Rear gear??

[smoked68]

Right now we are running the stock ratio which seems to be a 4:10, 3 grand + @ 60 mph. My son wants a 3:73 I on the other hand work with an old engineer who worked for DANA fo 30 yrs. He says to go with a 3:42. The super charger will be on it in the next mnth so... any help, ideas or opinions will be welcomed. Also which posi unit do you guys like, I always ran the detroit locker in my Fords. My friend says stay with the lim slip??? Ok lemme have it this is a soonest thing. Thanks Mike..........

[Longhorn Man]

What's the truck going to be used for?
What size engine?
What cam?
What size tires?
What tranny?
What stall converter?

Lotsa info needed to give an acurate answer.

[killer71]

hmmm. I was running 3000rms @ 60mph with 3.73 gears with 16 inch tires and changed to the 3.08 gears which made it quit screaming somewhat. How is the truck gonna be used mostly.

[LUV2XCLR8]

Supercharged? If it's a T350 Auto you might want to go lower than 3.42

['68 longbed]

you've got 3:73's now. my truck is doing the exact same thing. going to go to a tall tire first before i drop the bucks on different gears.

[Ackattack]

pop the cover off, there should be a stamp on the ring gear of the number of teeth....calculate the ratio. Sounds like an OD tranny may be a better way to go.

For any kind of street duty on street tires I would not go with a locker. I have an Eaton Limited slip in my pickup and have had no problems with it.

Also, I have a detroit true trac (gear type limited slip) in the front of my jeep. I've had some issues of spinning only one tire at a time, a little brake pressure helps that. This isn't really an issue when offroading, but may be one on the street so I don't think I'd use the true trac again.

[smoked68]

The truck is for play time only, it will be driven daily though. Right now it has a M-21 4 spd., 20 inch Rims, 4-bolt 350 bored over slightly, 300 heads, somewhat wild cam and as soon as possible the super charger. (It is sitting in my sons room as we speak). When I was running the stock 15's it was @ aboyt 3400 rpm @ 60. It has dropped since the 20's went on. I ask only because my freind told me the 3:73's will be about the same as I'm running now. So...is it 3:08's or 3:42's, Eaton lim. slip or a Detroit locker posi.??? Thanks, Mike.....

[Bucky71]

I have a Power Trax No Slip unit in the Dana 60 on my C20. I don't like it. On dry pavement the truck lunges to the left when I stomp on the gas. On wet pavement, even the most gentle launch spins the RR tire until the locker engages, then both tires kinda hop for a few seconds.
I want to switch to a selectable locker (ARB Air locker, OX locker or E-Locker).

[DoobieBrother]

I'm just starting to look at tranny, gearing, etc. and this thread has me wonderin about something. I see you guys talking about high RPM's at highway speeds with gears like 3.73. But I know one two of my current newer vehicles that's not the case.

For example, I have an `03 GMC Sierra (2wd) and a `99 Expedition (4wd). Both have 3.73 gears, and both run 265/70/17 tires. The Expy is turning around 2100rpm at 70 and the GMC is at about 2100 - 2200 or so at 70. When the Sierra had 255/70/16's on it, it was about 2300 at 70mph.

So, why the big difference other than tire size?

[67 cst swb]

Originally Posted by DoobieBrother

I'm just starting to look at tranny, gearing, etc. and this thread has me wonderin about something. I see you guys talking about high RPM's at highway speeds with gears like 3.73. But I know one two of my current newer vehicles that's not the case.

For example, I have an `03 GMC Sierra (2wd) and a `99 Expedition (4wd). Both have 3.73 gears, and both run 265/70/17 tires. The Expy is turning around 2100rpm at 70 and the GMC is at about 2100 - 2200 or so at 70. When the Sierra had 255/70/16's on it, it was about 2300 at 70mph.

So, why the big difference other than tire size?

Both your newer vehicles have OverDrive Trans add another demension to the equation.

[67ChevyRedneck]

Originally Posted by 67 cst swb

Both your newer vehicles have OverDrive Trans add another demension to the equation.

Yes, the Sierra has a 4L60 trans which has a 0.67:1 final ratio, where as these old trucks with stock trannys (manual or auto) have a final drive ratio of 1:1

The OD reduces the final gearing about 33%.

My 67 with a 350, 700R4, and 3.73 will spin about 21-2200 rpm @ 65-70 mph.

If this truck really is just a play toy, I'd leave what you've got in there. I'm assuming your adding the supercharger for more fun and power? Putting in higher gears is going to kill your launch time and make the money you've spent on the supercharger kinda pointless?

I'd put in a good set of 3.73 if your truck doesn't have them already (that seems a little unclear at the moment?)

[DoobieBrother]

Isn't that what the 700R4 is supposed to be about? Hence stuff like http://www.bowtieoverdrives.com. If not, how are we getting 100mph+ trucks for the drags that still jump out of the whole but aren't turning something like 8-9K RPM?

[71tahoe]

Originally Posted by DoobieBrother

Isn't that what the 700R4 is supposed to be about? Hence stuff like http://www.bowtieoverdrives.com. If not, how are we getting 100mph+ trucks for the drags that still jump out of the whole but aren't turning something like 8-9K RPM?

I have a '71 w/ 3-on-the-tree and 3.73 gears that would do 120+ @~5500 RPM with a slightly warmed over 350. I could even run 105 consistently with a worn out 267. The hole shot was non existent with the 267 though.

[Pl4yboy]

Originally Posted by 71tahoe

I have a '71 w/ 3-on-the-tree and 3.73 gears that would do 120+ @~5500 RPM with a slightly warmed over 350. I could even run 105 consistently with a worn out 267. The hole shot was non existent with the 267 though.

Yeah my mild 350 does 125 all day with 15s on it and a stock 3.73 rear end. When I put the 22s on it, it dropped to around 110 though. I'm running a hopped up TH400 though. With 15s I can spin the tires all day long in the back until halfway through second. The 22"s can barely break over. My 350 only has headers, an edleb 650, and a mild cam. You should have no problem going fast with 3:73s.