Speedometer 6 mph slow - The 1947 - Present Chevrolet & GMC Truck Message Board Network

skokie · 12-27-2003, 07:58 PM

My speedometer is showing 6 mph slow. Is this the tranny speedo gear? Tranny is a 400 turbo?

Thanks

Sweet72 · 12-27-2003, 08:16 PM

The correction can be made by changing the speedo driven gear. Check this website: bgsoflex.com
-Sam

Sweet72 · 12-27-2003, 08:19 PM

That address didn't work so try: www.bgsoflex.com/auto.html
-Sam

jimfulco · 12-28-2003, 05:01 AM

Depends on at what mph it's 6mph slow. Speedo calibration is done on a percentage basis. If it's 6mph off at 60mph, then it's 10% off, & you need to change your speedo gear ratio by 10%. If you're lucky, you can just change the drive gear (the one on the tailshaft) by 10%, or change the driven gear (the one the cable is connected to) by 10%. Otherwise, you could change both gears to get a 10% change, or use a geared adapter (expensive) that changes the speed of your speedo cable. You can usually get pretty close by changing just the driven gear, which is a $5 part at the Chevy place. If you're reading 10% slow, count the teeth on your driven gear (best to do this twice just to make sure), then get a gear with 10% fewer teeth. Here's a chart of Turbo 400 gears & color codes from TCI, & some more codes & calibration info from NovaResource:

http://www.tciauto.com/tech_info/speedo_gears.htm#400
http://www.novaresource.org/speedo.htm
http://www.novaresource.org/speedo2.htm

Watch out for color codes when dealing with old used gears, because the colors sometimes fade. My speedo gear would have needed a fractional amount of teeth to be exact, so I rounded down, making it read a little bit high, just to be on the safe side (lead foot, etc.).

gmc71 · 12-29-2003, 11:28 PM

Just go faster!

skokie · 12-29-2003, 11:38 PM

Quote:

Just go faster!

I did.

ChevLoRay · 12-30-2003, 12:05 AM

Our interstates in Arkansas are being re-done. Between my home town of Benton, and Little Rock, the speed limit varies from 55 to 60. They have some of those portable speed displays to show you how much over the limit you're driving. On my truck, original 3.07's, according to the manual I have, with 255R70-15's, and the stock TH400, I'm running at 55mph, when my speedo shows me to be doing 52. So, I adjust. I drive 33, to go 35, in town. When I want to go 65, I drive so it says 61.

I need a 700R4, so I can go faster with less RPM's.

12-27-2003, 07:58 PM	#1
skokie Registered User Join Date: Oct 2003 Location: San Jose Ca. Posts: 3,335	Speedometer 6 mph slow My speedometer is showing 6 mph slow. Is this the tranny speedo gear? Tranny is a 400 turbo? Thanks

12-27-2003, 08:16 PM	#2
Sweet72 Registered User Join Date: Nov 2003 Location: Alexandria, VA Posts: 1,534	The correction can be made by changing the speedo driven gear. Check this website: bgsoflex.com -Sam __________________ '72 GMC SWB C1500 Custom, frame-off in progress. 383 SBC, 9:1CR, Comp Cams XE262H, Scat internal balanced crank, Eagle SIR 6" rods, Keith Black dish pistons, Dart Iron Eagle 72/180 heads, Weiand Stealth intake, Stewart stage I water pump, Holley 4bbl vac sec, TH350 with B&M Shift Improver Kit. 12 bolt positraction.

12-27-2003, 08:19 PM	#3
Sweet72 Registered User Join Date: Nov 2003 Location: Alexandria, VA Posts: 1,534	That address didn't work so try: www.bgsoflex.com/auto.html -Sam __________________ '72 GMC SWB C1500 Custom, frame-off in progress. 383 SBC, 9:1CR, Comp Cams XE262H, Scat internal balanced crank, Eagle SIR 6" rods, Keith Black dish pistons, Dart Iron Eagle 72/180 heads, Weiand Stealth intake, Stewart stage I water pump, Holley 4bbl vac sec, TH350 with B&M Shift Improver Kit. 12 bolt positraction.

12-29-2003, 11:28 PM	#5
gmc71 Registered User Join Date: Aug 2003 Location: Buffalo, NY Posts: 115	Just go faster! __________________ 72 Chevy 4x4 350 55 Chevy Bel Air 350/350 53 Matchless G80s (motorcycle) 06 Trailblazer SS 400hp LS2

12-30-2003, 12:05 AM	#7
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Our interstates in Arkansas are being re-done. Between my home town of Benton, and Little Rock, the speed limit varies from 55 to 60. They have some of those portable speed displays to show you how much over the limit you're driving. On my truck, original 3.07's, according to the manual I have, with 255R70-15's, and the stock TH400, I'm running at 55mph, when my speedo shows me to be doing 52. So, I adjust. I drive 33, to go 35, in town. When I want to go 65, I drive so it says 61. I need a 700R4, so I can go faster with less RPM's. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

12-28-2003, 05:01 AM	#4
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	Depends on at what mph it's 6mph slow. Speedo calibration is done on a percentage basis. If it's 6mph off at 60mph, then it's 10% off, & you need to change your speedo gear ratio by 10%. If you're lucky, you can just change the drive gear (the one on the tailshaft) by 10%, or change the driven gear (the one the cable is connected to) by 10%. Otherwise, you could change both gears to get a 10% change, or use a geared adapter (expensive) that changes the speed of your speedo cable. You can usually get pretty close by changing just the driven gear, which is a $5 part at the Chevy place. If you're reading 10% slow, count the teeth on your driven gear (best to do this twice just to make sure), then get a gear with 10% fewer teeth. Here's a chart of Turbo 400 gears & color codes from TCI, & some more codes & calibration info from NovaResource: http://www.tciauto.com/tech_info/speedo_gears.htm#400 http://www.novaresource.org/speedo.htm http://www.novaresource.org/speedo2.htm Watch out for color codes when dealing with old used gears, because the colors sometimes fade. My speedo gear would have needed a fractional amount of teeth to be exact, so I rounded down, making it read a little bit high, just to be on the safe side (lead foot, etc.).

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