Alright guys and (possibly?) gals, I have a question that I know my ignoranimous will be showing in its wake:

In regards to an average joe manual transmission, specialty, or otherwise what is the difference between a granny gear and a your 1st gear? Secondly, what is the difference between your last gear and overdrive and final drive?

I'm just wondering so I can get a better grasp with the gears concept of transmissions and axles, etc.

Thanks friends

 

Not too sure on OD/final.

Granny gear is an extremely low first gear. Usually seen with the NP435 4 speeds in these trucks. Something like 6.88:1. So with my 4.10 rear end it's used for towing. I think my top speed in 1st is 8mph haha. But, it produces a large amount of forward torque and acceleration. So if you're trying to get a big load going, going up hills, or over rough terrain in 4x4, you need a slow steady power more than you need a fast jerking.

So taking off down the road I start in 2nd. basically having a 3spd. trans and a big rear end makes high speeds difficult, but for work horse it's a good setup.

 

I will agree with the above statement. I have the same setup in my 84 F250HD with 4 speed and 4.10 gears. 1st gear is useless when driving around unloaded. Not even sure if I could get 8 mph. haha Didn't know this was considered "granny gear" tho. Thinking that may just be a term coined to a gear ratio combination. not necessarily a specific gear.

as for the higher gears, I might be wrong here as Im new to the older fords, but I believe that your final gear (4th in this case) is usually close to a 1:1 ratio. Overdrive gears are usually less than 1:1 specifically for fuel consumption at high speeds and are usually under powered.

 

Typically, final drives are basicall rear end gears (in heavy duty trucks, semis, etc). I've also heard final drive as RPMs used to move a vehicle in a certain gear. Example: 1st gear = 6.88rear end gear) 4.10 = (final drive) 28.208. Make sense? It gets rear fun when u factor in transfer case ratios.

A NP435 is technically a 3spd with under drive (granny/creeper gear). 4th gear is 1:1, whereas overdrive in a ZF5 (I believe) & M50D is something like 0.76:1.

 

Sry my example got botched... Say 1st gear in a 2wd with a 4.10 rear end, you'd take (again, guessing numbers here) 6.88x4.10=28.208

Hopefully this helps, this is how I understand it, anyway.

 

I know what you're saying here, but I think it's going to confuse Xenthrax more, because the term 'overdrive' just relates to top gear.
Underdrive transmissions exist, but the NP435 & T18 etc, aren't examples of them.

In the 'old' days, & still in some cases, most transmissions were 1:1 in top gear - this meant that one revolution of the input shaft gave one revolution of the output shaft.
Transmissions in our trucks like this are the NP435, T18, T19, 3.03 etc.

As said, with O/D transmissions top gear (overdrive) is actually geared up, so that the output shaft is turning faster than the input.
This allows higher roadspeeds without higher engine rpm, as long as you have the power to swing it.

The term "granny' just relates to a creeper gear (IOW - extra low reduction).

A confusing aspect of rear axle slang, is describing low-speed diff ratios - eg 4.56:1 - as 'higher';
& high-speed ratios - eg 2.75:1 - as 'lower', in contrast to trans gears where 'low' is slowest, & 'top' is fastest.

 

Granny gear is nice for feeding hay by yourself. Put 'er in L, let out the clutch, jump out of the cab and into the bed, and drop hay while the truck creeps across the pasture on its own.

My old 300/6 '88 had some kind of manual 5-speed with a granny gear and (I think) an OD 5th. Living in a mountainous region, I seldom got out of 4th gear.

 

Not trying to be smart or snide, but I see you're from new Zealand. I heard you mention a Trans that I'd never heard of (3.03 maybe?). What is it? Also not sure what 'IOW' stands for.

Also, maybe it's a regional thing, but everyone I know locally calls 4.56's, for example, "low gears" (deep, big, slow) and 2.75 high gears (tall, fast, airplane gears). But, this does get confusing for us as well, as numerically 2.75 is lower than 4.56. I'm def not saying you're wrong at all, just a few things I've noticed that are a bit different

 

You should travel the world some, there are lots of differences amongst cultures.

 

Welllllll....I was born in Italy, raised in Ohio and I've been to every surrounding state and a few that are further away (I insist on driving, flying is no fun!). Quite a few subtle differences just over state lines!! I'd love to go to new Zealand or Australia to snag a cross flow head for my 300-6 lol

That and I want a right hand drive bully

 

"3.03" or "303" = Ford top loading, iron cased, side-shifted, all-synchro 3 speed manual introduced about 1962 in trucks, 1963 in cars. This was installed in almost car and light duty truck Ford made from 1963 to 1985 or 1986. Ford also sold them to GM for use in many mid to late '60s GM vehicles.

"IOW" = "In Other Words".

Also:

"A NP435 is technically a 3spd with under drive (granny/creeper gear). 4th gear is 1:1, whereas overdrive in a ZF5 (I believe) & M50D is something like 0.76:1."

