What does rear ratio in a semi means?

The number works like this. Some common ratios are 3.73, 3.55, 3.36, whatever that number is is how many times the driveshaft turns to turn the wheels one revolution. The smaller the number the faster it will go. Higher number, slower. It is a torque multiplication thing. There isn't a one size fits all solution. You also have to take into the transmission ratios and engine output. A quick example is a fleet truck 10 speed direct drive 1:1 from highest gear in transmission paired with a fast rear end 2.64 or something, the end result is the similar to say a 13 or 18 speed with highest gear 0.73:1 (input turns 0.73 times for 1 output revolution *overdrive*) paired with maybe 3.55. A heavy haul truck will typically have slower rears but more pulling power than a typical over the road truck that will never see heavy haul. If you were buying it to pull a 10,000 pound rv you could go with faster to achieve lower engine rpm at higher road speed. It is all a trade off and there isn't one single setup that suits all applications the same. Google "eaton road speed calculator" and play around with different scenarios. Just remember sure you can gear it to go 150 mph but you'd also need enough engine to actually make it happen. On the flip side you don't want something to top out at 55 mph at 1800 rpm.

Edited in bold

 

Yes it is a huge issue and needs to be spec'd for the engine and trans you are running for your duty cycle.

 

4:11s= company truck,,,2:88s,= O/O left lane truck.

 

This is exactly what I have. 2.64 and FRF-15210b. It's fine on the road, but it does 90mph in reverse, and can be quite difficult to get the truck moving on a steep grade in 1st. I'd take lower rears and more gears in the tranny any day of the week.

 

Start with the trans swap (may require some driveshaft modification) 13 os fine for almost everything, its a good happy medium.

If you do it yourself, even currently should be able to get away with a 7500$ swap with no core (i just swapped my 13 sp 169xxxx to a 189xxxx, shop let me use the 16 core for a 2000$ credit bringing me down to 4250$ out the door.


Then look at the rear end swap, theyre a bit more of a pain, but a bit cheaper. Highly recommend staying in the 3 to 3.75 range, dependent on the transmission final, 3.36 is my happy place, i wouldnt like slower than 3.55 and faster than 3.36 is too fast for tight docking when heavy.
I did also swap my front diff and locker out (was a bit sludgy and rusty on the inside) was about 2500 with 1250$ core credit, so both might be about 7500 with no cores.

These both were for good remans and a 1 year warranty from the builder (in house junkyard rebuilder) so far so good.

Trans took a day, diff took 2 days.

Or for 15,000... just buy a decent 60k truck specced how you want and keep yours as a spare

 

Depends where you run - if you’re doing flat I95 - lower gearing like 2.7 will really help you out on economy

If you do a lot of hills - you’ll find yourself shifting gears - but nothing really to worry about - as the truck has enough torque to get you over hills.
If you’re doing very heavy loads - go for the higher ratio

 

If manufactures could save even. 1/10th of a mpg by running a taller rear In direct they would do it, when it comes down to it, any gain is offset by useless rears.

tire size matters as well, 295/75 22.5s on 3.36 are going to do about the same as 3.90s with 11r24.5

 

I would go to a dealer of the truck you are looking at and see what the sales dweeb suggests as a start.

Here is a dumb question, you already a driver?