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Author Topic: Just had 700R4 installed  (Read 5456 times)

Offline 86swb

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Just had 700R4 installed
« on: March 20, 2008, 01:13:51 pm »
I just had a rebuilt 700R4 installed in my 1986 Chevy Short Wheel Base. The truck has a 350 engine, 3:08 posie rear, 235/75R15 tires and my MPG is now worse than with the 350 trans. I am thinking about going back to the 350. I really thought that 4th gear would help and it did not and is even worse. Has anyone else have this problem with the 700R4?
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'86 swb Siverado
'84 lwb Silverado (low miles)
1931 Model A 2-Door Sedan
  (in family since new)
I just gave the '31 to my Grandson

Offline Captkaos

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Re: Just had 700R4 installed
« Reply #1 on: March 20, 2008, 01:47:23 pm »
It would help if you moved your gear ratio up to like 3.42's.  Your effective final drive with OD is now 2.21:1 and it is lugging it down.  What size tires are you running on it?  My 87 has OD with 2.76 gears and OD and it gets 22mpg at 65mph on the highway.  About 15 in the city....  It is completely stock.
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Offline 86swb

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Re: Just had 700R4 installed
« Reply #2 on: March 20, 2008, 02:44:26 pm »
Quote from: Captkaos on March 20, 2008, 01:47:23 pm
It would help if you moved your gear ratio up to like 3.42's.  Your effective final drive with OD is now 2.21:1 and it is lugging it down.  What size tires are you running on it?  My 87 has OD with 2.76 gears and OD and it gets 22mpg at 65mph on the highway.  About 15 in the city....  It is completely stock.

So you have 2:76 and get 22mpg and I have 3:08 and NOT. Why are you not lugging with the 2:76 which is higher than what I have. My tires is 235/75R15. Thanks for reply
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'86 swb Siverado
'84 lwb Silverado (low miles)
1931 Model A 2-Door Sedan
  (in family since new)
I just gave the '31 to my Grandson

Offline eventhorizon66

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Re: Just had 700R4 installed
« Reply #3 on: March 20, 2008, 04:40:09 pm »
It's the 3.08s causing that poor fuel economy.  The motor is bogged down at cruise, so to maintain speed against wind drag (especially with a head wind) or up inclines, you have to apply more throttle than you would with lower gears like 3.42s or 3.73s.  For a stock 350 in a truck with OD, I've often heard to shoot for about 1900-2100 rpm at 65 mph.  Your current gearing has you around 1650 rpm, which is perfect for a Vette but not for something with the aerodynamics of a brick wall.
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'85 C10 SWB 350 700R4 TKO600

Offline VileZambonie

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Re: Just had 700R4 installed
« Reply #4 on: March 20, 2008, 05:03:32 pm »
I would swap in a 3.73:1 and you'll love it. Does it upshift too early?
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Offline Captkaos

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Re: Just had 700R4 installed
« Reply #5 on: March 20, 2008, 05:34:24 pm »
I was running 65-70mpg to get that mileage...
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Offline VileZambonie

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Re: Just had 700R4 installed
« Reply #6 on: March 20, 2008, 06:03:05 pm »
Quote from: Captkaos on March 20, 2008, 05:34:24 pm
I was running 65-70mpg to get that mileage...

I think you mean mph? lol
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline 86swb

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Re: Just had 700R4 installed
« Reply #7 on: March 20, 2008, 06:07:27 pm »
Quote from: VileZambonie on March 20, 2008, 05:03:32 pm
I would swap in a 3.73:1 and you'll love it. Does it upshift too early?

It goes into 2nd right off the bat. Then it goes into 3rd around 35 and 4th around 55mph. So it sounds like I need to swap out my 3:08 for the 3:73. I just so happen to have the 3:73 gear and ringpin. I hope this will help. Appreciate all the advice. Thanks guys.
Anymore suggestions welcomed but for now I will most likely put in the 3:73 gears.
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'86 swb Siverado
'84 lwb Silverado (low miles)
1931 Model A 2-Door Sedan
  (in family since new)
I just gave the '31 to my Grandson

Offline VileZambonie

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Re: Just had 700R4 installed
« Reply #8 on: March 20, 2008, 06:12:48 pm »
in the mean time tighten up your tv cable one click at a time until it shifts just where you want it.
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                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline 86swb

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Re: Just had 700R4 installed
« Reply #9 on: March 21, 2008, 12:10:39 pm »
OK, thanks....
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'86 swb Siverado
'84 lwb Silverado (low miles)
1931 Model A 2-Door Sedan
  (in family since new)
I just gave the '31 to my Grandson

Offline TexasRed

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Re: Just had 700R4 installed
« Reply #10 on: March 21, 2008, 09:00:39 pm »
same thing happened to me and my 305 when I went from a th350c to a 700r4. I have a slight cam in my 305, and I have a 3.42 rear end but I shouldn't lose that much vacuum that the rods pop up!
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