Technical Higher Rear End Ratio With Auto Trans?

I can't quite get my head around this, and google hasn't helped. If one of our old cars came with an automatic it might have like a 3.20 rear. Manual trans it might have been like 3.55. My old Ford with 3 speed and OD has 4.11. I basically understand everything except why the car can get rolling okay with auto and higher gearing. What's going on in the trans? Clutches slipping? Slush box effect in the converter, etc? Thanks!

 

Well, here’s a rabbit hole for you. My OT came with 2.77’s and a 700R4
But can’t answer your question, I have a ‘55 Chevy rearend that as I recall is 3.55? It was behind a power glide car. Maybe it is a 3.3x? I dunno.

 

Isn't there greater torque multiplication with an auto, which would allow a higher ratio rear. Also, maybe, the torque converter will slip until it reaches its stall speed, again allowing a higher gear, but slipping the clutch on a manual isn't recommended othr than at starts.

Chris

 

There is also converter slip as you can tell by the RPM in newer trans with a converter clutch.

 

A torque converter is designed to increase available TQ to the trans gear ratios. Multiplied, sometimes it can be close to 2:1 and that's probably closer to performance uses but the concept is similar. Pretty easy to grasp the difference when using a clutch vs a converter and the need for steeper gears if you look at it like that.

 

Yeah, it's called a torque converter, rather than a fluid coupling, for a reason.

 

I'm with you on that @Budget36. My square OT truck also came with, a 305, 2.70-ish gears and a 700R4 but gets 13-14 mpg on a good day.

 

The original Hydramatic had no convertor, a low first gear instead....In 4th gear it was said to have no slip

 

Torque converter; converts slippage into torque.
This is a table of OT factory converters. Higher stall (K factor) have more torque multiplication.
K=rpm/sqrt(torque)

 

I had an OT Jeep that had a manual transmission, and 3.07:1 gears.

If it had the automatic, it would have come with 3.55:1 gears.

 

Trans 1st gear also dictates rear ratio. Some manual ODs have 3.06 or even 3.20 1st gears. A turbo 400 is 2.48, a 'glide was 1.82 and some were 1:70-something.

 

....and heat.

 

From a rodder who drives a lot of highway miles, rear end ratio has everything to do with the transmission's high gear ratio.
A 700R4 final drive ratio is .70. Using a 3:50 rear end gives an effective ratio of 2:45. Great for low rpm cruising and decent gas mileage.

On the other hand, a street racer with a Saginaw 4 speed (final drive ratio of 1:1) A good differential ratio of 4:11 or lower would make for good acceleration.

To choose the perfect rear end, the car owner needs to decide how it will be used, what engine and mod's along with transmission will be used.

I hope this helps, some.

 

With an automatic transmission the converter performs a neat trick called "torque multiplication". Off a standing start, the multiplication is at it's greatest, a factor of 2 to 2.5. This is because the impeller and turbine are running at much different speeds (slip.... for want of a better term). This multiplication of torque allows the use of lower ratio rear end gearing while still maintaining the same acceleration. As the car's speed increases, the turbine more closely matches the speed of the impeller (engine rpm), and the effect diminishes. At cruising speed the multiplication is almost zero. In an overdrive transmission with a lockup converter, there is no slip when the converter is locked, and everything operates as if it was a manual in top gear.

By the way, your title says 'higher rear end ratio'. You have this backwards. It should be lower.
Low ratio = higher speed.
High ratio = lower speed.

 

I have a 3.08 gear in my coupe. It has a TH400 right now but it’ll be getting a 700r4. Not ideal but it’ll be a good road car. If it sucks I’ll change the gear. The low first gear of the 700 will be nice though.

 

3.08:1 is a nice sedate rear end ratio with a 700r4 transmission. It is what the Holden factory cars had down here in Australia. Good all-round choice for a daily driver.

If you like things a little more spicy, the 700r4 will allow you to up the rear end ratio to really get it accelerating quickly, while still having reasonable highway manners. Just as an example, you could go to a 4.40:1 rear end ratio and still have the same highway rpm as the TH400 in top gear.
A 4.40:1 rear end ratio would give a 700r4 first gear overall ratio of 13.46. That's wheelstand territory..........
Or at least tires disappearing in a cloud of smoke...

 

Mine was (now the kids) ot car, but on straight highway was close to 24.5. In town, ya, like 10-12.
But we have a 350!! Lol. Stop and go killed me.

 

28,
I think you're the one that has this bass akwards.
When you start out you put the transmission in 'low gear' and shift into 'high'!

I know this has been debated here and elsewhere many times. I usually don't get involved unless someone makes it a point. Not trying to change anyone's mind just putting it out there for consideration.
By the way, at least one Quickchange company agrees with you . I don't try to change their collective mind either.

 

A torque converter does increase torque in a manual trans it's called down shifting. You increase engine RPM in relation to ground speed as to slippage How many late trans do not have a converter clutch I realize that low cruse RPM getting in the range of stall speed is a lot of it. If the converter clutch goes out on a overdrive trans temps go way up because of slippage. As to the olded Hydrometrics and 4th gear slippage would the fact that those cars were at close the 3000 rpm not sub 2000 on the road.

 

Not to extend the pissing match, but what hot rodders and associated aftermarket vendors refer to as high and low gear ratios and what oems and transmission / drivetrain designers and manufacturers use for nomenclature are sometimes two different things.

Having said that, “highway gears” can be used with automatics because of the torque multiplication and stall speed characteristics of a torque converter. It lets the engine accelerate above idle without lugging, plus, it multiplies the torque. The “highway gears” allow lower engine rpm, and more relaxed cruising on the open road.

The manual needs steeper gearing to avoid clutch slippage at startup under load, since there is no torque multiplication other than what the gearing allows.

