Gear Ratios: Misinformation at its worst.

You can’t really compare a pre-emission engine to an emission one no matter what they are. Any emission engine is choked back so much it isn’t even funny. That said, I love mine, it’s about 700 pounds lighter than an X15, and as far as emission motors go I’m happy enough with it that I just ordered another one yesterday instead of taking a chance on a Cummins.

 

had one of those in my last truck , put 800K on it.

 

I have never driven a standard H pattern much. I put an RTO top plate on my 13. Flipping those back 2 around makes it impossible lmao. High gear is supposed to be against the dash

 

I have a '01 Cornbinder with a 500 HP 12.7 and a '20 T680 with a 510/1850 MX. We run 88,000 lb six axle The t680 shaved 45 minutes off the trip from Pocatello to Vancouver, BC, and does it with about .8 mpg better fuel (year over year IFTA).

The T680 has a 13 with 3.25's, and it is VERY driveable, no problems climbing, never drop below 35 mph (at 88K).

 

Mine's actually plated as an RTX-15615. Previous owner swapped it in but put the RTO shift bar assembly on it. A lot of confusion when I rebuilt it because I had no idea. Had to take all the shift forks back lol.

 

i think the whole direct drive/1:1 top gear in the trans + high rear axle gear vs. Overdrive and lower rear gear comparison is splitting hairs; i think direct drive would save a little energy but a very small amount.

If i was spec'ing out a whole fleet of trucks, would i do direct drive? Not sure.

What i think MIGHT be worth it is single screw with tag axle. At least there would be a significant weight savings?

 

Well as long as everyone’s talking about improper terminology. I have a question. I’ve often heard of a fast street Car described as having a higher or bigger rear end. Say 4.10,4.56,4.88 vs. a Lower rear end ratio of 3.23,3.55, or 390. The way I always understood things. The “higher” rear end has the higher number of teeth on the ring gear. Because the ring gear is larger, it’s often referred to as a bigger rear end. Though top speed is lower, it’s much quicker, for racing. Yet I’ve been told, quite a few times, that I’m wrong. Lower numbered ratio gears are considered higher rear ends. Because they’re capable of higher top speeds. Maybe I’m wrong? Wouldn’t be the first time. When I was a kid, my Father would explain such things. Mostly talking about his 4.10 Detroit Locker in his 65 Plymouth, along with curved dual point distributer, capped headers, stall converter and reverse shift automatic. And those #### 409 Chevys and 421 Pontiacs, Lol. So bottom line. 410 is considered higher than 390. Right or wrong? I’ve always considered my 13 as being double overdrive. Simply because of the .86 and .73 final ratios at the tail shaft. Now, I understand technically it may be wrong, but the (tail) end results are such. I think I’ve learned something from this thread, and appreciate the inputs on outputs. Yet I’m also a bit more confused. Further study is needed. On a side note, my 04 Jeep Grand Cherokee w/5 spd OD, got better mpg (17) than my Durango w/ 4 spd OD . Same 4.7 V8 in both. I had assumed they just added another final OD gear to the 4 spd. I recently bought another Jeep. ‘03, with a 5 spd. Reading up on it, compared to the 4spd. The difference is an extra gear between 2-3, with an OD final gear. Not sure about exact ratios of either, trans or rear. Still researching.

 

The lower the number the higher the ratio. As far as number of teeth on ring gear here are a few examples
4.56 are 41 tooth ring with 9 tooth pinion
4.11 are 41 tooth ring with 10 tooth pinion
3.73 are 41 tooth ring with 11 tooth pinion
3.54 are 46 tooth ring with 13 tooth pinion
3.55 are 39 tooth ring with 11 tooth pinion

the 3.54 and 3.55 are close enough to be ran together either in a 4x4 or a twin screw application. However the 3.55 is a stronger set.
It’s these little things most over look when gear ratios come into the conversation.

As a rule when you go to higher ratios yes the ring gets more teeth but the pinion gets more too. Which makes it bigger. So it gets pushed farther to the off side. Some carriers are offset more to one side for the higher ratio. Under torque this can cause a side load condition and a bad weak spot.



With a lower ratio running lots of overdrive the gear set will get hot due to excessive RPM. Again a weak spot so there is positive and negative to both you just gotta pick your poison

 

Thanks, I’m guessing the 3.55 is stronger because of less offset.