Overdrive vs Taller Gears

[money pit]

I have a 8 3/4 with a 3.23 gear and a 360 stock engine, 727 trans, no headers. At 60 mph it revs at about 2900 rpm. To reduce the rpms, should I go to 2.76 gears or convert to a overdrive unit?  I want to make this my daily driver but with the price of gas these days, I'm not sure. I drive mostly freeways.  Thanks

[Brads70]

I'd suggest the OD if you can. I love mine, makes the car MUCH more enjoyable to drive longer distances. Cheaper on fuel is a bonus. Depending on your camshaft selection , you might need a higher number gear though. I have 4:10 in mine now , it was 3:55. Best of both worlds, launches great and low rpms on the hwy.

[340Challman]

I have a 8 3/4 with a 3.23 gear and a 360 stock engine, 727 trans, no headers. At 60 mph it revs at about 2900 rpm. To reduce the rpms, should I go to 2.76 gears or convert to a overdrive unit?  I want to make this my daily driver but with the price of gas these days, I'm not sure. I drive mostly freeways.  Thanks

2900 rpm at 60mph sounds like much lower gears than 3.23's. Maybe 3.73's. Are you certain about your rear end ratio? What size tires do you have on back?

[Brads70]

Wrong stall speed, convertor slipping ?

[Bullitt-]

   Your RPMs to speed not matching up may be a tachometer issue or could be the speedo pinion is not matched to the rear gear// 
60MPH with 3.23 gear 26"tall tire should be 2504RPM

https://spicerparts.com/calculators/engine-rpm-calculator

[73chalngr]

I have 391's in my car 14 inch rims 3000 rpm's is 60 mph in my car .

[GoodysGotaCuda]

OD is worth every penny over trying to get lower cruising revs out of the rear end.

[money pit]

I bought a completely rebuild rear end and breaking system. The guy I bought it from told me it was 3.23. He replaced the rear end with a Dana. I have 225, 70, 14 inch tires.

[money pit]

i thought my speedo was off so I bought a digital GPS speedometer. I'm running both speedos. The digital speedo indicates the mechanical speedo is 4 mph faster than the digital. When I had the 7 1/4 rear end (gear ratio unknown) at 60 mph, the rpms were about 2400.

[Cudajason]

As I said in my Trans swap thread, link below,

I recently went for the first really big drive in the car 100 miles round trip mostly 2 lane highway.

What a treat! 55 / 60 mph most of the way, the old girl spins about 1600 rpm with he OD and LU engaged.   The cam loves it at the rpm, just smooth as silk.  it is so different from running over 2500 rpm.  I always find that the engine is screaming at the rpm and it makes things a little hectic, at under 2000, its much more relaxed.

Hands down the best ever mod I have made to the car and worth every penny and second it took to get it done.

https://forum.e-bodies.org/engine-transmission-and-rear-end/4/rh-transmission-the-result/12648/285

[Filthy Filbert]

I would go with an overdrive transmission if it were my car.   

In fact, that's exactly what my car will have.   a TKX 5-spd while maintaining 3.91 gears in the rear end.

[chargerdon]

If your thinking of OD to save gas...forget it.   I put an A518 and lockup converter into my car.   It cost me about $1,100...   Spent $300 on the A518 itself, then another $200 on a rebuild kit to insure it was in good condition.   Then about $210 on getting the axle shortened (the a518 is 4 inches longer than a 727), about $200 on the lockup torque converter...   NO u cant use the old one from the 727 as the spline count is different on the lockup a518 tranny, another $50 on a new u-joint, $50 on tranny fluid, $50 on new transmission cooler lines (a518 is larger than the lines on 727 3/8 vs 5/16), $40 on 14 gauge sheet metal to fabricate a new transmission cross member, and another $20 on a "gm style" tranny mount.    I also spent about 25-30 hours of my time pulling the (904 in my case), fabbing up the crossmember, doing the "refresh overhaul" of the A518 myself, and installing the A518 into the car.   

Add that all up and it is a little over $1100 not counting my labor, pay to have it done and your looking at another $1,000.    So, if you get say 11-14 mpg now, and the OD gets 20% better, it will be several years to break even on gas !!   

PS Putting in a gear vendors overdrive tail section will cost you $2900 just for the gear vendors unit!!!

If you want to do it save your ears with the lower rpm...then put in the A518 OD...or maybe go with gear vendors bolt on OD for the 727.    I probably would have gone that route, except that i had the lower capacity 904 sp auto and it was beginning to show signs of the clutches slipping ...my engine is also a LA 360 stroked to 408 so i had to change out the 904 anyways.   

You can pick up a 2.76 or 2.94 rear 8 3/4 "punkin" for far cheaper...   ive got a 2.76 for local pick up.     Yes, you will lose a good bit of performance, but, if its for daily driver street only and lots of highway...that is the cheapest and simplest alternative.   Unbolt the old one, slide in the 2.76 and reconnect driveshaft and your ALMOSt DONE...dont forget to change the speedo cable gear in the transmission or the speedo will be way off.   

[money pit]

I think I will wait until next year for the OD. I spent a lot of money on the paint and rear end.  Thanks

[ledphoot]

There are a few reasons I went with overdrive on my builds. 1) Better fuel economy, 2) Lower noise on the highway, 3) Ability to have a good starting ratio with improved highway cruising manners, 4) I live in the California Central Valley where the summer temperatures are routinely >100 and I want my engine to stay cool when it is going down the highway for extended periods of time.

It's worth the investment of time and money.

[Cudajason]

There are a few reasons I went with overdrive on my builds. 1) Better fuel economy, 2) Lower noise on the highway, 3) Ability to have a good starting ratio with improved highway cruising manners, 4) I live in the California Central Valley where the summer temperatures are routinely >100 and I want my engine to stay cool when it is going down the highway for extended periods of time.

It's worth the investment of time and money.

number 2 is one of those things you do not realize how nice it is until you experience it.  These cars are not quiet to begin with, but buzzing down the road at 65 mph and turning 2800 RPM is crazy loud.  When that OD kicks on and you are turning under 2000 RPM...you realize how loud it really is because all of a sudden things are much quieter and relaxed!