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.