Another thing that has not been mentioned is that with deeper gears (higher number), there is less strain on everything between the crankshaft and the pinion shaft. It may be something to consider if planning to tow heavy.

 

People that understand won't need an explanation; sometimes people that need an explanation won't understand. Don't ask questions and argue because the answers weren't wha you wanted to hear.

I used to have a 1995 F150 4x4 that originally came with a 351, an E40D, and 3.55 gearing. It came with 30" tires, which I swapped for 31" tires- not a big jump at all, but I could still feel it. I eventually had the engine rebuilt to a 393 stroker; and in time, changed the axle ratios to 4.10. The truck gained 300-400rpm at cruise if I remember correctly, and was much happier with the new gearing- even with the added torque of the stroker. It had more grunt in every gear and shifted a lot less often. My 1997 F350 has the 3.55's and a ZF5 with granny gear 1st. I Would love to have the 4.10 in mine to keep the engine in its sweet spot instead of shifting so much. Increasing tire size, or doing something else to alter the final drive ratio without rehearing only puts more stress on the transmission- no matter what kind of power your engine is making. Why on earth would I want to just use less gears instead of utilizing the full functionality of my vehicle? That's just silly.

 

With out using numbers here is how I see it.
The axle ratio should be of such to put the motor in or just above the sweet spot RPM when the trany is in high gear but it over drive or not.
Maybe a little below if over drive trany is used.


The trany is used to keep the motor in the sweet spot from take off from a stop to high way speed.
More trany gear ratios the better to keep the motor in the sweet spot at a given speed.
More ratios are also needed if the motor has a small sweet spot window.


Also the more load you have to carry or pull the lower the axle ratio needed.
That is why big rigs have any where from 9sp, 10sp, 13sp and 18sp tranys, a sweet spot window of about 500 RPM and most max out at 2000 RPM so they need all the trany gears to stay in the sweet spot.


That's my story and I am sticking to it.
Dave ----