Easy question, Help! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

nejeff · 07-10-2003, 11:00 AM

I'm no surgeon for brains so I need a little help---Are the speedo gears in the tranny the same in a manual and auto tranny?

Took rear end from a 3-speed truck and put it in my automatic truck and speedometer is way off. Thought if I took the speedo gears from the manual tranny and put them in the auto tranny I could be back on track. Does this make sense? Am I an idiot or are will this work?

Thanks for the help!

bigvinnie · 07-10-2003, 11:27 AM

the gear ratio (ring and pinion) on your new rear end must be different than the original, which would produce the problem you're seeing...

wnc mountain · 07-10-2003, 01:31 PM

I think he is asking is the tail shaft on an auto the same as on the manual trannies. I don't know but I'm sure someone will inlighten us.

nejeff · 07-10-2003, 02:31 PM

Someone had said that there is little plastic gears in where the speedometer cable goes into transmission. I was wondering if it were possible to switch those gears into the auto tranny from a manual tranny. I might not have been clear before. thanks

beautimus · 07-10-2003, 02:38 PM

no they aren't the same. in an ideal world, the idea would be to know what gears are in your rear axle, and proceed to the nearest parts house and ask for a speedo gear for transmission and the axle ratio that you have. you may have to make a guess, as to your axle ratio, and get a gear for that ratio. oh by the way those gears are pretty cheap, just plastic. they are simple to change, even if youve never done it. just unbolt the drive shaft. (chock the wheels first!!!) unbolt the tail housing, (5 or 6 bolts) and remove the gear, no sweat.
i had to buy a couple of different ones and through trial and error i got it right. good luck with yours!!!!!!
Tony

FrankHoy · 07-10-2003, 02:43 PM

You don't necessarily need to remove the tail shaft. The gears are fairly cheap but I would attempt to change just the one on the speedo cable end, before going into the trans and changing the companion gear. They do make several gears with differnet # of teeth.

71GMC_3/4T · 07-10-2003, 05:29 PM

Drive it at what you think is 60, going by what the speedo says. Measure how many seconds it takes to go a mile (section lines or mile markers). Do it a few times and throw out the bad numbers and average the ones that are close to the same. Divide 3600 by the number of seconds. That will get you your actual MPH. Then divide actual by indicated to see what ratio of difference. You will need to decide if you need to go up or down in teeth. Since you are dealing with the driven gear, if you want the speedo to read faster, use less teeth. Pull the old gear, count the teeth, multiply or divide by your ratio you calculated and go buy a gear with close to that number of teeth. If you're lucky, you won't have to change the drive gear (tailshaft), only the driven gear. If it jumps around or doesn't move, you have the wrong gear match and will have to change the tailshaft or buy a gear box from your local speedometer repair shop.

Either that, or go to Canada and check to see if your truck is calibrated in KM/HR now!

nejeff · 07-10-2003, 06:20 PM

Thanks alot guys for all the help!

Jeff

07-10-2003, 11:00 AM	#1
nejeff Registered User Join Date: Jun 2003 Location: Nebraska Posts: 63	Easy question, Help! I'm no surgeon for brains so I need a little help---Are the speedo gears in the tranny the same in a manual and auto tranny? Took rear end from a 3-speed truck and put it in my automatic truck and speedometer is way off. Thought if I took the speedo gears from the manual tranny and put them in the auto tranny I could be back on track. Does this make sense? Am I an idiot or are will this work? Thanks for the help! __________________ 72' Chevy Super Cheyenne LWB 72' Chevy C-10 (307 Parts Truck)

07-10-2003, 11:27 AM	#2
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	the gear ratio (ring and pinion) on your new rear end must be different than the original, which would produce the problem you're seeing... __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project

07-10-2003, 01:31 PM	#3
wnc mountain More hobbies than money! Join Date: Jan 2002 Location: Raleigh, nc, usa Posts: 655	I think he is asking is the tail shaft on an auto the same as on the manual trannies. I don't know but I'm sure someone will inlighten us. __________________ '70 cst10/swb 350, 3 on the tree, factory buckets. '70 GMC lwb 350/350 ps,pb '72 c-10 lwb 350/350 a/c, ps, pb '72 Blazer project

07-10-2003, 02:31 PM	#4
nejeff Registered User Join Date: Jun 2003 Location: Nebraska Posts: 63	Someone had said that there is little plastic gears in where the speedometer cable goes into transmission. I was wondering if it were possible to switch those gears into the auto tranny from a manual tranny. I might not have been clear before. thanks __________________ 72' Chevy Super Cheyenne LWB 72' Chevy C-10 (307 Parts Truck)

07-10-2003, 02:38 PM	#5
beautimus Registered User Join Date: Mar 2002 Location: woodstock, alabama, usa Posts: 938	no they aren't the same. in an ideal world, the idea would be to know what gears are in your rear axle, and proceed to the nearest parts house and ask for a speedo gear for transmission and the axle ratio that you have. you may have to make a guess, as to your axle ratio, and get a gear for that ratio. oh by the way those gears are pretty cheap, just plastic. they are simple to change, even if youve never done it. just unbolt the drive shaft. (chock the wheels first!!!) unbolt the tail housing, (5 or 6 bolts) and remove the gear, no sweat. i had to buy a couple of different ones and through trial and error i got it right. good luck with yours!!!!!! Tony __________________ i survived cancer and kidney stones, but my kids are killing me.

07-10-2003, 02:43 PM	#6
FrankHoy Registered User Join Date: Aug 2002 Location: Des Moines, IA Posts: 154	You don't necessarily need to remove the tail shaft. The gears are fairly cheap but I would attempt to change just the one on the speedo cable end, before going into the trans and changing the companion gear. They do make several gears with differnet # of teeth. __________________ 72 Cheyenne 2wd LWB 402/350 CC/PS/PB/AC Tilt/Tach (upgrading to 4X4 402/700R4 CC/PS/PB/AC Tilt/Tach PW/PL) 71 Custom Deluxe 350/350 Tilt/PS/PB 00 Blazer

07-10-2003, 05:29 PM	#7
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	Drive it at what you think is 60, going by what the speedo says. Measure how many seconds it takes to go a mile (section lines or mile markers). Do it a few times and throw out the bad numbers and average the ones that are close to the same. Divide 3600 by the number of seconds. That will get you your actual MPH. Then divide actual by indicated to see what ratio of difference. You will need to decide if you need to go up or down in teeth. Since you are dealing with the driven gear, if you want the speedo to read faster, use less teeth. Pull the old gear, count the teeth, multiply or divide by your ratio you calculated and go buy a gear with close to that number of teeth. If you're lucky, you won't have to change the drive gear (tailshaft), only the driven gear. If it jumps around or doesn't move, you have the wrong gear match and will have to change the tailshaft or buy a gear box from your local speedometer repair shop. Either that, or go to Canada and check to see if your truck is calibrated in KM/HR now! __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

07-10-2003, 06:20 PM	#8
nejeff Registered User Join Date: Jun 2003 Location: Nebraska Posts: 63	Thanks alot guys for all the help! Jeff __________________ 72' Chevy Super Cheyenne LWB 72' Chevy C-10 (307 Parts Truck)



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