) with the truck screaming over 2500rpm the entire time, I went with the 3.73's. MUCH better with my 5spd. I guess I just have to blame the horrible aerodynamics of a tag enclosed for the poor mileage. Oh well.

 

Slowing down will help tremendously if you can stand it. The wind resistance is approximately equal to the square of the wind velocity so any little bit really helps. My truck picks up about 2mpg if I go from 70 to 65mph.

 

I thought the lower RPM would be worth more than the wind resistance, but I guess I proved myself wrong when I switched the rear gears... I personally would rather lose the 2mpg and get there, but it's nice to know that I could get better mileage if I wanted to...

 

I totally agree with everyone else and would just like to re-iterate the fact that for me the difference is 65 to 70, like noted before, just backing off it a little greatly improves my mpg almost from 12 to 15 with a 8000 lb tag along, if i am puller a heavy trailer such as my 34ft gooseneck with around 18k lbs on it, i have to back off a little more from say 60-65 to get a difference from 10 at 65 to 12-13 at 65. just my 2cents

 

i tow just about 10k with my 2005 6spd. i hardly ever touch 1st gear, never- without a load.

i was also wanting to reduce gearing to reduce RPMs at highway cruise.. but dont even know what is on the truck. i read somewhere to have the back off the ground and make a mark.. count revolutions.. but i guess i couldnt comprehend. should i be able to tell with the VIN or something else, what would have came stock with it?

 

Put a bulb on the front of the trailer or a wing on your truck, or both

get smaller mirrors

Lower and dam it

Wheel discs to streamline sides

Make sure trailer axle alignment is perfect

Make sure speedo is calibrated and that you are getting accurate readings...

 

Should be a metal tag on rear axle cover on one of the bolts, front too if 4x4.

VIN will tell you, there is a vin decoder available on this site.

Jack it up, take out of gear. Put valve stem at bottom of wheel, mark drive shaft at drivers side on the side. Turn tire one revolution and count number of times driveshaft mark passes original spot. If 3.54 it will be about across from where it started, if 4.10 it will be just past where it started on fourth pass....

My guess from what you say...4.10's great for towing often.