Swap Meet Score !

[Dan-K]

I went to a local Swap Meet over the weekend and scored an Eaton Detroit True-trac posi unit with 4.10 gears . I would have preferred a 3 series to run 3.42 gears but it is a 4 series carrier (#913A315) . My question is how ridiculous will this be at highway speeds ? . My rig is a 71 C10 with a 350/350 combo with 255-70-15 tires . The majority of my driving is in town anyway and I think the 4.10s would add some pep but I am worried that highway driving would be almost a death sentence to the drivetrain . Give me your thoughts / opinions .

[geezer#99]

Punch the numbers into here shows sub 3000 rpm at 60 mph.
That won’t hurt it.


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

[72c20customcamper]

My 72 has 4.10s and it’s not bad on the highway . It will cruise at 70 - 75 all day . In between fuel stops that is. 265/75r16 tires

[truckster]

Originally Posted by geezer#99

Punch the numbers into here shows sub 3000 rpm at 60 mph.
That won’t hurt it.


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

I looked at the calculator and I'm curious... wouldn't the transmission final drive ratio be needed for those calculations?

[jocko]

Dan, your biggest issue is that you have relatively short tires (at least compared to what the factory put out with 4.10s - generally 3/4 and 1 tons came equipped with 4.10s, and they had a bit taller tire than a 255/70/15).
Below are your rpm values at 55, 65, and 75 mph for your combo (Tire Rack says this tire size is 28.94" tall, before wear).
Workable for town cruising, but maybe not so fun at the higher highway speeds - but it's certainly doable depending on what your priorities are - i.e. burnouts vs highway cruising.
From a wear perspective, it will probably "feel" more uncomfortable for you than it is for your truck - because we're all used to overdrive transmissions and low highway rpm these days - but if you drive the highway at 75 mph very much, it would be a bit much I think - personal opinion only. Like I said, comes down to your preference and goals for the truck. You could run it this way for a while and then consider a taller tire if it's really annoying (for example, the 265/75/16 tires mentioned above, a 31.6" tire) would run about 300 rpm lower at 75 mph - so it can make a difference). If you ran a 15" tire in that diameter range, you might find it a bit more comfortable. But I don't know your setup and what you have room for, what look you have on your truck etc.
I'm curious, what rear end ratio does your truck currently have?


EDIT: truckster, that's a good question, I think that web calculator is probably only valid for a 1:1 final drive trans, no O/Ds allowed (unless you do the math and apply the overdrive ratio to your rear end gears before entering the data).

[geezer#99]

Originally Posted by truckster

I looked at the calculator and I'm curious... wouldn't the transmission final drive ratio be needed for those calculations?

It’s always 1.00 to 1.00.
For an o/d trans just whip out the calc to decipher final ratio in o/d.

[Dan-K]

My truck has a 3.08 open at the moment .

[72c20customcamper]

Originally Posted by Dan-K

My truck has a 3.08 open at the moment .

The 4:10 will be nice from a standing stop over the 3:08s

[jocko]

yes, that will be night and day. You'll be very well suited for exceptional acceleration from the stop light straight to the gas pump

[72c20customcamper]

My 72 gets the same milage as my 85 which has 3:23 and a 700r4 about 13. My 77 Suburban with 3:08s got 14 . Hard to get mpg out of a brick

[tdangle]

You will also need to add 3-400 RPM for converter slippage to those charts. My C20 with 4.10 and TH350 seemed always to be 3000 + on the highway. While that's what the motor did when it was new, We have all gotten use to OD 2-2200 RPM cruise on the hwy, not to mention the noise factor. I went with a 4 speed OD auto trans, best of both worlds and did pick up a few MPG.