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06-21-2013, 02:47 PM
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#1 |
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Registered User
Join Date: Jul 2010
Location: So Cal
Posts: 421
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Highway RPMs
Hey guys, will there be a huge difference if i change rear end from 3.73 to say 3.08 ?
I am currently at about 32-3300 RPM going 60MPH. How much of a change will it be if i change the rear end to a 3.08 and what would the trade off be ? |
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06-21-2013, 02:56 PM
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#2 |
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Cluster King
 
Join Date: Nov 2012
Location: Junction City, OR
Posts: 5,263
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Re: Highway RPMs
http://www.coastdriveline.com/htmlfo...lculator2.html
Use the link and plug in the numbers. Looks like just over 400 RPMs at the same speed. So your current 3200 would drop to 2800 It will be a little more sluggish from a light with a 308 vs the 373 but your RPM drop at 60 will save you a little fuel and wear on the drive train. |
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06-21-2013, 02:58 PM
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#3 | |
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Registered User
Join Date: Jul 2010
Location: So Cal
Posts: 421
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Re: Highway RPMs
Quote:
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06-21-2013, 02:59 PM
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#4 |
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Registered User
Join Date: Jun 2010
Location: Upland Ca
Posts: 4,147
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Re: Highway RPMs
Yes, that is a huge difference. With my 3.08s I'm at about 2,400 at 70. Tire size will change it a little also.
Posted via Mobile Device
__________________
1972 C10 SWB, Air, PS, PB, 350/350THM. Second owner. 1965 Corvette roadster, 44K miles, 327/365 SHP, 4 speed, side exhaust, knockoffs, teak, second owner (bought in 1970), Have ALL numbers matching components. My frame off restoration thread: http://67-72chevytrucks.com/vboard/s...d.php?t=556703 |
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06-21-2013, 03:14 PM
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#5 |
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Senior Member
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,593
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Re: Highway RPMs
17.5% difference in the ratio, tire size is another matter
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06-21-2013, 03:35 PM
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#6 |
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Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: Highway RPMs
__________________
Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban |
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06-21-2013, 05:16 PM
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#7 | |
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Registered User
Join Date: Jul 2010
Location: So Cal
Posts: 421
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Re: Highway RPMs
Quote:
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06-21-2013, 06:25 PM
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#8 |
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Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: Highway RPMs
That would be about a 30" tire and a direct drive trans. The numbers are right in line with what I was getting from my Suburban with L78 tires. Did you try the calculator I posted the link to? No matter what someone else has in the end it will be your numbers that matter. Just vary the numbers for tire sizes and axle ratios to see how different things will be. But an I-6/3OTT will never be as happy with tall gears as a BBC/TH400.
__________________
Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban |
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06-21-2013, 07:50 PM
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#9 |
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Registered User
Join Date: Jun 2010
Location: Upland Ca
Posts: 4,147
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Re: Highway RPMs
SS Tim is right on the money, 30 inch tire, 295-65-15. THM350.
Posted via Mobile Device
__________________
1972 C10 SWB, Air, PS, PB, 350/350THM. Second owner. 1965 Corvette roadster, 44K miles, 327/365 SHP, 4 speed, side exhaust, knockoffs, teak, second owner (bought in 1970), Have ALL numbers matching components. My frame off restoration thread: http://67-72chevytrucks.com/vboard/s...d.php?t=556703 |
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06-21-2013, 09:13 PM
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#10 |
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Registered User
Join Date: Jul 2012
Location: Super South, Texas
Posts: 173
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Re: Highway RPMs
^
I can verify that too. 275/55/18 (29"), TH350, 3.08's. 2500 even @70 |
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