how to fix it????????????????? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bestchevyfan · 02-20-2003, 07:17 PM

my speedometer is about 20 miles to fast I HAVE A 350\350 WITH A 308 REAR. WHAT HAVE I GOT TO DO TO FIX IT ? THANKS, BUDDY

adamls · 02-20-2003, 07:23 PM

You can change the output gear in the transmission to correspond with the gearing. Someone on here should know where to get the parts or a local transmission shop should be able to fix it.

*Piston* · 02-20-2003, 08:30 PM

You'll need a stop watch, 2 miles of highway with mile markers first off. Time exactly how long it takes you to drive 1 mile while YOUR speedometer is reading 60mph. You'll probably be in the 75.5sec time (guessing) which would give you a difference of 15mph that you have to compensate for.

A general rule of thumb:
Drive gear = 10mph changes per tooth
Driven gear = 2.5mph changes per tooth.

If your 20 fast, I doubt you'll be able to cover that by only changing the driven gear. It will require you to remove the drive gear (remove drive shaft, then tail shaft housing), count the number of teeth then go to a local transmission shop (or dealer) and buy a driven gear with 2 less teeth than your current one.

From there you can "dial in" your exact MPH by changing the driven gear. Most of the time it wont be exact so error on the side of +. meaning, make your speedo read about 1-2mph higher than it actually is. I used a few websites that have you use tire rpm and a few other equations which didnt work for me, so I talked to a transmission shop we use and this is the method he prefers.

02-20-2003, 07:17 PM	#1
bestchevyfan Registered User Join Date: Jun 2002 Location: new market, alabama Posts: 1,330	how to fix it????????????????? my speedometer is about 20 miles to fast I HAVE A 350\350 WITH A 308 REAR. WHAT HAVE I GOT TO DO TO FIX IT ? THANKS, BUDDY

02-20-2003, 07:23 PM	#2
adamls Gotta Startem Early Join Date: Oct 2001 Location: Amarillo, TX, USA Posts: 364	You can change the output gear in the transmission to correspond with the gearing. Someone on here should know where to get the parts or a local transmission shop should be able to fix it. __________________ 67 SWB In Progress, one piece at a time, maybe 3 years I will be finished

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02-20-2003, 08:30 PM	#3
Piston Account Suspended Join Date: Oct 2002 Location: Olathe, KS Posts: 2,300	You'll need a stop watch, 2 miles of highway with mile markers first off. Time exactly how long it takes you to drive 1 mile while YOUR speedometer is reading 60mph. You'll probably be in the 75.5sec time (guessing) which would give you a difference of 15mph that you have to compensate for. A general rule of thumb: Drive gear = 10mph changes per tooth Driven gear = 2.5mph changes per tooth. If your 20 fast, I doubt you'll be able to cover that by only changing the driven gear. It will require you to remove the drive gear (remove drive shaft, then tail shaft housing), count the number of teeth then go to a local transmission shop (or dealer) and buy a driven gear with 2 less teeth than your current one. From there you can "dial in" your exact MPH by changing the driven gear. Most of the time it wont be exact so error on the side of +. meaning, make your speedo read about 1-2mph higher than it actually is. I used a few websites that have you use tire rpm and a few other equations which didnt work for me, so I talked to a transmission shop we use and this is the method he prefers.



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