So I found a pretty good deal on a complete set of front and rear axles out of a 3/4 chevy diesel with 3.42 gears. I RARELY ever tow and if I do it would be a small utility trailer or a 2 horse bumper pull at the very most. I currently have 4.10's in my truck right now and have gotten 20mpg a few times (been right around 18mpg lately. I run a Firestone Transforce 235/85R16 tire right now (actual specs are 32"hx9"w).

So my question is, do you think it would be a wise choice for me to swap out axles? I can pick up both the front and rear for $300 and I am heading that way this weekend.

If I did my calculations right, I would be right around 1,900rpm at 70mph in 5th gear which I feel would be great for fuel mileage. In 4th gear I would be a hair over 2,500rpm, would it be adviseable to tow in 4th gear then (if I had to)???

I love a nice set of highway gears in a truck, they do wonders for fuel mileage in my experiences :)

I have 3.42s and dont get any better mileage. Although my injectors are old and worn and im sure the timing chain is too. Those are getting changed soon. I wish I had 4.10s sometimes, but i'm gonna change injectors and timing chain and see how the towing performance is then. It might be fine then I dont know. I tow about 5-6k.

That's a great price on a set of axles. It is tempting. You won't improve your mileage much, but the truck will probably be quieter on the highway. I've run 4.10 and 3.73 rear gears and definitely prefer the 3.73 for the best all around use. The 4.10's are great for towing though. IMHO, the 3.42 are just to tall for a truck to be very useful.

Jake

If I did my calculations right, I would be right around 1,900rpm at 70mph in 5th gear which I feel would be great for fuel mileage. In 4th gear I would be a hair over 2,500rpm, would it be adviseable to tow in 4th gear then (if I had to)???

Towing in 4th is fine, its actually better for the trans, there is no power flowing through the gears which will make it run cooler and less wasted HP since no power is lost through gear mesh/friction. Also gives the problematic 5th gear/nut a break. I ran 5th gear too, flatland, downhill, empty, ect..

He'll see an improvement, he dont have the power sucking 4L80. It'll be fine with those trailers. My 92 towed my 24' stock trailer over 100k miles, 4th gear will outpull a 4.10 truck in OD.

Towing in 4th is fine, its actually better for the trans, there is no power flowing through the gears which will make it run cooler and less wasted HP since no power is lost through gear mesh/friction. Also gives the problematic 5th gear/nut a break. I ran 5th gear too, flatland, downhill, empty, ect..

He'll see an improvement, he dont have the power sucking 4L80. It'll be fine with those trailers. My 92 towed my 24' stock trailer over 100k miles, 4th gear will outpull a 4.10 truck in OD.

Yeah, I have to agree with you on seeing the improvement. Heck, I will be running a whole 500rpm lower at interstate speeds which SHOULD save me a lot in fuel consumption and could actually afford to do 75mph without having to hear 2,700rpm roar through the cab. I personally didn't even know what it would be like towing in 4th gear and 3.42's so thanks for the input and it sounds like it wouldn't be that bad. Gotta love the 5 speed!!!

So do yall think a consistent 22 +/-mpg would be achievable with 3.42's in my rig?

I have 3.42s in my daily driver 92. I tow 10,000 on a regular basis.No problems at all doing 70+mph towing. I'm only getting 18mpg empty on interstate runs at 80mph. I have no idea how many miles the injectors have on them,but have no black smoke.

So do yall think a consistent 22 +/-mpg would be achievable with 3.42's in my rig?

I would think so as long as everything else is in good condition. I have made 22 with mine before (when the turbo wasn't working and neither was the lift pump, it stayed if defuel mode most of the time) and its not hard to get 20 with it now (with piss poor old worn out injectors, and worn/stretch timing chain and turbo master)

I'm really hoping for 22 or more with ease with new injectors and timing chain and maybe exhaust when I go on my trip to Texas in June.

I have 3.42s on my 6.2 (non turbo). It rocks for highways speeds, 2000rpm @ 75mph. I can stay in the fast lane almost the entire trip down I5.
I can get 20-22mpg on those trips.

However with a 2500lbs trailer doing mountain pass climbs, the steepest parts I end up in 1st or 2nd gear doing 18-35mph and netting 15mpg.

I had a chevy with 4;10s and never got over 15mpg in the 10 yrs i owned it. Pulling a trailer or emty it averaged 8-12.
If your getting anything near what you say you are I'd be keeping those 4;10s...... and i'm with the the thought of the 3;42s being to tall.

i just completed 3 heavy runs truck averages 6830 from previous weight tickets

my truck has a manual trans and 3.73 gears.

1st run was with a v-6 gasser c1500 ticket on door says 3400 lbs150 miles.
was able to maintain 65mph 5th gear egt's 650 to 800 except for the hill climbing. boost ranged from 4 to 10 lbs


2nd run was a local trip 60 miles with weight tickets to the iron scrap yard
total gvw was 16060 truck, trailer, load
note the trailer did have toooooo much tongue weight and it did porpuse (sp) on me alittle bit.
egt's on this local run did exceed 1000 for the short trips up overpass's and dropped right back down.
boost from 5 psi to 11 psi
the issue was i could not even keep speed on the flats in 5th gear without egt climbing over 1000. therefore the run was in 4th gear.
the load did have alot of aero drag along with the fact that it was 7000 lbs of scrap steel
the trailer this run was at 2800 lbs..

3rd run was the heavy long distance
3200 lb trailer with 8100 lbs of 2 field pump units heavy aero drag. 6 foot high units on 2 foot high trailer.
definatly not able to use 5th gear ran 340 miles in 4th. fuel milage dropped to 14mpg
egt's on acouple of bridges from houston to houma, la did spike at 1100 boost most of the run was 6 to 10 psi topping at 12 on one long high bridge in 3rd gear. total load was almost 18000 lbs

my notes
this is a newly installed motor i think i have to much advance on ip. need to adjust back just a little bit.
this run cured me of wanting the 3.42 gears even tho i do not generally make heavy load runs there was just no fun on this run. it was real work constantly watching egt, boost and engine temps. (temp never over 190 new ho pump, radiator)

i do have another run this comming week to pick up 4 6.2's
2 in redstone arsenal in albama
1 in dover del
1 in warner robins ga.

this is going to be a long run.

have a safe weekend. i have a truck to scrap out bad engine, trans and not one good piece of sheetmetal except for the roof and that has been spoken for.

wes pierce
houston, tx

I loved my 3.42's in my K5 Blazer. That thing would move let me tell ya. I had the 700r4 trans modded with HD parts, and the HD torque conv. would lock up at 35 mph providing prodigious amounts of pulling power. It was so good that the truck would never downshift above 60 mph, all it would do is gain speed if you punched it. Told my dad how sweet it was on highway drives. My dad (Ford man) thought I was full of !@#)(*! until he drove it on a 14 hour road trip. That truck rolled down the highway nice and sweet. He looked down, had the speedo pinned on 85+ Always had to watch your speed cause it would just creep up on ya. I regularly towed a 21 foot boat with it (old Grady White w/ 302 engine, weighed a good 5000 lbs) got about 13-14 mpg towing that. When injectors were newer, got 23-25 mpg. As they got a little worn it dropped to around 21.