3.73 vs 3.91

Discussion in 'Ask An Owner Operator' started by raj_tiwana, Aug 26, 2019.

  1. raj_tiwana

    raj_tiwana Bobtail Member

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    Aug 26, 2019
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    Hi everyone I'm looking for a new truck and can't decide what rear gear ratio to go with. I'm going to be driving through the Canadian mountains and some flat land as well with super b flat decks loaded 63500kg. I was leaning towards the 3.73 because I want to save some fuel but I'm not completely sure how much of a difference will it be. Also finding a truck with a 3.73 has seemed to be impossible but I have found a few with 3.91. What is the best ratio?
     
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  3. snowwy

    snowwy Road Train Member

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    2.6ish is the new norm these days. With 10sp or automatic.

    The old 13 with 3ish is getting to be less and less these days. 18's are even lesser.

    But then. Daycabs with drop axles are also hard to find.
     
  4. Superhauler

    Superhauler TEACHER OF MEN

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    3:90 would be perfect for what your going to do. Plenty of 13 and 18 spd out there.
     
  5. uncleal13

    uncleal13 Road Train Member

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    Frontline Freightliner in Saskatoon has four Coronados right now with 3.73 46,000 lbs rears.
    I have 3.91 now, had 11R24.5 tires. It ran about 1380 rpm at 100 kph.
    I felt that it wanted to spin a little faster so I switched to 11R22.5, brought it up to about 1460 rpm at 100 kph.
    It seemed to increase the performance/fun factor. But fuel economy went down about 0.2 mpg. Which pulling super b-train can add up over time.
    One of our guys got one of those new Coronados with 3.73 11R24.5, the company new Mack’s are out pulling him, but Detroit’s are kind of sluggish when new until after 80,000 kms.
    If you are on the highway all the time I’d go with 3.73, if you didn’t like it you could go down to 22.5s and still have the same rpm as 3.91
     
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  6. Snow Monster

    Snow Monster Medium Load Member

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    Consider this.

    Since you will be pulling loads over 90,000 lbs the rules are different than pulling less than that weight.
    The engine manufacturers will specify a higher cruising rpm, roughly 1500 rpm rather than 1200 - 1300 rpm.

    Knowing this, you should try to gear your truck so it runs 65 mph at around 1500 rpm or even a little higher.
    What I see most common for super B's in the west is engines 500 hp and up with 3:91, 4:10 and even 4:30 ratios with an 18 spd on 24.5 rubber.

    A 3:73 on 22.5 is nearly the same as 3:91 on 24.5.
    If you're going to be in the mountains most of the time or on soft ground a lot, a 4:10 with 24.5 rubber or a 3:91 with 22.5's might work better, more steady pulling and braking power, less shifting.

    If you can run steady in the engines power band between 55 and 65 mph all day you can make a lot of miles and get good economy.
    Having said that, I know a dude who pulls a set of B's with 3:36 rears on 22.5 low pros and an 18 spd, but the truck has a 700 hp Cat and runs strictly on the flat part of the prairies.

    Edit:

    An afterthought.
    If you want the best of both worlds and more flexibility in your gear selection consider a 2 spd differential.
    I know Eaton's fastest 46k diff is 3:70 on the high side and around 4:88 on the low side, can't remember.
    The same or faster ratios can be had with a Super 40 diff.
     
    Last edited: Aug 26, 2019
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  7. Minion120

    Minion120 Bobtail Member

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    Would it help to split down on the top notch? Ive run 3.90 with a regular 18 speed, (.73 over on the high side)and 22.5s get me about 1450 at 62mph, I've wondered if the .87 overdrive is hurting economy. With a N14 460 celect plus I've been getting 4.2 to 4.5 per imperial gallon pulling max super b weights. Poor economy in my opinion. Ive had it bumped to 525/1850 since the last set of b's I pulled about 3 months ago, thinking of going back to them.
     
  8. AModelCat

    AModelCat Road Train Member

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    I'd rather the lower ratio myself. More bottom end grunt. Getting 63,500 kgs moving is hard on parts. With that kind of weight, fuel economy isn't gonna be good anyways.
     
  9. FoolsErrand

    FoolsErrand Road Train Member

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    The deeper ratio axle gears will take load off the clutch, trans and driveshafts by the percentage deeper compared to the other gear. If you go 5% deeper in the diff, thats 5% load reduction on all components ahead of the ring and pinion. Deep gears are easier on everything but fuel. However profit isnt just this weeks fuel, its also repairs and replacement parts long term. Deep gears were meant for big loads on big grades. Feed it whatever fuel it needs.
     
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