Gearing Question #2: What does a .67 overdrive mean?

[Noriaki]

Heheh Noriaki's Gearing Question #2.
(Thanks to those that helped me with the rear end gearing question)

What does it mean when you say a car has a .67 ratio overdrive?

Does that mean that 2/3s (.67) of a turn coming into the transmission is a full turn going out?

(A guess from my new found understanding of rear end gearing )

What kind of a ratio would an average 1st gear have?

 

[PG]

Does that mean that 2/3s (.67) of a turn coming into the transmission is a full turn going out?

Bingo.

What kind of a ratio would an average 1st gear have?

It can really vary. Older automatics were about 2.5 to 1. Newer ones might be slightly deeper, say 3 to 1. Most car manual transmissions are in the range of from 2.7 to 3.5 to 1.
Most truck transmissions are lower than in cars, (higher numerically), and some truck manual transmissions have a "granny" 1st gear that is extremely low. I had a NP435 4 speed tranny in a '75 F250 4X4. The first gear on that was about 6.69 to 1.

 

[Demon-Xanth]

The 1st gear depends on the application and the total number of gears. The T-56 6 speed in the F bodies and 'vettes has a gear set something like: 3/2/1.7/1.5/1/.75/.5 Where as the NV-5600 (in Dodge Ram pickups) has something along the lines of 9/6/4/2.66/1.33/0.866 (yeah, it's quite low, 3/2 ratio on every shift)

 

[Noriaki]

9:1!!!

Holy crap lol. You must be able to pull a small house off it's foundation with that kind of gearing

 

[Demon-Xanth]

Noriaki: in my Dad's 99 Dodge diesel, 1st gear redlines at 5MPH@3250RPM, put the 4wd in low range and you're looking at 2MPH.

Edit: and he didn't go for the 4.11 rear gear option, otherwise those would be lower (iirc, he has 3.55)

 

[Noriaki]

hahah still even with only 3.55 in the back end in 1st you're looking at like 32:1 from the egine to the axle.
That's a hell of a lot of Torque (though obviously 5mph isn't a lot of speed )

 

[PG]

Noriaki: in my Dad's 99 Dodge diesel, 1st gear redlines at 5MPH@3250RPM, put the 4wd in low range and you're looking at 2MPH.

And people wonder why trucks are cool.


Actually, the NV5600 doesn't quite have a 9 to 1 first gear. It's 5.63, but that's still real deep, especially when coupled with the diesel. Here's a great page on that tranny: here

It has a cast iron case and weighs 360 lbs. LOL!

 

[Noriaki]

Well 5.63 1st gear gives a more reasonable ~20:1 from engine to axle, that doesn't sound nearly as fun as ~32:1.

 

[PG]

Keep in mind that the transfer case low ratio is 2.72 to 1. http://dodgeram.org/tech/specs/tranfer_case.htm

So, 5.63 x 2.72 X 3.55 is 54.36, but that's in 4WD Low.
In normal 2WD, you can have 5.63 x 3.55 = 19.98.

Some hard core rock crawling off roaders mount 2 transfer cases together!

 

[Noriaki]

Yeah I fogot about the transfer case, so for 4x4 I guess you get a moderately fun ~55

2 transfer cases!
Egad!

 

[Demon-Xanth]

Looking on that NV5600 site, it makes me wonder if they really stuck a 4.11 in my dad's truck. The speeds listed seem a bit on the high side (even when adding the effect of the 10% oversize tires)

 

[PG]

Looking on that NV5600 site, it makes me wonder if they really stuck a 4.11 in my dad's truck. The speeds listed seem a bit on the high side (even when adding the effect of the 10% oversize tires)

You should be able to figure it out, but it would take some work.
On my old truck, there was a sticker, or maybe it was a plate, on the driver's door or the door frame. It had a code for the axle ratio. I think the axle itself had a tag with the same code. It was just a "P", but it stood for 4.10 gears. I had to look it up in a Ford manual.
Or you could go to a dealer's dervice dept. and maybe they could help you out.
I'm not 100% sure how Dodge labels the axle ratio, but they must do it somehow.