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05-14-2007, 08:54 PM
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#1 |
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Registered User
Join Date: Feb 2007
Location: Austin, TX
Posts: 3,930
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Crazy Speedometer
I just bought a 1977 GMC Sierra Grande. Pics to come as soon as I get the truck all cleaned up.
I drove it home yesterday and when I got out on the highway the speedometer was reading 95 and was pegged at 100 when I was passing people. I was following my dad the highest he said he went was 75. I looked at the option sticker in the glove box and for the transmission, the code is M40 which translates to a turbo 400, but looking under the truck it's obvious that it's a turbo 350. I'm led to believe that this transmission isn't original, which could mean that the speedometer gears are wrong. What do you guys think the issue is ? Is my speedometer just screwed ? Are the speedometer gears in the transmission way off ? |
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05-14-2007, 09:09 PM
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#2 |
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Dark Heart Motorsports
Join Date: Apr 2007
Location: 29404 DKFX
Posts: 683
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Re: Crazy Speedometer
tranny speedo gear...
__________________
How Deep Can YOU Bleed..? 1985 Chevy Blazer K5- 6.2 N/A, 400 turbo trans CUCV, 12v converted... a new chapter 2005 GMC Sierra Crew Cab 6.0 powwwaa!!! custom jumpseat install w/ captain seats, Hypertech tuner, MSD Blaster Coils, CB and twin antenna's, all electric interior, touch screen in dash. custom center console, with jump seat replacing original console. 1500 HD! |
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05-16-2007, 01:58 AM
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#3 |
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Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Re: Crazy Speedometer
A truly crazy speedometer will read anywhere between 62 and 102 when you're cruising on cruise control at ~75mph. I yours is CONSISTENTLY wrong, your problem is probably gears. Go to www.novaresource.org/speedo.htm and do the math. You'll need your rear tire diameter and your axle ratio.
Once you find out what speedo gear ratio you need, use the chart to find what gear set you need. If you're lucky, you might be able to use one of your current gears. They're cheap at the Chevy place. http://tciauto.com/Products/TechInfo/speedo_gears.asp has a tire diameter calculator that you plug your tire size into. Last edited by jimfulco; 05-16-2007 at 02:00 AM. |
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05-16-2007, 02:27 AM
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#4 |
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Senior Member
Join Date: Sep 2004
Location: CANADA
Posts: 1,681
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Re: Crazy Speedometer
mine bounced from 100 km/h to 140 km/h on the highway and stayed at the 140 until i stopped.The speedometer head was buggered up.replaced the speedo and it worked good
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05-16-2007, 03:39 AM
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#5 |
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Chevys Kick A$$
Join Date: Jan 2005
Location: Zootown, MT
Posts: 12,699
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Re: Crazy Speedometer
I second the transmission speedo gear.
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*HOODS is what I answer too* -'79, '77, '88 Pickup Sold, '85 Camaro, '83 T/A, '81,'83,'90 K5 Blazer All Sold  -'79 3/4 ton "Big Yellow Bananna" Lifted 4spd. 39.5 TSL Swampers, The money Pit   -'86 K5 Blazer Silverado 6.2 4" lift 35"s -'95 Ext. Cab Shorty 4" Tuff Country rolling 35" M/T's -'83 Monte Carlo T-Tops. 126,500 Original Miles -LATER  I would rather push a Chevy then drive a ford!!
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05-16-2007, 09:17 AM
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#6 |
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Robert Olson Transport
 
Join Date: Oct 2002
Location: recent transplant to NC USA
Posts: 20,299
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Re: Crazy Speedometer
im sure its the trans speedo gear i had the same kind of thing happen to me on the 86 when i swapped trannys
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Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!! |
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05-17-2007, 09:43 PM
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#7 |
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Registered User
Join Date: Sep 2006
Location: Bow Island, AB, Canada
Posts: 166
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Re: Crazy Speedometer
i got the same problem i just took a buddies GPS figured out where all common speeds are on the guage so i can generally guess my speed
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05-18-2007, 01:27 AM
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#8 |
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Senior Member
Join Date: Dec 1999
Location: Knightstown, in, usa
Posts: 2,525
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Re: Crazy Speedometer
I've replaced speedo cables, and fixed erratic speedo problems. Maybe I just got lucky?
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1970 long fleet, 355 4 speed, primer, walnut goodies |
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