Technical Higher Rear End Ratio With Auto Trans?

Not sure I believe that chart. A large converter has more torque multiplication than a small one. Big tow converter’s multi torque almost 2:1 vs a 9 inch that would be maybe 1:1.

 

I have a 3.54:1 rear end with tall 235.75.15 tyres, Approx. 2k rpm @ 60mph. Rule of thumb I subscribe to is engine rpm should be between 1800 and 2100rpm @ 60mph

 

Gear ratios in the trans it's self have a lot to do with the rear end ratio. Most of the 3 and 4 speed car trans have a 2.2/2.5 first gear.
A 700 R4 has 3.06, 1.63, 1, 0.7, and 2.9 (2.9 being reverse)
Turbo 350 has First gear: 2.52 to 1. Second gear: 1.52 to 1. Third gear: direct drive, i.e., 1.0 to 1. Reverse: 2.07 to 1.
Turbo 400 2.48:1, 1.48:1, 1.00:1, and 2.07:1
Power glide 1.78/1.82 low and 1.0 drive.
M21/22 Muncie 2.20, 1.67,1.22, 1.0
Saginaw 3 speed 2.54, 1.88 and 1.0

The AX15 I have for my 48 runs 3.83, 2.33, 1.44, 1.00, 0.79,
With the 3.54 rear end that I have for my 48 and 27 inch tall tires it would hit 3084 rpm at 70 with the 3 speed that is in it, with the .79 od in the AX15 it will run 2430 rpm at 70.


The deeper first gears in the 4 speed overdrive trans let you run a bit taller gear in the rear end and really cut the rpm down in 4th gear. Or the overdrive lets you run a 3.3, 3.5 or 3.7 and still cruise along at 70 or ?? a comfortable engine speed.

 

I'd suggest that taller final drive gearing with automatics was to compensate for converter slippage under cruising conditions.

Converter slippage and torque multiplication aren't the same thing. There is torque multiplication only when the stator is held stationary by the sprag. If we could measure it, we'd find that torque multiplication would be proportional to the holding torque on the sprag: it is greatest at launch and then diminishes, reaching zero at the moment the stator starts to turn. When that happens, torque multiplication is 1:1, but there is still slippage.

At cruising speeds, the stator is happily spinning at something under engine speed, but the turbine is still turning a bit slower than the impeller. That difference is slippage, and it means that the transmission's mechanical direct-drive top gear ratio is really closer to 1.15:1. We need a taller final drive to translate that into the same road speed as we'd have with actual, mechanically-fixed direct drive.

So, while torque multiplication tends to increase the effective ratio spread of the transmission, slippage tends to decrease it. The overall spread on a transmission with 2.48|1.48|1.00 ratios might be 2.48/1.00 x 1.8 x 0.87 = 3.88, depending on the converter specs. That net increase in spread is why automatics have tended to have one ratio fewer than their manual alternatives.

Compensating for slippage through taller final drive gearing was viable and cost-effective under the conditions for which the automatic transmission was developed: cheap, plentiful fuel; considerable fuel sovereignty; rapid urban expansion; and a government commitment to a programme of spatial development which offered the automobile to people who couldn't care less about cars, driving, or learning to shift gears as pretty much the only mobility option, leading to cumulative cartelization of the North American motor industry from c.1934 on. These conditions (at least the first three) didn't obtain in Europe, hence far slower adoption of automatic transmissions for any but the most luxurious high-end cars, until lock-up converters made most of the slippage penalty go away.

Despite this bit of nefarious history I regard the torque converter to be a thing of beauty and wonder. I've speculated about combining a converter with manual shifting, e.g. here.

 

Actually, Mother Mopar did have a torque converter in front of a geared transmission the 54 only Plymouth torque converter that shared engine oiling in front of a 3spd standard.

 

I forgot about that one!

 

It’s all about matching the torque output of the engine, weight of the car and then intended use. Our 8BA with overdrive happens to LOVE a 3.00:1 geared differential. With the 30” tall tires the flathead cruises on the open road at 65-70 mph all day long getting pretty fair gas mileage. Because first gear is pretty low it accelerates from a stop without problems.

 

Yes, and N0...... Everything depends on tire diameter, and rear gear ratio....... Will 1'st be super low, as in shifting in 50 ft, but will cruise nicely down the highway, OR, could you drag out 1st for a while= accelerating quickly, but then when OD is applied, your not even in the torque curve, or at least the beginning of it, and lugging the engine at highway speeds.....A lot of variables to consider.....Proper planning and math with all the given charts should help. I guess it all comes down to what the engine will/will not handle as far as the "sweet spot"- cake and eat it too kinda thing.........In my case, the 700R is coming out, and the T350 is going in........
It's only going to bump my 1to1 600/700 rpm higher in drive, but will put my engine in the curve so to speak..... and all this at 70-75mph. Results and opinions will vary.......... Good luck with your quest.

