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Build it right the first time
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Discussion Starter · #1 ·
In response to my last post (Best Budget Heads for my combo....?) someone posted to put steeper gears in the rear end first to really wake up my truck, all the specs for the motor and the truck are in that thread, currently it has 3.08 gears in it, and going down the interstate I usually cruise at about 2700-2800rpm, what would my 65mph rpm be if i were to switch to 3.73 rear gears? also now its only one wheel peel, I would like to have possi (spelling?) with the 3.73 rears, how hrd would they be to change out and about how much would it cost?....I'm a n00b when it comes to gears....thnx
 

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A 3.73 Possi shouldn't be that hard to find, they were/are pretty common.
Often the 4x4 boys wil pull 3.73's to put steeper gears in. You Might pick up a rearend from one of them. I wouldn't get rid of the 3.08's though untill I figured out if I could handle the difference in fuel milage.

A little math work should let you calculate pretty close to the rpm increase at highway speeds. This site might be just the ticket http://www.ecst.csuchico.edu/~traverso/hotrodmath.html
One more for me to bookmark.

Man' I wish I could find a 71 3/4 ton rear end with 3.08's
I have stump puller gears that give performance off the line but eat the gas like crazy.
 

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You still have to take the size of his rear tires in to consideration to get the radius of the tire for the equation.

gear ratio X tire radius X MPH = rpm

3.08 X 12.25 X 65 = 2745.6 rpm


3.73 X 12.25 x 65 =3325 rpm

If you ran the engine up to 3325 on the highway what would the speed be.
That would be how fast the engine thought it was going with 3.73's as compared to 3.08's. It would be running at a speed comparable to 79 or 80 with the 3.08's

That is if your tires are 24.5 inches tall which was my calculation judging by the info you gave. Jim's calculations were pretty close too ;)
 

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Discussion Starter · #5 ·
the tires are P235/75R-15, I don't know the diameter of those but thats what I run at the moment.......

3300, would that be ok to run for lets say 120 miles sraight at that rpm???? on a 10:1 388 stroker?????

thnx
 

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kuhn-77 said:
3300, would that be ok to run for lets say 120 miles sraight at that rpm???? on a 10:1 388 stroker?????
That's up to you, do you feel comfortable running it at that RPM for that distance? I drove my 4.10 geared Blazer almost 1200 miles from Wyoming to Western Washington and most of the time the engine was running between 3200 and 3400 RPM. I averaged about 12 mpg on that trip.

You could always just drive slower and keep the engine at an RPM you're comfortable with. That is of course if you don't mind driving under the speed limit. Nowdays if I go some place on the highway I usually bring the engine to 3000 RPM and cruise it at that speed. For me that's about 55 mph, usually 5-10 mph under the speed limit. If I'm on a multilane highway I just stay in the farthest lane to the right and cruise. Single lane roads I just pull off every now and then to let people by. I figure it puts a little less wear on the engine and saves a lot of gas.

You just have to ask yourself how fast you're willing to spin the engine and how fast you're willing to drive for extended periods of time.
 

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Tire size, gear and rpm, calculators are HERE. The P235/75R-15 comes out at an industry standard 28.9 inches tall. Depending on the manufacturer yours could be a bit taller or shorter.

Plug the tire size (28.9) and MPH (65) in the RPM calc and it says, 2819 RPM. The calc does not take torque converter slip into consideration so your actual RPM will be a bit higher.
 

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71C10 said:
The calc does not take torque converter slip into consideration so your actual RPM will be a bit higher.
I have read that torque converter slip tends to be in the range of about 10%. In other If the engine is spinning at 3000 RPM then the transmission input shaft is spinning at about 2700. Does this sound about right?
 

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3.08 was never in the 3/4 tons

3.42 was the highest ratio ever offered and that was only in the 80s 3/4 tons

otherwise 3.73 was the tallest

the axles were the 14 bolt 9.5" and 10.5" after 80 and after 72 respectively

and the dana 60 and the eaton HO52 were before 73

3.54 was one of the ratios in the dana 60 axles, along with 4.10 and 4.56

these were pretty common where old trucks still may exist, ive found one so far, under a 67 gmc C-2500 i got it for 100 bucks and it had powrlok in it even

i shoulda kept it, hmmm



good luck
 
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