What is the point of overdrive? I know your probably thinking to help lower RPMs for less fuel consumption but the point that I want to make is with towing. Almost every manufacturer recommends that when you tow, you lock overdrive out because the gear is so small that you may damage it. That basically means that OD is the weak point in every transmission.

I would think that you would be able to make a transmission stronger by giving all of the gears in a transmission bigger gears (higher numerically) with that last gear being a ratio of 1:1, meaning that the engine and drivetrain are directly connected, saving wear and tear on the transmission. And then to be able to obtain the same RPMs for the highway, put taller gears (lower numberically) in the differentials. I would think this would make the drivetrain more bullet proof.

Give me your thoughts on this subject because I am extremely curious what you guys think.

Gary

 

the trannies already have 1.1 in third.

 

In an automatic OD trans, THIRD gear is 1:1, and fourth or OD is .67:1 (AOD/4R70W) or .73:1(E4OD/4R100). The toruqeshift has 5 gears, with two different OD ratios, one for towing, and one for normal cruising. Some of the manuals have and OD as well, but I'm not sure on the ratios. I'd assume they are all between .67 and .75:1, and they are primarily used for fuel economy. I wouldn't really call them a weak point. When you're towing a heavy load you are asking a LOT from the whole drivetrain, not just the trans. You put greater stresses on the ring ang pinion in the rear end, greater stresses on the engine, brakes, suspension, and even the frame of the truck. The reason most manuals say to lock out OD is because in OD you may not have enough horsepower to maintain speeds with out the trans constantly hunting for gear. This makes the transmission get hot, and burns the fluid. The burnt fluid then in turn tarnishes bands, and clutches inside the trans, which leads to failure.

 

I know third gear is already 1:1. My point was to make that the final gear, put shorter gears throughout the transmission and put taller gears in the rear end to help make the the drivetrain a little stronger. Its my understanding that when the transmissions are in 3rd (or whatever the 1:1 gear ratio is on your vehicle) there is no stress on the transmission because the engine is connected directly to the driveshaft (when the torque converter is locked up), therefore there is no load on the transmission which means no extra heat is being built up.

This would mean that you would be able to run in the 1:1 gear towing a trailer and be running the same RPMs as you would be in a normal vehicle with OD and there would be less stress on all of the components. Correct me where needed.

Thanks

Gary

 

point of OD, so i can run down the highway at more than 67

i don't have OD, i wish i did.


DT

 

Putting taller gears (lower ratio #) won't necessarily help things out.

If I went form 3.73s in my 2.3L Ranger to 2.73s, my mileage will be terrible getting up to speed, with or without an OD tranny.

The idea of the tranny, whether manual or slushbox, is to have the driver or computer shift to the proper gear for a certain situation.

In the old days, there were 2 speed and 3 speed trannies without overdrive and taller rear gears. Fuel mileage was terrible for that among other reasons. Carbs, crummy tuning, etc.

You can get away with towing in OD to a point. I wouldn't say that point is very high, but you can get by.

Basically, in the real world, there is always a trade off. A tranny with OD that might be DQ'ed in towing situations or not having OD at all. It's a lot easier to shift a tranny than swap rear axle gears out.

Good thought provoking post, but I like cruising at 1950rpms in the Ranger and probably about 1500revs in the Camaro (when it gets finished, of course) and shifting into 3rd or 4th when I feel the need to pass someone.

 

Hello Gary,

I asked 2 different Ford tranny techs about using OD for towing. They said it is just as strong as the other 3 gears, it is just a 4th gear. The problem with using OD for towing is the ratio (0.7:1). It puts the engine at low rpms and way below the hp range when coupled with 3.55 diff gears so your engine will not have enough power to pull very much. If you continue to step on the gas trying to pull a heavy load the tranny will start shifting in and out of OD which generates excess heat which is not good so Ford put the OD off button there to lock out OD so you can tow a heavy load in 3rd at partial throttle and not have the tranny shift into OD and lose all your power. You could put 5.13 gears in the diff and end up with a true 4 speed tranny with OD at almost 1:1 just like 3rd is now.

 

OTR tractors usually run a system you are proposing here (No OD)

 

Random question: Is it called overdrive only when and because the gear is less than 1:1? Just out of curiosity. So if the last gear was 1.02 or something like that, it wouldn't be considered OD since it was still greater than 1:1?

 

The most important reason to lock out OD is for cooling. The increased engine speed increases the circulation of the transmission fluid through the cooler, keeping the transmission temperature down. Air cooled engines on lawn and garden equipment are designed to run at full throttle for the same reason. It's not for the horsepower but for the increased cooling.

 

You have it exactly right.

In the old days, overdrive was a separate gearbox, behind the transmission. They went out of favor when automatics and four speed manuals started to take over in the 1960's. A typical application was a 1954 Ford Mainliner with a straight six and three-on-the-tree plus optional overdrive. It was mostly for the few that did a lot of highway driving and cared about gas mileage back then.

Jim

 

Almost all Studebakers and Hudsons had overdrive in the early 50s. The Studebaker Champion was known and bought for it's fuel economy. The disadvantage of the old overdrive was the lack of any engine braking. They freewheeled like they were in neutral when going downgrade or slowing down. Brakes of that era were poor and it made many people not want or use their overdrive.

 

People actually cared about gas mileage back then? Wow. lol jk.
That sucks about it freewheeling though, that would have convinced me not to use it as well.

 

I believe that it's to stop the truck from hunting for gears when towing, since you're only SUPPOSED to be going 55 mph. I'm sure everyone here does that, right?

 

Gary, I'm assuming when you say "almost every" you're included all of them except Ford. Ford doesn't say that anywhere in the manuals.