 

The poster ( @Dan Hay ) says he has a 3.08 gear in his coupe with a TH400 transmission. I would assume he means a 3.08:1 ratio rear end. His coupe does not look like it has large diameter tires on the rear. I think he will be just fine swapping in a 700r4. If he posts up his rear tire diameter, I am more than happy to give him the rpm figures he will get with the new transmission.

 

I wanted to add that I can tailor the shift points in my org reply, but I can't change the drive ratio's.... I can swap gears, but, it's always a give and take somewhere........

 

1959 Lion's Dragstrip 4:56 Positraction gears... 3 Speed transmission

Hello,

Rear gear ratios are just that… makes you get better traction and power transfer as per activity. Just replacing the rear gears from a 3:55 to a 4:11 makes a lot of difference. Whether it is a stick shift transmission or “just” an automatic transmission, the change in gears is noticeable. 3:55 is usually accepted as highway gears, as it makes the car run smoother out on the open highway. For most in town driving, it is a fine selection of gears. Going to a 3:78 gearing makes some difference, but for acceleration and more power to the wheels, a 4:11 gearing is/was used.

Our 58 Impala had special order 4:11 gearing, the set of Positraction gears was perfect for the power of the 348 motor at 280 hp for the street and at the drags. The drags were fine as it was the driver that made the difference, since we all had or was supposed to have, the same motor from the factory in the stock class. The stick shift 3 speed was standard and it was fun competing. Quickness at the line gave the edge.

The 4:11 gears worked fine, until the top guys started using 4:56 gears to win the class on a regular basis. Just that much changeover added to the quickness of the driver made more wins, until the rest of us in the class also got our own 4:56 factory gears. (a complete 3rd member unit)

Now, the competition was back to quickness off of the starting line and holding on without missed shifts down the lane. The competition was a lot closer. Tuning also played into the winning combination, but without a quick reaction time, it was usually all over just past the orange timing tower.


Jnaki

Now, driving around on the local streets for daily high school and jobs was another story. The 3:55 gears gave it plenty of power for flat land driving and the 348 motor had enough power to go up into the steep elevations of the local mountains. But, it just did not have the quickness/power while using the 4:11 gears. So, for most of the daily driving in the 58 Impala from the spring of 58 to selling the Impala in 1965, the overall gear choice was the 4:11.

In 1960, we had the opportunity to have a C&O Stick Hydro transmission put in place of the 3 speed transmission. It was one of the first C&O Stick Hydros installed in a street sedan that could be used for the drags, too. But, now it was no longer in the factory stock class. The same thing was noticeable of the gearing. We did not try the 3:55, but for overall daily driving and getting around, we used the 4:11 Positraction gears.

When we knew of a big cruise to other drive-in locations or a round of hot rods was gathering somewhere, we put in the 4:56 gears, just in case something popped up. Those gears were good for around town and to school cruising, but one had to get used to the high revving motor with the 4:56 gears. If we knew we were going to a longer Southern coastal drive to Laguna Beach/La Jolla/San Diego or North to Santa Barbara, we had the 4:11 gears for easier highway cruising.
A wolf in sheep’s clothing …
Thanks for the close as possible look… @themoose

The custom car look with the modified Buick Skylark wire wheels installed, gave everyone an “easy to win” appearance. No one suspected a mild custom car/local cruiser to have the quickness and power to blast past everyone in any encounter. By this time, a Racer Brown Cam/Lifter Kit was installed and some mild head work provided a little more power for the relatively stock 348 motor. It was more than the stock 280 hp to the rear wheels with the modifications.

We had the choice of starting in manual or automatic. In the manual mode, first/second gear was very short and powerful. Most of the time it was always in automatic or “D” for everyday cruising. Even when an encounter at various locations was concerned, “D” was the winning mode. There was too much to get down in preparations for the quick start and not worrying about shifting was preferrable. Just time the drop of the elbow or end of the yellow light and off we were in the lead. YRMV

Remember, it was “ a quarter mile at a time…”

 

Thats true, but changing the rear gears (numerically higher) will give you better acceleration and help keep your engine in its power band.....and make more use of that power. Then when you get to cruising speed, your engine can stay at a slightly elevated rpm as you desire and still keep up with todays 70-85 mph traffic.

Swapping to a turbo 350 doesn't accomplish any more than just keeping your current trans in its 3rd gear selection as both are 1to1 ratios. Put your trans in 3rd and get on the xway and drive somewhere. You basically accomplish nothing by changing trans, although there is some difference in the gear spreads in each trans. I believe the 700 actually has the lower 1st/2nd gears.

Look at todays cars and what do you see.......transmissions with more gears. I just think you are making a mistake to go to fewer gears and wish you would rethink it. Best of Luck whatever you decide.