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 Post subject: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 10:58 am 
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Joined: Tue Apr 18, 2006 10:36 pm
Posts: 173
Location: spring creek nv
what the difference is in rpm and response between the ring /pinon gears of 3.73 and 3.90 at 70mph with a 700r4 and 25 inch tires? Which do you think would be best to use?
this is for a driver[non race]350sbc with a small cam/headers/intake/650cfm carb.used for car shows and cruises but like to have a little more balls for burnouts without missing with engine.
friends car just trying to find rpm for him and i don't know myself how to figure it..help


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 Post subject: Re: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 11:08 am 
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Joined: Wed Apr 19, 2006 4:11 pm
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Location: Shepherdsville, KY
There would be a 4.5 % change. If cruise rpm at 70 is 2,800 now it would go to 2,925. I don't think it would make enough difference to make it worth the change.

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 Post subject: Re: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 11:19 am 
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Joined: Tue Apr 18, 2006 10:36 pm
Posts: 173
Location: spring creek nv
need to reword this.
what would rpm be with gears of 3.73 or 3.90 at 70mph with a 700r4 and 25 inch tires? car now has 2;3? gears and just idles going up highway and no balls in town so wanting to go lower to help town performance still have descent cruising rpm on highway
this is for a driver[non race]350sbc with a small cam/headers/intake/650cfm carb.used for car shows and cruises but like to have a little more balls for burnouts without messing with engine.
friends car just trying to find rpm for him and i don't know myself how to figure it..helphotrodharley

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Joined: Wed Apr 19, 2006 3:36 am
Location: spring creek nv
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 Post subject: Re: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 12:22 pm 
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Location: Shepherdsville, KY
I think the 700r4 has .7 overdirve. If that's right with a 2.38 rear it would be turning about 1700 at 70 mph with a 25" tire. That would go to about 2,650 with a 3.73 and 2,775 with a 3.90.

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Chuck Woloch

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 Post subject: Re: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 1:34 pm 
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Joined: Tue Apr 18, 2006 10:36 pm
Posts: 173
Location: spring creek nv
gearhead1011 wrote:
I think the 700r4 has .7 overdirve. If that's right with a 2.38 rear it would be turning about 1700 at 70 mph with a 25" tire. That would go to about 2,650 with a 3.73 and 2,775 with a 3.90.



chuck;;
thanks// that's what we needed to know.
don
badtoyzz racing


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 Post subject: Re: rpm to gear ratio's
 Post Posted: Tue Jan 05, 2010 2:17 pm 
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I didn't think of it until after I posted but those numbers are assuming that you are still locking up the torque convertor. If not the rpm will be a little higher depending on the convertor.

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