Speedometer gear advice wanted - TriFive.com, 1955 Chevy 1956 chevy 1957 Chevy Forum , Talk about your 55 chevy 56 chevy 57 chevy - Belair , 210, 150 sedans , Nomads and Trucks, Research, Free Tech Advice

ol55 · 04-17-2011, 11:37 PM

My '57 speedo is off by about 20 mph @55 (shows@75). I have 15" wheels and a Turbo 350. I assume I need to change the driven gear off the trans to the speedo cable. Is there a chart somewhere to tell me which (driven) gear color and ratio I should use to correct this.

Thanks,
Larry

ChuckG · 04-18-2011, 07:15 AM

Ah, the old math problem again.

Your speedo shows 75 MPH when you're really doing 55 MPH.

So, you need a driven gear with MORE teeth to slow the speedo down.

75 MPH / 55 MPH = 1.36

SO, here's what you do. Pull the driven gear in the trans now and count the number of teeth on it. You'll need a gear with 1.36 times the number of teeth on your current gear.

IF your current driven gear has 30 teeth.....

30 X 1.36 = 40.9, or you "round up" to 41.

That's the gist of how you figure things out. NOW, if you cannot find gears with the correct number of teeth, you might be forced to buy one of those speedometer "adapter" that you see in the catalogs to correct the speedo.

Chuck

carls 56 · 04-18-2011, 09:33 AM

www.classicchevy.com has the gears and the formula. ................#drive gear teeth (inside tailshaft) x axle ratio x 20.168 divided by outside of tire in inches = number of teeth needed. got that :stupido3:

p.s. don't shoot the messanger

markm · 04-18-2011, 09:48 AM

Quote:

Originally Posted by ol55

My '57 speedo is off by about 20 mph @55 (shows@75). I have 15" wheels and a Turbo 350. I assume I need to change the driven gear off the trans to the speedo cable. Is there a chart somewhere to tell me which (driven) gear color and ratio I should use to correct this.

Thanks,
Larry

GM parts manuals have charts with gear ratio, tire size etc.

ol55 · 04-18-2011, 09:15 PM

Thanks, Now I have two ways to figure it out!

I found this site had the color chart
http://www.tciauto.com/Products/Tech..._gears.asp#350

Thanks again,
Larry

BADD55 · 04-18-2011, 10:35 PM

The lazy & simple way for me, was to get my wife to drive along beside my chevy, with her new jeep, and drive 60mph steady. Cell phones come in handy doing this!!-- I summized, the chevy speedo was in error by showing me I was running 85mph!!--- I called my local speedometer repair shop, told him, I was showing 25mph to fast.--- He said no problem and made me a correction adapter, it screwed on the transmisson, the cable screwed on it! Cost me $75.00, worked perfect, problem solved!!--As I said the lazy way!!!

joninomader · 04-18-2011, 10:39 PM

Ther is no such thing as the lazy way if the results are good.

ChuckG · 04-19-2011, 05:03 AM

Quote:

Originally Posted by joninomader

Ther is no such thing as the lazy way if the results are good.

Absolutely right.

Another way to do it is to use your Garmin or other GPS device.

Those devices will show you your speed on the read-out screen, and you can compare those numbers with your speedometer.

Chuck

04-17-2011, 11:37 PM	#1
ol55 Member Join Date: Apr 2011 ol55's Garage Location: Northern VA Posts: 4 Member #17868	Speedometer gear advice wanted My '57 speedo is off by about 20 mph @55 (shows@75). I have 15" wheels and a Turbo 350. I assume I need to change the driven gear off the trans to the speedo cable. Is there a chart somewhere to tell me which (driven) gear color and ratio I should use to correct this. Thanks, Larry

