Just bought everything for 3.73 gears

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zac_F71

zac_F71

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Hey guys I bought everything to swap my 3.23's to 3.73's mostly based on suggestions from a few fella's on here - so I want to hear from guys that run 3.73's or 3.91's even.. will I be able to drive the car on the highway, what are your cruising rpm's, what tire size are you running?

8 3/4 741 case
Motive 3.73 gear set
Motive gear bare install kit
36t speedo gear
29 spline 7260 yoke
275/60/15 tires (28")
 
I've got 4.10 gears in my dart and I love them! I really don't drive on the highway much so it doesn't bother me. I'm running a 28 inch tire, and it's about 3500 rpm on the highway, I forget the speed because I didn't change my speedometer gear yet.
 
I see I drive all highway pretty much in my area 60mph + and don't want to be at 3,500rpm at 60mph let alone 70mph
 
Going off that calculator -

Automatic
in drive gear
3.91 gears since 3.73 isn't an option

29% throttle
75% converter efficiency
71mph = 4189rpm
YIKES!

24% throttle
75%
60mph = 3555rpm
Not as bad but still not good

I drive 30 minutes to work one way, it has a good cooling system, but I can't see it liking that kind of rpm for long...

Well we'll see I guess somewhere between 3,500 and 4,000 cruising rpm is my guess for 70mph
 
I have the same gear ratio in my Barracuda, got carried away when I had RRP build me an
8 1/4 SG unit. After much soul searching I opted for a GearVendors overdrive unit and a custom built 79 904 based transmission with a custom non-wire lockup torque converter. I am seeing numbers in the 2400-2500 RPM @ 70 MPH range with the different calculators. Hope it works out as my plan for the car involves cross country driving and the Hot Rod power tour.
Good luck with your solution, mine was go back to 3:21.1 in a brand new rear end or pony up the treasure and deal with it.
DR--------:coffee2:
 
The Vexer calc in my experience is accurate. S'wy I took the time to post it... My calculations say 3000rpm @ 60 using a 26.5" tire and a 6% converter slip.

This sounds right. I'm at 3400 @ 60 with a 23.5" tire and the same gear.
 
zac_F71 said:
Going off that calculator -

Automatic
in drive gear
3.91 gears since 3.73 isn't an option

29% throttle
75% converter efficiency
71mph = 4189rpm
YIKES!

24% throttle
75%
60mph = 3555rpm
Not as bad but still not good

I drive 30 minutes to work one way, it has a good cooling system, but I can't see it liking that kind of rpm for long...

Well we'll see I guess somewhere between 3,500 and 4,000 cruising rpm is my guess for 70mph
Click to expand...

The reason the rpm's looked so high is you had the converter efficiency set at 75%. The junkiest converter ever built would normally be better than that. Reset it to 93-94% and you'll see a huge difference.

I had 3.73's in my car for a while with 275/60/15 tires and IMO their are pretty low for much highway cruising but everything is a trade-off. You want the car to be pretty quick out of the gate and you don't have a stroker making 500+ lbs ft. of torque you gotta have low gears.
 
I would not try to figure "converter slip" into the equation. It is a real "unknown". But yes I agree, 25% slippage would be excessive.
 
69_340_GTS said:
I would not try to figure "converter slip" into the equation. It is a real "unknown". But yes I agree, 25% slippage would be excessive.
Click to expand...

had a 92 gmc w/ a real hot 406 sbc in it, (estimate of 600h.p.) 3;73 gear, 3800 tci convertor, 30" tall rear tires, truck weighed 3500 pounds w/o me in it. 70mph was around 2950 rpm. believe it or not, I got 16.1 miles a gal. on the hiway "if" I kept my foot out of it completely. high eleven second truck too ! got so pissed off that I couldn`t keep it from leaking oil, I traded it off. think it had a hole that would open a small amount when hot .:wack:
 
Please post your RPMs at 75mph when you get them in.
I am contemplating the same swap to 3.73 with the same tire.
With 3.23 I am running about 2800-2900 @75mph.
 
