Gears, mpg, could use some wisdom

[96-1500]

Hey,
I have a 1996 Chevrolet K2500 eclb 5.7 vortec/4L80E, K&N CAI, slight exhaust work, tires are Kelly Safari TSR 265/75R16s and I'm running between 12-14 mpg pretty consistently at mostly 80-90 km/h with some 4 lane 110 km/h, or 50-55mph and occasionally around 70mph. Doesn't seem to make much difference how fast I'm driving, it's usually around 13 miles per gallon.
Currently I'm running 4.11 gears, but I've found a set of housings I could swap in with 3.73s and I'm wondering how much of a difference this will make.

For the most part the old 350 is pounding away at over 2500 rpm and I can really tell she's labouring to get up some hills on the highway at any speed. Let off the throttle and it's like you throw out an anchor from the trailer hitch. I'm just not sure how 3.73s would compare as I've never had them before. My old half ton had 3.42s and it would haul everything I wanted it to and more with good mileage, but the truck just wouldn't take it. Too light duty, which is why I bought the 3/4 ton, but I'm not impressed with it's fuel mileage. My half ton would get for the most part closer to 20mpg.

I realize 3.73s aren't going to be quite as good as 3.42s mileage wise, and I know that I've got a bigger, heavier truck than my old one, but I'd really like to target closer to 16 or 17 mpg without spending a lot of money. I also realize that this isn't a Honda Civic or Toyota Corolla lol.

What do you guys think? Am I wasting my time, or would it make a difference enough to be worth the work?

Thanks a lot in advance, I've read a lot about this stuff recently but it seems to be really vehicle specific so that's why I started a thread on this site.

 

[DRAGGIN95]

Well on flat ground the 3.73's will drop the engine rpm, but if the truck is laboring up hill's now with 4.11's then it will be much worse with 3.73's.

 

[LoudnLifted88]

my dads 99 2500 burb use to average 18mpg on the highway with a bone stock vortec 350 410s and 245s

 

[oohunter04oo]

Just put a set of 285's on it.

 

[96-1500]

Well on flat ground the 3.73's will drop the engine rpm, but if the truck is laboring up hill's now with 4.11's then it will be much worse with 3.73's.

To clarify, I should have said it will get up the hills no problem if I walk the gas to 'er, but I try not to as you can almost watch the gas gauge dropping when I do. Usually I hold a pretty consistent rev, a little higher on the hills but not much. I work pretty hard to get the mileage I do. How much do you think the 3.73s would lower my RPMs?

my dads 99 2500 burb use to average 18mpg on the highway with a bone stock vortec 350 410s and 245s

Wow. That's pretty impressive. I don't understand how mileage can be so different sometimes. My old half ton has gotten as much as 21 mpg on a completely highway trip, and yet some guys on this site get less than I'm getting in my 2500 with stock 1500s.

 

[96-1500]

Just put a set of 285's on it.

Really? How much of a difference do you think that'll make?

Thanks for the quick responses guys

 

[SixSpeedSS]

I know your pain. My completely stock '96 ECSB 8 lug with 4.10s only gets 9-10 mpg around town (with a freshly rebuilt vortec 5.7L). On the only highway trip I have taken with it, I was able to get 13mpg on that tankful (not all highway miles on that tank though). I think the combination of excessive weight and lack of torque is the problem with our trucks. I bought a 5.7L vortec truck hoping I wouldn't get the 7.4L vortec's poor mileage. Turns out I get the same or worse mileage and have less power. My truck runs pretty decent, but still struggles when attempting to go up a hill. Fortunately I don't rely on my K2500 as my primary transportation.

I don't think the 3.73s are really going to help enough to offset the cost/hassle of the change. Plus the truck will probably struggle more when pulling out from a stop. I was planning to put a set of 285/75-16s on my truck to raise the final drive ratio (numerically lower) of my truck. Mine only has 245/75-16s on it now. The difference from 245s to 285s will probably be about the same as switching from 4.10s to 3.73s.

I don't think the cost/effort is worth the 1-2 mpg you may get with the swap.

 

[SixSpeedSS]

my dads 99 2500 burb use to average 18mpg on the highway with a bone stock vortec 350 410s and 245s

I call shennanigans on this. Average 18mpg?????? I could see maybe on a straight highway trip at 55mph across the middle of the country where it is flat as can be, but not an average between city/highway. These trucks just don't get that kind of mileage.

 

[96-1500]

I know your pain. My completely stock '96 ECSB 8 lug with 4.10s only gets 9-10 mpg around town (with a freshly rebuilt vortec 5.7L). On the only highway trip I have taken with it, I was able to get 13mpg on that tankful (not all highway miles on that tank though). I think the combination of excessive weight and lack of torque is the problem with our trucks. I bought a 5.7L vortec truck hoping I wouldn't get the 7.4L vortec's poor mileage. Turns out I get the same or worse mileage and have less power. My truck runs pretty decent, but still struggles when attempting to go up a hill. Fortunately I don't rely on my K2500 as my primary transportation.

I don't think the 3.73s are really going to help enough to offset the cost/hassle of the change. Plus the truck will probably struggle more when pulling out from a stop. I was planning to put a set of 285/75-16s on my truck to raise the final drive ratio (numerically lower) of my truck. Mine only has 245/75-16s on it now. The difference from 245s to 285s will probably be about the same as switching from 4.10s to 3.73s.

I don't think the cost/effort is worth the 1-2 mpg you may get with the swap.

Sounds like we've got the same truck only mine is a long box. I don't know the specs off hand but I know she's heavy.
Probably doomed to poor mileage I guess. The truck is my daily driver, I have to travel around 100kms round trip about 3 days a week for work. My other job is right handy in the small town I live in. But the fuel bill is just killing me.
It wouldn't be the cost but the time I'd be strapped for. The set of rearends I'm looking at buddy wants less than $200 for everything, but the rear needs new seals (and probably at least axle bearings), plus the time swapping the stuff into mine. I work 7 days a week at 2 jobs plus odd jobs on the side.

I don't know what to do right now. Wouldn't be a fun job by any stretch of the imagination. Especially where mine works as it is.

As for the tire swap, the truck came with an almost new set of 265s so I won't replace them until they are worn out. Which might not be for awhile yet.

I'll do some more research and calculations and be sure to post up what I find.

Thanks a lot for the insight SixSpeedSS, any further advice is always appreciated

 

[Blue95]

How about a full tune up, muffler, tune? much better, easier, more bang for buck and time