NO NO NO NO NO. "Technically" a NP435 is a FOUR SPEED. "Practically", it is a "three speed" as far as usable road gears.

Stop calling granny and OD four speeds "three speeds + OD" or " three speeds + granny". They are not "+" anything, they are 4 speeds.

Three and four speed transmissions plus OD DO exist, an the distinction should be maintained to avoid confusion.

For example, there is a world of difference between a T85 with an electrically shifted OD on the tail and a SROD. One is a 3 + OD, the other a 4 speed.

Rant over, I'm foaming at the mouth.....

 

So a "three on the tree" probably had a "303", correct?

 

And a NP435 is actually considered a wide ratio 4spd, so I stand corrected. I'll also point out that my definition of "final drive" is also (probably more commonly) known as "crawl ratio". Sorry for any confusion

 

Granny vs any other first gear:

A granny gear is usually a really low ratio first gear. This gear can be synchronized (helical-cut) or un-synchronized (spur-cut). A synchronized gear you can shift into while moving. An un-synchronized gear, you have to be at a dead stop to shift into.

4th gear can be direct drive (1:1 ratio) or overdrive (less than 1:1 ratio, or anywhere around 0.60 - 0.80:1). To understand direct drive and overdrive easier, understand final drive ratio.

Final drive ratio is the total gearing in the drive line of the vehicle, dependent on what gears your in.

For example, NP435 1st gear is 6.69:1, my transfer case is 2.6:1 low range, and a rear end ratio in my truck of 4.10:1.

To calculate the final drive ratio, you multiply.

6.69 x 2.6 x 4.10 = 71.31:1 crawling gear.

If I were doing normal driving:

Second FDR: 3.34 x 1:1 (transfer case high range) x 4.10 = 13.69:1

Third gear FDR: 1.76 x 1:1 x 4.10 = 7.26:1

Fourth gear FDR: 1:1 x 1:1 x 4.10 = 4.10

If it was overdrive: 0.76:1 x 1:1 x 4.10 = 3.11:1

Overdrive gears, being less than a 1:1 ratio, simulate having a higher set of rear end gears. As you can see, overdrive with 4.10 gears is the same as having 3.11 gears in the rear end with no overdrive (or, a 1:1 4th gear).

Your last gear is your highest ratio forward gear that you can drive in. For example, in a 4 speed with no overdrive, it's 4th gear. In a 4 speed WITH overdrive, it's still 4th gear. With a 5 speed it's 5, with a 6, it's 6, and so on.

Gears themselves are torque multipliers. That's their purpose.

So, say you have a motor making 250 lbs-ft of torque at a given RPM. In my last gear, FDR of my truck being 4.10, you would multiply 4.10 x 250 lbs-ft to get around 1000 lbs of torque to the ground at the given RPM.

If I were in 4 wheel low and 1st gear, 250 lbs-ft of torque, here is what you'd have:

Torque x Transmission Gear x Transfer-Case Gear x Rear-End Gear Ratio = Torque to the ground at peak torque RPM.

So, let's say peak torque is 2000 rpm, at 250 lbs-ft.

250 x 6.69:1 x 2.6:1 x 4.10:1 = 17,828 lbs-foot of torque to the ground at 2000 rpm.

Now you can see why some trucks are known as stump-pullers. It's because they can!

 

^^^ This!

Speed's that a transmission can have are counted as Forward gears. You don't count Reverse gears.

And yes, they aren't + OD or + Granny gear, they're counted by the forward speeds.

You can have overdrive "units" that are basically the same thing as a transfer case mounted to the tail-shaft of the transmissions. These are "overdrives". They are also the traditional form of overdrive. It wasn't until the right thought processes came along that manufacturers started to put the overdrive gear in transmissions as a forward motion gear instead of a rear-mounted unit in the drive line.

A transfer case is an example of what would be an "underdrive" unit. Underdrive increases the ratio of the gear, overdrive decreases the ratio of the gear. Underdrives used in trucks are for pulling/traction, overdrives in trucks and everything else are made to change RPM at a given speed for maximum torque/pulling or maximum fuel economy. Undrives can also be mounted "traditionally" on the tailshaft of a 2wd vehicle to give increased gearing possibilities.

So say your cruising along in an old car or truck that has an overdrive unit bolted to the back of the transmission. You can be in 1st, 2nd, 3rd, or any other gear if it has more than three speeds. Along with that, you can be in 1st + OD, 2nd + OD, 3rd + OD, etc. The + OD is known as "over". So you can be in 1st over, 2nd over, 3rd over, 4th over, etc.

With an underdrive, it's the same principle. 4th under, 3rd under, 2nd under, etc.

It's all about keeping the motor in it's power-band for maximum pulling or maximum fuel efficiency. Generally you want your cruising RPM to be where the motor makes peak torque. Having more gears lets you do that.