I just went from 3.23's to 3.91's. Tire is 25.4 tall size 235/40R18
Was running right at 3K with the 3.23's at 70 mph now at 3600k at 70 mph
Lots of fun light to light but pretty much stuck at 60-65 mph on the freeway
Going to run it for a while but thinking I will end up back to the 3.23's or 3.55's
 
I have 3.73s in my demon right now. 3,500rpm at ~75mph (my speedo gauge is not accurate) 295/50/15 tires. For around town and short jaunts on the HW I love these gears. The problem is I live in Lubbock, Texas now and most car shows and meetups are 2-3 hour HW drives away. So I am trying to come up with a solution...
 
Why not just run the 3.23s first and see what you think of them?
 
As for the calculator, I did worse case scenario LOL, I wasn't sure how to calculate converter slip, as I haven't dealt with a build like this before

Cruising is part throttle, it has a Hughes 3500 stall, there will be a little to much slip with the 3.23's I would think

I might just run the 3.23's for the first drive and see what they're like. I really want to get the most out of the car, as it will have a 450ish hp big block in it..
 
If the Hughes converter is a quality unit, you shouldn't have any trouble with the 3.23s. I had a 10 inch GER converter that flashed to about 3800 in my 3.23 geared 68 Satellite. Street car that I drove all the time. Ran 12.35 at 107 with an 8.5 440 , 906s, Comp 292 501, Torker, 750dp, 28x9 slicks. 1.69 60 ft. While the GER performed okay, it was a poorly machined piece of crap that would crack a flexplate about every year. Replaced it with a 9.5 Dynamic that was built for the car. I'ts been in the car for about 12 years now. No issues whatsoever. Flashes to about 4500. Drives fine but is a bit looser than I would recommend. Unfortunately no track times with the Dynamic as I have had other projects going on and the Satellite really needs some things done to it before it would probably pass tech again (leaky oil pan, leaky ps, rusted out floor pan, etc) and the engine is getting a bit tired. In short, use a good cooler and you will have no problems. You already have the 3.23s, you might as well try them.
 
zac_F71 said:
Cruising is part throttle, it has a Hughes 3500 stall, there will be a little to much slip with the 3.23's I would think
Click to expand...

You'll be fine, Hughes like most off the shelf converters are wound tight.

Be at 6-10%. Don't worry bro, gear up!!!
 
Woods74 said:
You'll be fine, Hughes like most off the shelf converters are wound tight.

Be at 6-10%. Don't worry bro, gear up!!!
Click to expand...

Agreed. Hughes is a good quality converter but they usually don't stall as much as their rated at. Slippage shouldn't be much, if any more than normal with 3.23's
 
200.gif
 
While the GER performed okay, it was a poorly machined piece of crap that would crack a flexplate about every year. Replaced it with a 9.5 Dynamic that was built for the car.
Click to expand...

GER was the worst converter company on the face of the earth.



I've always used this RPM calculator, it's very simple and accurate.
https://www.ringpinion.com/Calculators/Calc_RPM.aspx

Original poster, what motor are you running?
I've read alot of threads about highway gearing and have one of my own and for the most part they're just a pee'ing match since it's all about personal preference. Some guys are fine with roaring down the highway at 3800 RPM's. I am not ok with that as it just simply annoys me.

My Duster has a 500" big block with a 727, 9.5" Dynamic converter, and an 8.75 with 3.55 gears along with rear tires that are 27.5" tall. I'm at 2800 RPM's at 65mph, yesterday I installed 3.23 gears which should bring it down to around 2400 RPM's.
When I have the energy, hopefully soon, I want to pull the 9.5" Dynamic and sell it or trade it for a tighter 10" converter. I drive my car alot and the motor has so much torque that the 3.55 gears feel like 4.10's so I'm thinking I'm gonna like the 3.23's.
 
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