I'm thinking about changing my 4:10 gears out on my '03 2500 w/5.7 Hemi.
I never seen double digit fuel economy, and I'm considering going to 3:42. Anyone have experience in the area?

 

With a heavy framed truck like a 2500 you would see dramatically worse fuel economy in every situation except when cruising on the open highway at 70 mph or so and above. Will put more strain on the transmission and will have to give it way more throttle to accelerate. It's true that moving down to a mathematical lower gear will reduce engine rpm, but too low of an rpm takes away a mechanical advantage and performance. The modern Hemi makes most of it's power and torque in the mid-high rpm range, running a heavy truck at lower rpms will cause the engine to work harder and the transmission to constantly shift to try and attain the proper gearing.

Unless this truck is used to travel a good bit on the open highway, UNLOADED and NOT TOWING, re-gearing to a mathematically lower gear would cost you more money in gas, not to mention the expense of re-gearing.

Single digit is kinda low though, unless you live in constant city driving. Your money would be better spent in keeping the truck tuned up, changing filters, keeping the TB clean and a move to synthetic lubricants wherever possible. Tire pressure is also a HUGE fuel economy robber, keep the tires maintained at 80% of the max. PSI as indicated on the side of the tire.

11-13 mpg should be attainable in all but constant stop/go driving and 15-16 is not out of the question on the open road.

What size tires are you running? Do they have a fairly aggressive tread?

 

I get 9.5 mpg according to the overhead and it works out within tenths when figuring it manually. I have a heavy foot and make no apologies for it. The truck performs the way I like it but it costs me gas as a result.

I do need to finish my plug replacement but I don't expect it to make huge improvements. I see 14.5 or so on the highway.

 

The only way to get double digits in a 2500 is with a cummins

 

Completely agree, a numerically higher gear will get you moving easier/faster with less strain on the engine and in an end result gives you better mpg. a numerically lower gear results in the engine working hard and ends up burning more gas. My mpg went up about 1-2mpgs swapping from 3.92's to 4.56's. The best rpm range to run is roughly 2200rpm +/- when cruising

 

I'm currently running 325's for tires, however when I previously ran 305's the mpg was the same. I use only synthetic oil, the truck has 42k on it and I purchased it new. I have a True Flow intake system and keep everything tuned, so I'm at a loss for the poor economy. It's got to be the 4:10's and the limited slip rear.

I do see your logic regarding the power deficit caused by the higher gear ratio, and I had concerns about the transmission too. I've lived with it for the last 8 years so it's not going to kill me now. I have several other vehicles so it's used on a as needed basis.

Thanks for your opinion.

 

Well, hell.
If you're running 35's or 37's with 4.10's thats your problem.
Your gearing is too low RIGHT NOW.
You need 4.56's or 4.88's to get your MPG back, not 3.55's.

 

If you have another vehicle to use that's better on gas, then you got it made. It's a 3/4 ton truck and gets ballpark what most are seeing with a 2500 gasser, a bit on the low end of the scale, but on the scale.

Use it as a truck only when you need a truck.

 

I picked up a project car last night. Towing 5600lbs netted 10.5 mpg highway which still beats my in-town average!

 

Yeah, I was still fishing bass tournaments for about a year after I bought my current Ram, not a lot of weight, prolly just over 3000# or so with gas and gear in the boat and I didn't have any appreciable loss in fuel economy than when on the highway not towing it, maybe 1-2 mpg topps.
My 2nd Gen, which I used for a good three years for towing all over the southeast for tourneys was pretty much the same way (it was a pig with or without the boat).

I did use the ex's Grand Cherokee (4x4, 4.0 I6) a time or two to tow the bassboat, usually for a trip where she went along and we combined fishin' with a little vacation or camping. It REALLY was dragged down towing the boat. Usually got right around 22 mpg on the highway with it and towing the boat, I'd see as low as about 13-14 mpg on the open highway. Forget pulling hills, the thing was single digits pulling the north Georgia mountains the one time I took it up there.

Now of course my bassboats had next to no wind drag at all, very aerodynamic. I would imagine towing something like an enclosed trailer, even with next to no weight would really effect gas mileage...