looking to change my axle ratio

Have you been on a dyno? My truck had 45 more horse running in direct vs OD. Show me math that the Hp at the wheels drops.

Tire rpm is simply your engine rpm*trans ratio*rear ratio
A/B/C=D
a,b,and c can be whatever you want as long as they = d

for 1400 rpm (A)

1400/.73 (overdrive)/3.55=540 tire rpm
1400/1.00 (direct)/ 2.64=530 tire rpm

As you see, rpm is close. the big difference is startability, which i mentioned. Yes you would need to use low as 1st when real heavy and on a hill, but you could still make it in 1st most of the time.

Show me any math that suggest you loose HP by running with a lower number rear.

Dyno results don't lie. technically you should be able to dyno in any gear, but they choose dirrect to get the most power to the wheels. I asked them to dyno in top gear on my 13 and in dirrect. I had 45 more hp in dirrect. The trans is a huge loss.

Melton and mavrick have been running 2.64 for years and are buying more trucks with that spec. If any one needs startability, its flatbedders, and they feel that ratio is ok. Most of there drivers dont even know, so as far as driving, if speced with a direct trans they run just fine. With an od 13, then you would not use the top 2 gears normaly

 

Ahh, fleet truck mentality, it makes sense now.
And flatbedders need "startability"? Really? Whats the difference between a flatbed and freight hauler as far as that goes? 80k lbs or under, is 80k lbs or under, the engine knows no difference. Now do the same thing at @100k lbs or more, starting on a 2 lane road's red light, uphill, and then you will see what "startability" really means.
I would hazard a guess their drivers dont know, because their average experience is under 2 or 3yrs, if that, and they have driven nothing else.
It's time to give up, especially when you start questioning folks who obviously have more experience in these things than yourself. Dyno time? I have personally had loads of it over the years, in different trucks, as I am sure the person you questioned has also.
As you MAY learn over the years, the dyno is only half the story. Real world driving, in different terrain with different weights, will tell a more truthful tale. Chasing numbers to brag about to the nearest other fleet driver is fine, until someone with lower HP goes flying by you up a long grade. It happens to me every time I run with a certain guy we have here, with @120HP less than I have. Gearing.....

Martin

 

Tire RPM, can we at least agree that means Revolutions Per Mile? Good, I thought so, so explain to me how gearing in the transmission or the rears has any affect on the revolutions per mile of a tire. The rpm of tire is based solely on its circumference, nothing else can or will dictate how many times it goes round and round in a mile.

 

We start on really hilly back roads that van drivers generlay dont go on. we also go offroad a lot more at shippers. I never said to go direct carrieng over 80k. The startability becomes to big an issue. Oscar may have more overall experience then me, but I dought he has more experience researching and testing direct drive trucks then I do. He seems to be an old school trucker who is not open to these kind of ideas. That is certainly fine and if he doesn't want to run a 2.64 rear, he doesn't have to. That doesn't keep me from giving advice on a subject i know a lot about. Im sure he gives advice on areas i know less about. Just cuz he has tons mroe years of trucking then i do, doesn't mean he knows more about everything.

FYI, what you say about dynos used to be true. The one at Pittsburgh power has hydraulic arm that hooks into the fith wheel. Varring settings on resistance in the rollers and weight on the 5th you can simulate power to the wheels in much more realistic situations. (including but not limited to, startability, and hp in direct.)

As I said before, if any of you can show me math why i'm wrong, i'd like to see it. Real world experience doesn't count if you've never driven a 2.64 rear truck. It also doesnt count if anything else is different when comparing to a different ratio. (ie a pete cat with 3.55 vs a FL detroit 2.64) Math never lies, show me the math and if its correct, i'll change my mind.

 

revelutions per min. Yes tire size dictates how fast you end up going, but im talking about the rotation speed. I see how the use of tire rpm can be confusing. Should I say axle rpm.

Revolutions per mile is a tire spec that does affect gearing and speed, although not a ton. A larger tire will have less startability and higher top speed, just like being in a higher gear or a lower rear end number. (I say number as to not confuse with a lower geared rear which would be a higher number.)

But, since this discussion has just become about rear end, i left tire specs out of the calculation for now. The eaton rear calculator shows all this (posted link before)

Now, a 22.5 tire will have a lower rr then a 24.5 tire. So, if he is gonna switch the rear, id go for a spec that would work with 22.5 tires so he can get the best option when he eventually replaces tires.

 

2.64 gears, Maverick trucks, Pittsburgh power, show me the math. This all reminds me of a certain infomercial that runs on Sirius/xm weekends from 3-6.

 

The Kevin Rutherford lemmings are out in full force today!

 

First off, the HP part is just flat out wrong. Gearing does not affect HP. What gearing does affect is torque. But, let me see if i can clarify this:


In a normal od setup
.78 trans ratio DECREASES TORQUE, 3.55 rear ADD A LOT of TORQUE back
we need to add more torque back because we lost more by using OD on the trans

In a direct set up
1.00 trans ratio TORQUE REMAINS SAME, 2.64 rear ADDS SOME TORQUE BACK
We don't loose torque through gearing in the trans and thus need to add less back in the end.
This is only a slight disadvantage for startability, because 1st gear is not as low.

If all you care about is tourqe, stay in first gear all day.

In the first od example, why would you take torque away just to add it back? Doesn't make much sense, but its that way we've been doing it for years. You can accomplish the same thing by changing the rear.

 

Why bother.
It used to be "text book genius'", but Google has taken that role now, along with XM, I guess.
Like I said, explain, in your mathematic genius, why a truck with less HP outpulls me on every hill, regardless of what gear I am in......
Never mind, you're right, everyone else is wrong.

Martin

 

What rear end do you have? Pulling hills isnt only horse power. (how do you even know the other guys HP) Truck aero, rolling resistance on tires, bearings...prettymuch everything that effect mpg will also affect how well you climb hills. A big luggy tire need more hp to climb a hill then a low rr wide based. Also, some engines are just off spec. No matter what you do they were built wrong and wont ever profirm 100%. That is way more likely then the thought that you truck defys the laws of physics and math. My truck at TMC was like that. No mater what it just didnt run 100%. They set the head, checked a bunch of other stuff, the engine was just a little bit of a lemon.