What to run? 3:42 or 3:73?

[Toad455]

I'm getting my rear diff ready and I need to get parts. I'd like to know if anyone prefers the 3:42 over the more common 3:73 for normal driving and highway, no towing or hauling ever. Truck is a 78 C10 with a 6.0 & 4L80. I "think" I'm going to run 275-60-15 tires on the rear. I played with the online speed/rpm calculators and there isn't a tremendous difference between the two on the highway. Any real life experiences out there with the two? Thanks.

 

[TotalyHucked]

4.10 minimum. The 4L80 sucks so much power up, you need more gear. I had a 3.90 with my 5.3/4L65 and 315/35/20s and it was perfect, the 60 sucks alot less power out of the drivetrain. There's only a few hundred RPM difference between 3.73 and 4.10, it'll cruise the hwy just fine with 4.10s

 

[Toad455]

Thanks,I'll consider that. How tall are your tires? I'm going to run stock rallies with 28" tire.

 

[TotalyHucked]

Same, a 28" tire physically measured. That put me just below 2300 at 70. But a 4L60/65/70 has a .69 OD, you have a .75 I believe. I always forget the 80 doesn't have as much overdrive. So you'd be more around 2500 at 70mph. That might be a bit much if you're gonna be on the highway a ton. 2200-2300 is a perfect cruising speed.

You'll have to decide for yourself how much backroad/in town driving you're doing vs highway. I've got 55k miles on my truck since I built it and half of that or more is highway. I went from Ga to AZ and back for Dino's Git Down twice now and from Memphis to Flagstaff, we ran 90 just about the whole way. That put me right at 3k, that was annoying with the volume of the exhaust. If you're gonna be primarily hwy, then maybe 3.73 or 3.90 would be better. If you're mostly gonna be backroads/in town with just the occasional highway trip, I'd say 4.10.

 

[Toad455]

Thanks. I'm focusing more on hiway, I don't really want to spin over 2200-2300 at 75 mph really. If I win the lottery I'll slap a gear vendors behind it with 4:10.

 

[bucket]

2200 at 75 mph is pretty tall. In a truck shaped like a brick, lower rpm at higher speeds often does not equate to better mpg.

 

[Toad455]

Right, ha. Lot of wind resistance.

 

[TotalyHucked]

2200 at 75 mph is pretty tall. In a truck shaped like a brick, lower rpm at higher speeds often does not equate to better mpg.

Not for an LS though. They don't make any torque below that. That'll keep ya cruising in OD even up grades

 

[idrivea2002golf]

It's probably not apples to apples, but I ran the 5.3 with the 4l60 in my old sierra and it had 3.42 and 265/70R17 tires. It struggled to maintain 85 on flat land through New Mexico and parts of Texas. At a minimum I would go with 3.73. If you have the slightest inclines I'd go 4.10.

I agree with TotalyHucked, the LS motor won't make much torque below, i think their curve starts at, 2,000 rpm. My Sierra ran 75 at about 1,800 and any change to road condition was immediately down shifted to 3rd.

 

[Grit dog]

Thanks. I'm focusing more on hiway, I don't really want to spin over 2200-2300 at 75 mph really. If I win the lottery I'll slap a gear vendors behind it with 4:10.

You do have a 6.0 so more torque than a junkyard 5.3 and you don’t mention what’s been done to the motor.
If it’s bone stock I’d want to see at least 2500rpms at 75mph. Maybe what you said if it’s got good headers allowing a quicker bump in hp/tq.
By the same token, you have a LS and overdrive and all the time and money spent to add it. Half the reason should hav been how they like to spin and run so effortlessly at higher roms. May as well use it. If you don’t want it to be fast then you could have spent $0 and left the old (fill in the blank) stock engine in it. Or dropped in a cheap 350.
My vote is 4:10s if you’re going through all the trouble of regearing it.