Difference in Hiway RPM-3.73 to 3.42
 

I’m wondering what the difference in in Highway RPM would be if I was to change my Rear End gear from 3.73 (factory peg leg) to 3.42 (new Yukon Posi unit).
Currently, it runs well over 3000rpm at 65-70
My ultimate goal is to get a lower Highway cruising RPM, when I change to the Posi unit.
Many will say “just put a 700r trans in”, but I don’t want to do that.
I’m just wondering how much difference there is between to two ratios.
I’m not expecting “Better Gas Mileage”, but just want to lower the RPM to go easier on the engine revs.
I know the change to 3.42 won’t make a HUGE difference, but I think if I went to a 3.23 it would affect the performance (I.E. Turn it into a Toad)
Thanks in advance

Re: Difference in Hiway RPM-3.73 to 3.42
 

I don't know the RPM drop and you know that won't be huge. But I do know, or feel, 3.42s are a sweet spot for retaining the grunt of lower gears while giving the best hiway results. 3.42s will be best for final straight drive while also working well with O/D if you go that route later. Lower number ratios are too high for O/D if you ask me

Re: Difference in Hiway RPM-3.73 to 3.42
 

About 250 rpm less.
I use this handy calculator.



http://www.advanced-ev.com/Calculators/TireSize/

Re: Difference in Hiway RPM-3.73 to 3.42
 

I did this swap (except stayed a peg leg) at 65-70 I am still turning 2700ish on the highway. I am now really considering pulling my M21 4 speed for a 5 speed to help even more with the highway rpm

Re: Difference in Hiway RPM-3.73 to 3.42
 

Leave the 3.73 and step up to a 5 speed. A swap to 3.42s is only a 9 percent difference in RPM. A 5 speed will net you about 25 percent depending on which overdrive trans you go with

Re: Difference in Hiway RPM-3.73 to 3.42
 

My 69 C10 with a mild 350 has a 3.08 posi diff. I certainly wouldn't call it a toad, because it will spin both tires in low gear. But the engine is still a little busy for me at a constant 70mph, even with 29.5" tall tires.

I also had an 82 C10 with a mild 350 and a 2.73 open diff. It was great on the highway, but acceleration was nowhere near what you'd get with a 3.73 or 3.42. Still, the 2.73 was just fine for cruising around town and doing pickup stuff.

Both of those engines were very torquey at low-mid RPMs, which is what you need for anything lower numerically than 3.23. If you want better acceleration and have a non-OD trans, you'll have to live with higher revs on the highway.

What are you engine specs, including camshaft duration at .050" tappet lift?

Re: Difference in Hiway RPM-3.73 to 3.42
 

About 8%

Re: Difference in Hiway RPM-3.73 to 3.42
 

Thank all y’all for the responses and information.
In regards of “Putting a 5spd/overdrive” in.
Not right now, but it’s a viable option, and could happen later.
Because I don’t go on the Interstate much, as I mostly drive on the back country roads are my house, I can’t justify the cost of the OD swap, YET.
My “loose plan” is to purchase another Complete Rearend, then rebuild the center, and do a Disc Brake conversion on it. After that I’ll just do the swap.
Or I’ll just take the one I have, disassemble it and do everything to it.
(It’s all factory and I’ve never rebuilt it, but the bearings are good, so I’ve never had a reason to)
End goal will be to have 4-wheel Disc Brakes and the Posi Differential :smoke:

Re: Difference in Hiway RPM-3.73 to 3.42
 

3.73 to 3.42 isn’t a drastic change - I’d submit not worth doing if that’s “all” you’re doing. Different story if you happen to grab a junkyard rear end that happens to have 3.42s in it - no problema, that’d work just fine. I’d also submit that, since you hinted that you didn’t want to do an o/d trans swap “yet” - that also means that you might consider it someday. If that is a future option you’re interested in, I’d keep the 3.73s and save those $ to be used toward the future trans. 3.73 is a near-perfect ratio for an o/d trans, there’s a reason so many of the factory o/d light duty GM trucks came with 3.73s.

Re: Difference in Hiway RPM-3.73 to 3.42
 

Jocko is correct...rule of thumb is 1 full point to be worthwhile...4.56 to 3.55 for example

Re: Difference in Hiway RPM-3.73 to 3.42
 

IMO, I think it is a great upgrade. You don't want drastic changes when altering gear ratio's with the same engine, trans etc. The 3.40 became a common ratio in the square door years, but, as I mentioned in earlier posts on the same subject, the '67-'72 era left major gaps in gear ratio coverage in both 1/2 ton and 3/4 tons.

Re: Difference in Hiway RPM-3.73 to 3.42
 

3.73 minus 3.42 equals .31.
.31 divided by 3.73 equals .0831
.831 times 100 equals 8.31 percent.

Lot of work and money for 8%.

29 minus 27 equals 2. Divided by 27 is 7.4% So swapping to a 29” tire from a 27” gets you almost same result.

Re: Difference in Hiway RPM-3.73 to 3.42
 

3.42 divided by 3.73 = .91 less than 10 percent

Re: Difference in Hiway RPM-3.73 to 3.42
 

Not to long ago I changed from a 3.42 to a 3.73 in my 1994 truck. Has a 4L60E and larger than stock 32" tires. I did the rebuild because the rearend gears failed. Was hoping the lower gears would increase milage as the trans went in and out of OD a lot. Milage is the same, the OD hunting issue is gone and it feels a bit better around town. Highway rpm went up about 100 RPM.

Re: Difference in Hiway RPM-3.73 to 3.42
 

If you are going to want to swap in an OD trans at some point, you'll want the 3.73's. I had 3.08's and a 700r4 and overdrive wasn't viable below 70mph.

If you don't plan on going to an OD trans, 3.08's don't do too bad in these trucks. Chances are, you could find a complete 3.08 rear end because a good portion of the automatic trucks were 3.08

I run 3.08's in my truck currently. I have a TH400 in it now and was debating swapping to a 4L80E, but for as little as I drive it, the 3.08's will do. They are NOT as nice on the highway as an OD trans with lockup, but they are tolerable. It's also easy for me to remember to let up after the 2-3 shift because the truck shifts out of second gear at 90. and I don't care to go faster than that. :devil: