What does rear ratio in a semi means?

Discussion in 'Ask An Owner Operator' started by TRUCKER101ROOKIE, Jan 25, 2023.


    TRUCKER101ROOKIE Light Load Member

    Jan 25, 2023
    detroit, MI
    what does the rear ratio mean?
    is it better to have a high ratio or a low ratio?
    I am looking to buy a semi-truck to haul with a flatbed, what is the best ratio for that?
    does ratio really have a big part to play?
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  3. gokiddogo

    gokiddogo Road Train Member

    Mar 5, 2012
    Ontario Canada
    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
    Last edited: Jan 26, 2023
    Reason for edit: Lower=faster higher=slower
    kenn2632, GYPSY65, AModelCat and 12 others Thank this.
  4. Siinman

    Siinman Road Train Member

    Mar 5, 2017
    Kansas City, MO.
    Yes it is a huge issue and needs to be spec'd for the engine and trans you are running for your duty cycle.
  5. 201

    201 Road Train Member

    Apr 16, 2014
    high plains colorado
    4:11s= company truck,,,2:88s,= O/O left lane truck.
  6. God prefers Diesels

    God prefers Diesels Road Train Member

    Jun 26, 2020
    South Texas
    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.
  7. skallagrime

    skallagrime Road Train Member

    Apr 10, 2012

    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
  8. Tigerfishinc

    Tigerfishinc Light Load Member

    May 28, 2021
    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
  9. abyliks

    abyliks Road Train Member

    May 2, 2010
    ludlow MA
    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
  10. Ridgeline

    Ridgeline Road Train Member

    Dec 18, 2011
    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?

    TRUCKER101ROOKIE Light Load Member

    Jan 25, 2023
    detroit, MI
    no. looking to be one
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