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Old 02-20-2011, 06:19 PM
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th350 gear ratio and engine RPM questions

My pickup has a th350, 3.73 rear end and 27.6" tall tires. Torque converter is a 2000 stall.
I have done all the math, and if anyone would like to double check me, please feel free.
I SHOULD be at 4k RPM at 88 mph, 3k RPM at 66 mph, 2k RPM at 44 mph. (while in 3rd gear which is 1:1 for the th350)
What I am actually getting is 4k RPM at around 50-55 mph. Which is waay off what the math is telling me. So something is not working correctly and I'm not sure what.
I checked to make sure that I am actually shifting into 3rd when I think I am. Disconnected the kickdown, just in case. I shifted manually through the gears to ensure I was in third. All good there, shifts fine and at the appropriate RPM
Is my converter malfunctioning? I know there will be some slippage, but this seems pretty extreme.

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Old 02-20-2011, 06:37 PM
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Actually everything sounds right. At 6000 rpm you should be at 110 miles an hour. Have you verified your speedo is correct? That is the first thing you should do.
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Old 02-20-2011, 06:43 PM
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I got 5k at 110 mph. Hmm, it seems very high to me
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Old 02-20-2011, 07:08 PM
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thats right , I was wrong at 6 grand.5 grand for 110 mph.
Still, you need to verify your speedo.
with a 3.73 the motor is gonna rev higher at cruise speed.Once you calibrate the speedo,then it is what it is.
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Old 02-20-2011, 08:06 PM
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You might be right on with the speed. To be perfectly honest I was guesstimating my speed. The speedo is a dakota digital that is all wired up and ready to go, however I have not yet calibrated it. It just seems way off. However it may be my internal sensor! I will update tomorrow after I calibrate the speedo with a GPS.
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Old 02-24-2011, 07:27 AM
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To check and calibrate your speedometer
http://www.bushnell.com/products/oth...ed-gun/101901/

To check and calibrate your tachometer
http://www.princessauto.com/workshop...ser-tachometer

5000 rpm at 110MPH is just about right for 3.73's with a 27.6" tire.

MPH x 336 / tire dia x gear ratio= rpm

For stock low stall converter with foot to the floor substitute 355 for 336.

For a high stall converter foot to the floor use 360 for 336 in the formula.

Last edited by F-BIRD'88; 02-24-2011 at 07:33 AM.
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