Need some advice...replace tires or regear?

Discussion in 'Trucks [ Eighteen Wheelers ]' started by DocHoof, Dec 14, 2014.

  1. DocHoof

    DocHoof Light Load Member

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    Just bought an 00 Pete 379 EXHD. 3.7 rear Spicer rears, Eaton 8 sp. The truck originally came with 11r 24.5, but some genius opted to put Low Pro 22.5 on it somewhere along the way. My problem is that the speedo is 6 MPH off and with the smaller tires, fuel economy is kicking my wallets ##### over 68 mph. I've been looking around the Internet at replacing the tires and rims to go back to original specs. The prices I'm finding are in the $750+ per wheel range for a tire and rim. 750x10=$7,500. Motherless goat!!!
    So, as I drove home last night, I asked myself, "Would it be cheaper to just regear the rears to drop the rpm range about 400 rpm?" And came up with another question, "If so, what gear ratio would I need to put in to match the trans and tires?"
    Anyone have a suggestion besides the inevitable "drive slower"? 6 mpg at 65 mph ain't so bad, but 4.5 mpg @ 75 mph and 3.5 mpg @ 80 mph stinks.
     
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  3. Oscar the KW

    Oscar the KW Going Tarpless

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    Put 11R 22.5's on it. Eaton 8spd, that was a typo right?
     
  4. Blind Driver

    Blind Driver Road Train Member

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    Stop driving so fast :biggrin_25526:

    I drove a 379 (C15, 9 sp, 3.70 gears) that would get 6.2 at 65 and 5.0 at 75. I got it up to 7.0 by slowing to 62 and getting a moderate run on the hills.
     
    LGarrison Thanks this.
  5. DocHoof

    DocHoof Light Load Member

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    Nope, not a typo. 8 speed is correct. My understanding is, if you take an 18 speed and remove the splitter (low range high and high range low), you have an 8 speed. Not a big fan of it.
     
  6. DocHoof

    DocHoof Light Load Member

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    I downloaded the Peterbilt speedo dipswitch setting manual. The difference in diameter of the low pro 22.5 and 11r24.5 is 4.5". Further research and the difference in the 22.5 low pro and 11r22.5 is about 2.0". That's about halfway there. Still gonna be about 250 rpm above optimal at 75 mph. For the cost, of (x10)11r 22.5 and the minimal gains, it would be worth spending the extra for the 11r 24.5 tires and wheels. Still over the $4,000 price mark to gain a meager 2 mph at optimal engine rpm.
     
  7. GrapeApe

    GrapeApe Road Train Member

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    What model # trans do you have? Get the actual # off the trans, you may have a direct final drive trans. If that's the case, an 13 or 18 overdrive trans may be a better option overall for you.

    If you have an 18 speed, it's probably just a matter of putting the right shift knob on it and running a couple of air lines.
     
  8. Oscar the KW

    Oscar the KW Going Tarpless

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    If your speedometer is off, how are you checking your mileage and getting any kind of number that is believable?
     
  9. DocHoof

    DocHoof Light Load Member

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    GPS reading. Speedo was 6 MPH off. I've adjusted by the manual and now have it 1 mph off. The odometer gains about 4 miles from GPS reading over 600 miles.
     
  10. Oscar the KW

    Oscar the KW Going Tarpless

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    Ok, so you're in the ballpark then.
     
  11. Pedigreed Bulldog

    Pedigreed Bulldog Road Train Member

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    Run those tires until you need to replace the tires anyway. Then, buy the 11R24.5 tires & rims. Your only expense above & beyond that which you'd already otherwise be spending is the price difference between the lo-pro 22.5's and the 11R24.5's...and the rims. In other words, if you'd spend $300/tire for lo-pro 22.5 tires, you'll spend $3000 putting tires on it anyway whether you do the swap or not. If the 11R24's are $400/tire...and if you can pick up some used/refurbished steel rims for $50/ea, then you'll be paying $4500. However, when you factor in the $3000 you would've spent anyway, it'll only cost an extra $1500 to do the swap. If you're wanting brand new aluminum rims, it'll be more expensive. If you want Michelins, it'll double your tire cost...but the cost for the lo-pros would also be higher if that is the case.

    This is an expensive game we're playing. No matter which way you look at it, though, the gear swap is going to be more than a tire/rim swap.
     
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