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12-11-2004, 03:04 AM
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#1 |
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Registered User
Join Date: Jul 2000
Location: Mo
Posts: 963
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speedometer question
I have a 83 C20 originally a 4 spd that I put on a 70's frame with 79 203 transfercase and 3:42-1 axle ratios that all came from the same suburban. My question is the suburban had a 85 mph speedo and my 83 has a 85 mph speedo but the speed shown is 10 mph slower than what i am actually running. Is there a difference between these speedos that is causing the problem or what. I asked the guy I got the burb from and he said the transfer and axles were original to the burb. And I am running the same engine tranny combo that the burb had too 350/350/203. He also said that to his kinowledge the speedo was right in the burb. He bought it new and is a GM mechanic so he performed any maintenance that it may have needed. But never changed anything related to speedo on it. I am guessing the prob is my truck speedo but not sure. Thanks for any help, its just aggravating at times.
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New daily driver 96 Chevy K3500 crew cab dually 53,000 miles 350 Vortec 4L80 Nice ride 67 Chevy C-10 LWB Built 350 TH-400 Wifes ride and family rig: 2001 Furd Expedition 5.4 Liter  Kickin back in Rolla,Mo. USA "Do the thing you fear most and the death of fear is certain" Mark Twain |
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12-11-2004, 03:19 AM
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#2 |
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Garage Queen Material
Join Date: Oct 2002
Location: USA
Posts: 4,129
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what size tires are you running?
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12-11-2004, 03:34 AM
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#3 |
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Gentleman Jim Driver
Join Date: Nov 2002
Location: Poulsbo, WA
Posts: 1,553
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It's probably not the speedo, but the drive and driven gears in the tranny don't match the gear ratio in your axle and/or your tire size like Eric said.
You can change the driven gear in your transmission to correct the speedo.
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Joe '75 GMC Gentleman Jim '84 Chev C10 Short Wide - Super duper plain (manual steering, manual brakes, no dome light, no cig lighter) '85 Chev C10 Short Wide - Super plain Vortec 4.8 4L60E trans also: '81 K30, '83 C30 Crew Dually, '84 M1028 CUCV, '85 M1009 CUCV, another '85 C10 SWB, '89 R3500 Flatbed |
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12-11-2004, 03:35 AM
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#4 |
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Registered User
Join Date: Jul 2000
Location: Mo
Posts: 963
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It was 10mph slow with 31's since I have put 35's on it but I want to get it closer to being right than it is. LOL I know the tire size makes a big difference. So I want to get it close to right for 31's and go from there, I can't afford no fancy custom made speedometer correction dohickey but I have some access to different speedometer drive gears and such. My speedo gear is in the transfercase. I know I can change it but why was it 10 mph off with 31 inch tires. I have run 31's on other 4x's and only been off 3-4 mph. Now I know I am actually off more than 10 mph with the 35's but if I can get it closer to correct for 31's I have a starting place to correct it for the 35's. I am probably confusing the heck out of you guys I am sorry.
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New daily driver 96 Chevy K3500 crew cab dually 53,000 miles 350 Vortec 4L80 Nice ride 67 Chevy C-10 LWB Built 350 TH-400 Wifes ride and family rig: 2001 Furd Expedition 5.4 Liter Kickin back in Rolla,Mo. USA "Do the thing you fear most and the death of fear is certain" Mark Twain Last edited by Stano; 12-11-2004 at 03:39 AM. Reason: cause i forgot something |
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12-13-2004, 03:01 AM
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#5 |
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Registered User
Join Date: May 2003
Location: Shreveport LA
Posts: 3,170
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Take your axle ratio & tire diameter to http://novaresource.org/speedo.htm & plug them into the formula, then pick the gear combo that you need. A Chevy dealer should have them.
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