Gear ratio / RPMs at speed

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2shelbys

2shelbys

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I have a question for anyone that has a big block (especially 383) 4-speed car with 4.10s.

I am thinking of pulling my 3.54's and going to 4.10's since I really do not do much highway driving. What RPM do you turn at 60-65 MPH and how tall are your tires?
 
60 MPH with a 3.91 to 1 with 27.1" tire equals 3000 rpm with a 727. 1 to 1 in third gear in auto would be the same as 1 to 1 in 4th gear manual with no overdrive.
 
The guide is good but there is slippling in a auto trans-even the tightest convertors have some slippage where you will have a true lock up with a four speed.I played around with this on my cars and depending on the cruise rpm,stall speed of the convertor ect. it can be a couple hundred rpm difference-not real significant but every hundred counts.
 
lead69 said:
The guide is good but there is slippling in a auto trans-even the tightest convertors have some slippage where you will have a true lock up with a four speed.I played around with this on my cars and depending on the cruise rpm,stall speed of the convertor ect. it can be a couple hundred rpm difference-not real significant but every hundred counts.
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Yes, but I have a 4-speed. No slop in my drivetrain.
 
I know-just some thinking out loud for those who might have automatics-I like to bang gears myself!.
 
An amazing but true story... Back when the Lenco trans first hit the track a lot of guys were switching over to it. Ronnie Sox (Mr. 4-speed) did and it actually slowed him down. He ran better times with his 4-speed.
 
Excellent link! I have 3.23s and 25.5" tyres, the calculator =70mph@3,000rpm my indicated speed is nearer77-78mph, is this due to convertor slippage?
 
TimDart,

I don't think your problem is due to converter slippage. Slippage would cause your RPM to be higher for the indicated MPH, your is the other way around.

You probably don't have the right speedometer drive pinion gear for your tire/rear gear combination.

:burnout:
 
2shelbys, I have problems trying to find it as well sometimes so I know how you feel.

TimDart, I dont know what the problem is with your car so I cant help you there. But that wedsite is only calculating a relative RPM at which your car would run, it can be off as lead69 said.
 
Tim, I double checked my actual speed with a cheap GPS after figureing out what speedometer gear I needed with my combo. Speedometer is bang on.
 
demon seed said:
Tim, I double checked my actual speed with a cheap GPS after figureing out what speedometer gear I needed with my combo. Speedometer is bang on.
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This is the best way I've found to verify if a speedometer is reading right also. I don't own a GPS but a buddy of mine does and he always lets me borrow it to check my speedo's when I need to.
 
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