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RPM's

Discussion in 'The Garage' started by fireplug, Jan 26, 2006.

  1. fireplug

    fireplug 1/2 ton status

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    Hi guys, I'm about to order gears and having been using the calculator on 4lo but I'm putting in too many aproximations and don't want to bugger this up.

    I'm going to be running a new motor soon, probably a 383, a 350, and 37" (not certain the actual diameter) Trxus MT's.

    What RPM's should I expect to be spinning on the highway running 65MPH and expecting a decent balance between power and gas mileage?

    Your help is very appreciated, TIA. Jason.

    If you just want to throw in your own suggestions for gears please go ahead!
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    With a TH350 I'd run 4.10's. Freeway rpm's would be ~2500 (@65 mph)or so which will give you a nice balance of power IMO. Fuel economy is gonna be suckish no matter what...:D

    Rene
     
  3. 74757685k5

    74757685k5 1/2 ton status

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    I had 38.5 boggers w/4.10 400ci/th350 . it was a little under powerd but good for mud. I would go 4.56 if you plan on any bigger tire.
     
  4. fireplug

    fireplug 1/2 ton status

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    Wow! I'm really surprised you guys think 4.10. I was running my '89 TBI burb with a 700r4 and 3.73 and the same tires and thought it was sluggish!

    My '76 Blazer (the truck in question) has a tired Quadrajet350/350 4.10's. I got it with 35's, and when I put on my 37's there was a big decrease in acceleration.

    Hmmm. So maybe the engine is even more tired than I thought. What RPM range is just short of too high would you say?
     
  5. MTBLAZER89

    MTBLAZER89 3/4 ton status GMOTM Winner

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    I run 4.11's with 37" SSR's and a TH350. I spin ~2600 on the hiway at 65. Like Rene said fuel mileage sucks either way. I get the same mileage with the TH350 as I did with the 700. In 4hi it could use more ummph for sure, but it does well in low. I pretty much have to use low to spin the tires unless it's in the mud. It doesn't feel sluggish on the road at all, but I wish I had 4.56 for off-road thats for sure.
     
  6. fireplug

    fireplug 1/2 ton status

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    I just read through a ton of posts. Helped a little I guess.

    By no means am I really worried about mileage. I guess what I really wanted to get across is this. I drive to my wheeling and can expect weekend trips to be up to 4 hours away. I want to be able to cruise at 70 without killing my engine.

    Judging by the number of people that think the 4.10 's are ok I'm thinking maybe the sluggishness is the motor's lack of power. I just feel like I'm killing the tranny getting the truck rolling. Maybe I should get the 383 in first and then worry about the gears. Any idea what rpm drop I can expect if I use a lock up convertor?
     

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