04-18-2011, 07:15 AM	#2
ChuckG Senior Member 1 Gold Star Join Date: Apr 2007 ChuckG's Garage Location: Beverly Hills/Pine Ridge, Florida Posts: 1,543 Member #2597	Ah, the old math problem again. Your speedo shows 75 MPH when you're really doing 55 MPH. So, you need a driven gear with MORE teeth to slow the speedo down. 75 MPH / 55 MPH = 1.36 SO, here's what you do. Pull the driven gear in the trans now and count the number of teeth on it. You'll need a gear with 1.36 times the number of teeth on your current gear. IF your current driven gear has 30 teeth..... 30 X 1.36 = 40.9, or you "round up" to 41. That's the gist of how you figure things out. NOW, if you cannot find gears with the correct number of teeth, you might be forced to buy one of those speedometer "adapter" that you see in the catalogs to correct the speedo. Chuck __________________ 1956 Chevy 150 2 Door Sedan, Red/Saddle Cloth, 355CI, 4 Spd, Vintage Air, Neat "Old School" Hot Rod 1963 Corvette Roadster, Riverside Red/Black Vinyl, 327/360HP FI, 4 Spd, NCRS Top Flight 1969 Corvette Roadster, Monaco Orange/Black Leather, 427/400HP, THM400, NCRS Top Flight. 2006 Corvette Roadster, Velocity Yellow/Black, LS2 Automatic.

04-18-2011, 10:35 PM	#6
BADD55 Senior Member 1 Silver Star Join Date: Mar 2008 BADD55's Garage Location: Marietta,GA. Posts: 909 Member #5166	Just another way of doing it!!!! The lazy & simple way for me, was to get my wife to drive along beside my chevy, with her new jeep, and drive 60mph steady. Cell phones come in handy doing this!!-- I summized, the chevy speedo was in error by showing me I was running 85mph!!--- I called my local speedometer repair shop, told him, I was showing 25mph to fast.--- He said no problem and made me a correction adapter, it screwed on the transmisson, the cable screwed on it! Cost me $75.00, worked perfect, problem solved!!--As I said the lazy way!!!

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04-18-2011, 09:33 AM	#3
carls 56 Trifive Leaders Club 2 Ball Pool Champion! Alien Invasion 2 Champion! Altex Champion! Anti Aircraft Gun Champion! Baccarat Champion! Block Buster Champion! Cliff Diver Champion! Get-A-Head Champion! Ninja Turtles 2 Champion! Pong Champion! Super Kickups Champion! Ultimate Ping Champion! Weetabix Games:Running Champion! Clash`N Slash Champion! Graveler Gunblast Champion! Cone Crazy Champion! Bug Hunt Champion! PelPet Champion! Hamster Race Champion! pursuit Champion! Weetabix Games:Cycling Champion! Red Kens Pigeon Cull Champion! Lotads Lament Champion! Desert Dash Champion! Bubble Blubbs Champion! Ed, Edd, Eddy Champion! Snowboarding Santa Champion! piljard Champion! Mini Putt 2 Champion! Weetabix Games:Swimming Champion! J&B Snowman Punch Champion! Roving Reporter Champion! Cone Crazy Champion! Extreme Racing Champion! Pepsi Bucket Launcher Champion! Springfield Snow Fight Champion! Lumpy Artist Champion! Dribble Worldcup Champion! Flyswatter Champion! Subzero Champion! Starfrosch Champion! Tobby Stick Balance Champion! Flash Golf Champion! Fruity Shuffler Champion! Spy Hunter Champion! Juice Squeezer Champion! Spell Catcher Champion! Klingemon Champion! Harry Potter: The Marauder`s Map Champion! Chansey Champion! JigSaw Puzzle Dog Champion! Renault Mr.X Champion! Grab-A-Snack Hurdles Champion! Slider Puzzle Champion! Plops Champion! Conundrum 3D Champion! Join Date: Feb 2008 carls 56's Garage Location: brandy station, va Posts: 33,144 Member #4923	www.classicchevy.com has the gears and the formula. ................#drive gear teeth (inside tailshaft) x axle ratio x 20.168 divided by outside of tire in inches = number of teeth needed. got that :stupido3: p.s. don't shoot the messanger __________________ "DRIVE AND ENJOY THEM WHILE YOU WORK ON THEM, LIFES TO SHORT" my toys: 56 4dr 210 & 57 4dr belair ARMY NAM VET (so proud)

04-18-2011, 09:15 PM	#5
ol55 Member Join Date: Apr 2011 ol55's Garage Location: Northern VA Posts: 4 Member #17868	Thanks, Now I have two ways to figure it out! I found this site had the color chart http://www.tciauto.com/Products/Tech..._gears.asp#350 Thanks again, Larry

04-18-2011, 10:39 PM	#7
joninomader Trifive Leaders Club Join Date: Aug 2010 joninomader's Garage Location: Central Coast California Posts: 7,942 Member #14655	Ther is no such thing as the lazy way if the results are good.

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