 

The simple way is to say "numerically" when talking gears. Yes, 4.88 is a low gear and 2.78 is a high one. This is found in the same book that separates "motor" from "engine"

 

Yep, sorry, you guys are right.......... well, you are right in the fact that there are two ways to describe it.
I will change it to what I still believe to be correct......... as best that I understand it anyway.

Low numerical ratio (eg: 2.78:1) = high geared = higher speed at a given rpm. A high gear has a low numerical ratio.
High numerical ratio (eg:4.11:1) = low geared = lower speed at a given rpm. A low gear has a high numerical ratio.

The thread title states "higher rear end ratio" so I was talking in terms of numerical gear ratios, in which case I still think he has it backwards....

 

drag racers and land speed racers use the opposite terms to describe "high" and "low" gears. It can get confusing if you don't add another word or two, such as "ratio", "numerical", "speed", or something to let the other guy know exactly what you mean.

 

OP, you've got the right answers from others.
The two I've quoted make me wonder why it was referred to as a converter? Why not multiplier? It doesn't convert anything in my way of thinking.

 

I had one of those too. 14 in town. Almost 20 at 70mph


I had another OT car that had a whopping 2.41 gear ratio. The car ran almost the whole 1/4 mile in 1st gear

 

Thanks guys!

 

Below I try to explain gear ratios and the decisions about choosing a gear.

My '51 Coupe has a 3.73 rear gear and the standard manual 3 speed. The '50 F-1 I used to have, also had the same rear ratio, 3.73.
Mated with the manual 3 speed, my relatively low HP flathead is very peppy down low and can run very well at 65-70MPH. Sure, it's got a little more, but why go there?
It's a great all-around combo for that flathead coupe.

My F1 with the OHV '68 302 was a rocket (adults only) with that gear ratio and the straight shift (manual) 3 speed.
I consider 3.73 to be the best middle or all-around rear gear ratio for a hot-rod. It's perfect for a manual, although it may be a tad bit low for an automatic. It depends on what your goals are.

The little '64 Fairlane Coupe I'm building (when I get back to it) is a more specialized car.
This car will have...
'68 302 with a little pepper, 2-5500 RPM powerband.
Early C-4 with 2500 stall.
The rear axle is a Ford 8" with an (Detroit) Eaton True Track "Limited Slip" at 3.80 ratio.
The 3.80 True Track rear may be a little stout for an automatic car, I'll explain why I chose that ratio for this combination.
Figuring tire size, camshaft and all that stuff...the factory 3.00 rear axle was going to be iffy for this car. Although equipped in millions of "automatic '60s Ford Cars", the 3 to 1 is about as exciting as a bread sandwich.
Also, what style driving will I actually do?
Pretty much the same style of driving that I do with my '51 coupe with an important exception.
I will do a lot of town driving and 55-70 MPH Hwy driving. With the stock "high" 3 to 1 ratio, I have a narrow comfort zone. More gear gives me more of a comfort zone for my application.

By comfort zone I mean, I don't want to be cruising around below stall RPM in "high" or 3rd gear. This can cause heat and it's not efficient. The important exception mentioned above is I may race the Fairlane in Nostalgia events. It's not a "racecar" per say but it may see an occasional '60s stock class event. It's mild enough to drive as a fun street car, and it may even see the track.

One thing for sure it will not be a long-distance cruiser. It's just a little car I may drive to the track and maybe even run.
I need to get back on it and I will not know how this works out until the rubber quite literally meets the road.

 

Stator...

 

Right now I have a 3.00 in behind my stock 302 AOD combo, and it's not low enough. It struggles to stay in OD at 55-60 MPH, constantly shifting on every little grade. I have a 3.55 in another rear I need to take out and install to give me justa few more engine RPM to get me above that in and out point since that is where I mostly drive. OD doesn't make that much difference in RPM, looks like about 250-350 RPM on the tach, so right now I just use Drive unless I'm on mostly level ground.

 

Think about it in some simplistic general terms rather than technical terms.

If you have ever driven a standard manual transmission car/truck, your leg has been educated to slowly release the clutch as you take off so that you don't either squeal the tires OR kill the engine. Its an aquired skill.
There has to be some transition in getting the vehicle moving rather than just removing your foot from the pedal and letting it fully engage. This is for normal driving, not racing.

An automatic transmission has slippage built in, inside that torque converter thingy. So your engine can have its rpms increased for take off and the torque converter smoothly applies the added power........essentially taking away the need for a clutch to provide some slippage.

Later versions of automatic transmissions have a "lock up" feature so that when your vehicle reaches a certain speed, it locks the torque converter so there is no longer any slippage.........just like when a manual clutch is fully engaged.

Originally automatics were for "family" type cars and gas mileage and over the road speeds were major considerations. The automatic worked just fine in them, and a lot of people didn't like shifting gears themselves. So a higher gear ratio (numerically lower ) (2.50-3.00) was used with them. I think they simply mean that the gears are for higher speed driving.

Trucks and drag race vehicles on the other hand found the a lower gear ratio (numerically higher) (3.70-4.56+) provided better performance for racing and pulling heavy loads. (it provided more torque multiplication at LOWER speeds).

Then they began making transmissions with more gears in them, and then even overdrive gears and people found they could run lower gears (higher numerical) in their rear ends and get that benefit.......and the overdrive transmission would still let them get higher speeds and/or better gas mileage.

The best combination of gears will depend on the vehicle you have and what you want it to do. In general if a vehicle is to be daily driven and see any time on todays expressways, I'd shoot for a combination that will give you 80 mph when your engine is turning about 2200 rpms. If you use an overdrive that gets that for you, then generally you will have decent performance in the lower gears in a daily driven vehicle with a decent V8 for power.