3.73 or 3.42 gear
 

I just installed a overdrive trans with lock up converter. What would be a good gear for a nice cruiser? Right now I have a 3.73 and don't know if it would be worth changing to a 3.42. I am running a 28" tall tire and with the 3.73 it rpms 1600 at 55mph. Haven't had it to 70 yet but the math says it should rpm at a little over 2100 where a 3.42 gear would put it at 2000.
This is just a cruiser with a mild 400.

Thanks Mike

Not worth the hassle or expense IMHO. 2100 RPM at 70 MPH is just putting around for a 400.

x2 the above

3.73 will match the 400 in OD.
3.43 will match the 455 in OD.

With the overdrive fourth gear of .67:1 your 4th gear ratio with the 3.73 gear would be 2.49:1
4th gear with the 3.42 would be 2.29:1

Your 400 can easily use handle either gear. 2.49 is plenty high enough of a gear to make a big difference in MPG!!!

Does your new converter retain the lockup feature?

What is your engine combo? Some engines may like a slightly higher cruise and be more efficient.

Might be of interest....

Rear end and overdrive tranny combo

https://forums.maxperformanceinc.com...ruising&page=3

At what rpm does your engine make peak torque at ?

Somewhat related, see my post number 50 in the link above.


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The thing that stinks dropping to lower number gears with overdrives is the gearing percentage of the overdrive becomes less effective, changing the RPMs less, and at the same time loose a bunch of the 1st gear advantage. 3.73 to 3.42 swap only drops the OD gearing .2, but you changed the rear end gearing .31. At the same time, you lose nearly an entire point in the total first gear reduction with the .31 change (roughly 11.4 to 10.4) Most people overlook what you loose in first. I think 3.73 is a good place to be.

Yes lock up converter. 1800-2000 stall. I am in the prosses of building the 400 for it with parts I have. RPM intake ,750 Holley, 1.75 headers, milled 6x heads, 9-1 compression. Leaning towards Summit 2081 cam.

Right now I have a 455 with about 500hp in it.

That cruise RPM is a nice place to be. 2000 RPM at 70-ish is what I generally shoot for.

Like Jay said, dropping the rear gear will affect 1-2-3 gears more than it affects overdrive. So it really becomes a matter of whether you want more effective gear to accelerate the car or not, because overdrive doesn't change much.

That's what makes the overdrive transmissions so attractive. You can run that rear gear you really want for best acceleration, and then go to sleep on the highway. No compromises.

You already have the 3.73:1 and it's in and functioning. That ratio would be my choice anyway because I like the little boost in 1st gear. Only thing not mentioned is the expense of changing the ratio. Doing it yourself, it's still a near $500.00 deal for parts. Ring and pinion, bolts, install kit, pinion bearings, gaskets, seals, lube. Having it done somewhere, $1200.00-1500.00. That's allot of $$ for very little change IMO. Now , if it already had a 3.42:1 installed, I would still be inclined to leave it alone for the cost reasons stated above. Let us know what yiu decide.

Im curious about a similar plight of mine from the opposite direction. I currently have a 3.08, but if I follow through in putting in a GV setup I want to do a gear change. 3.42 and 3.73 in 8.5s are all over the place so those are my obvious choices. What would people do there? Is the 3.42 not enough of a jump to be worth it and you would go to the 3.73?

My car ran 12.2 with good weather and the 3.08s. Im hoping to get 2 tenths out of a gear change. Really want to be able to say I have an 11 second car.

A gear vendor only offers about a .78 overdrive ratio so I don't care to run too much rear gear in them.

I'd do a 3.42 with the Gear Vendor if only for the reason of highway cruising.

Reason being, doing the math, 3.42's with a 27" tire and the .78 overdrive only yields about 60 mph at 2,000 rpm, and you still have to figure in converter slippage because you obviously won't have a lockup converter.

If you jump to 3.73's your mph drops to 55 at the same rpm.

If you're worried more about quarter mile times than highway cruising, then 3.73's would help. Cruising with those still isn't horrible with the gear vendor. It'll do about 70 mph at 2500 rpm. That's not bad, it's just not what is seen as overdrive territory.

What would be interesting though, is some dragstrip passes with the Gear Vendor before the rear gear swap. With 3.08's I'm betting you could cross the finish line in 2nd gear with the Vendor engaged. See what that's worth. 3.08's and a 1.48 2nd gear, with gear vendor engaged it turns 2nd into a 1.15 It'll do 114mph in 2nd overdrive at 5000 rpm.
Probably help you on the back half of the track, but you're still limited to the 2.48 first gear and only 3.08's for the first half of the track, so 60's aren't going to change.

I think a lot of what makes a noticeable difference in cruising RPM’s is the exhaust on your car too. If the exhaust system can handle a few extra RPM’s down the highway and not start droning or too loud of exhaust tone you could certainly pull off the 3.73s and overdrive like a GV. If that is the case I will take the 3.73s pretty much every time. To me that 3.73 to 4.10 range seem like the sweet spot on most overdrives.

3.73 are what my GMC with it’s 4L80E has with a 30” tall tire, I run it in drive a lot towing. It won’t pull the load in OD most of the time. It is still super tolerable cruising in drive with it’s quiet exhaust system.

28" tall tires and the 373 are fine.
Funny how the subject of torque comes up as a driver, But try to get a dyno to measure it below 3 000 rpm.

My friend has a 700R4 behind his 650 HP LS motor in a 85 caprice
with 4:10 gear and a 28 inch tire. It's a TCI 2-stage supose to handle
700 HP and 600 LBS, His converter is a 3500 with lock-up it works so
good I got me one for my 57 pontiac wagon project I'm getting ready
to start on only I'll be running 3:73 gears motor should make 600 +.
You might want to check into that tranny. He got his from JEGS.

GT

Ithe chassis dyno out here started @ 1500. Was rather impressed with 300 ft lbs @2k rpm.
My readings where a little jumpy till 2k so I would assume there is some trouble getting a solid reading at lower rpm?


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I'm running 3:73's with a 200r4 and 27 inch tall tires and love it. Perfect combination to me. I haven't run fuel economy numbers yet, but I'd venture to guess high teens to low 20's at least. No tach on the car for rpm's, but calculator says 2100 at 70 mph. Goes into overdrive around 46-47 mph, and does not lug. Engine is a 389 that made 400 hp and 460 tq on the engine dyno.

Kind of amusing what not having a tach fix’s sometimes.:poke:

I bought a piece of machinery 1000 miles away several years ago. I had room on the semi trailer for my wife’s Ford Focus RS so I loaded it on the trailer and took it with us to make a vacation of the trip. We got to our destination, and took off in the focus, let my wife drive. Speed limit was 80mph, it seemed like the car was making a little extra noise, I thought it was the road we were on. I fell asleep, and woke up about 30 miles later, that vibration the car was making was still there. But the road looked like it was freshly paved now. :confused: I pondered what it was for several more miles. Maybe it had a low tire. Then I leaned over and looked at the gauges. The engine was turning 5500 RPM, and the 5 speed was in 3rd, that was not what I expected. Lol. Amazing the difference the quieter car makes. I think my wife’s sense were shot from riding in my loud semi. :bolt:

LOL, I helped a single Mom co-worker, some, with her Focus.
I got on the internet to see what the potential problem might be.....she got rid of it about a year ago, Thank You, Lord!!!