looking to change my axle ratio

Get rid of the 15 and replace with RTLO 13 or 18..........problem solved!

 

BTW SQ100 are very inexpensive to swap.........less than $2000... if you install yourself.

 

Well if you changed your rear end, you could run a smaller tire without red line and get better mpg. Others will disagree, but a 2,79 rear would be great for you. You could run 62 in dirrect drive saving mpg and getting more power up a hill at 62. When you need more speed you use the overdrive gears.

 

not if he keeps a 3.9 rear the problem is not solved.

 

He will be running less than 1600 rpm @ 62 mph..........so yes......HIS......problem, will be solved!

 

is it a 15 overdrive or is the top gear dirrect. If the 15's top gear is direct then switching to an od trans would help, but would cost almost as much as switching the rear.

 

A 2.79 rear end ratio? Good Lord. I thought the fleets that ran those are all seeing the light now.....
On flat land and nothing else, maybe, on a hill or mountain?
Some of us can afford the extra pulling power, and indeed pull the weight and the terrain to fully justify normal gearing.

 

For the last time.....Pulling power is the same. It will be more in dirrrect gear since your not loosing 30-50 hp just to trans losses. Only problem is startability. It does make you 1st gear a little higher. Once your rolling though, there is no difference in power aside from when running in dirrect gear where you get more. If you decide to cruise in direct by specking with a 2.64 or 2.79, you will downshift less, becuase you have more hp going to the wheels in your cuise gear.

 

At the flywheel, but not at the rear wheels.

Where is it written that there is that much difference in parasitic lose? You don't even loose that much in total thru ANY transmission, let alone there be difference of that much from one to another.

When loaded low would have to be the new first gear.

Again, with a gear that tall you WILL NOT put the same amount of power and torque to the ground as you would with a set of 3.90's

WRONG, just because you are running in direct, does not translate to more power to the wheels. It only means that the engine could run more efficiently because it is turning less gears inside the transmission.

You really should actually research some of this stuff, its pretty simple physics, a taller gear takes more power to turn one revolution than a shorter gear. Its the same thing as using one pulley to lift something versus using two or more pulley's to lift the same thing, it will always be easier when you add more pulley's.

 

A quick lesson...
Torque from the engine gets multiplied through the transmissions lower gears. This torque gets multiplied yet again through the axle ratio.
So a 1650ft lbs torque engine running in direct 1:1 high side gear puts 1650ft lbs of torque to the rear ends (this is of course not counting the parasitic loss).
Now, for that direct gear, a 2.79:1 rear end will put 4603.5lbs to the wheels. The same engine with 3.73's will put 6154.5ft lbs to the wheels.
There is, and always has been, only two real ways to get more power to the ground,. Gearing and horsepower. Rear ends are for the most part, the most reliable way of getting more to the ground. To calculate ft lbs of torque from HP is pretty simple. 500HP at 1500 rpm's is 1750ft lbs of torque as a rule. HP x RPM's / 5252 = ft lbs of torque. This is of course only a guideline, and some may differ slightly. As you may of guessed by now, I have indeed dabbled in this a time or two......But by all means tell me I need to do some more research.
For the weight I pull, and the terrain I drive, 3.73 or 3.90 would make a massive difference to my truck, instead of the 3.58's it currently has. My only issue with swapping them out is my OEM warranty I have. Hills would be a LOT more easier to pull with the deeper rear end gearing.
I have of course, ran with folks with different rear ends than I have had over the years, with otherwise the exact same truck, trailer and load. On the same hill, with the same load, the truck with the lower (numerically higher) gearing will always get to the top ahead of the other one, all the time while using less fuel/boost, and giving the transmission an easier day to boot.
Your best case scenario's ASSume flat roads, light loads, and big horsepower and thus torque. Before you knew how to drive, we were hauling heavy (100k lbs+) loads with under 400HP. How did we ever get to the top of that 8% grade back then? Gearing, pure and simple.
Martin