What size speedo gear do I need?
Thread Starter
Burning Brakes
Joined: Mar 2011
Posts: 862
Likes: 9
From: Communist party of MD
What size speedo gear do I need?
I have no clue what size/color speedo gear to order for my trans. I have 3.08 rear end, 275/60/15 rear tire size and a 4spd trans in my '69.
Thanks.
Thanks.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
Look up the rolling diameter for the tires you have. Look up the rolling diameter for the tires that came on the vehicle. Figure the distance that would be traveled by one revolution of the tires. Use the ratio between those to determine what gear you need. Mutiply the ratio by the number of teeth on your old gear.
Thread Starter
Burning Brakes
Joined: Mar 2011
Posts: 862
Likes: 9
From: Communist party of MD
Look up the rolling diameter for the tires you have. Look up the rolling diameter for the tires that came on the vehicle. Figure the distance that would be traveled by one revolution of the tires. Use the ratio between those to determine what gear you need. Mutiply the ratio by the number of teeth on your old gear.
stock tire size was: 235/70/15
current tire size: 275/60/15
rear end gears: 3.08
Instructor
Joined: Jan 2012
Posts: 103
Likes: 0
From: Sonoma Ca.
i came up with trans dr. gr. 8 teeth, speedo cable dr. Gr. 18 teeth, go to tri5chevyrides.com/speedo.php
Last edited by anips; Feb 14, 2012 at 12:02 PM.
Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
Once you get everything together and head for the highway, take along a GPS. More accurate than having a buddy drive along side with his cell phone telling you his speed.
I have found that, on average, changing the plastic driven gear 1 tooth will change your speedo 3 mph.
I have found that, on average, changing the plastic driven gear 1 tooth will change your speedo 3 mph.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Once you get everything together and head for the highway, take along a GPS. More accurate than having a buddy drive along side with his cell phone telling you his speed.
I have found that, on average, changing the plastic driven gear 1 tooth will change your speedo 3 mph.
I have found that, on average, changing the plastic driven gear 1 tooth will change your speedo 3 mph.
For example, 70 indicated mph / 63 actual = 1.111;
1.111 x 18 teeth = 20 teeth needed for the driven gear in the trans.
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Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
Another thing I found when comparing my speedo with the GPS is that my speedo output is not linear. I have installed a combination of drive and driven gears to make it accurate at 70 mph. It is off by as much as 3-4 mph when going much slower or faster. I chose to make mine accurate at a speed where I think I'd be likely to get a speeding ticket. If I get caught doing 135 in a 65 zone, a difference of 3-4 mph won't matter much.
Heel & Toe
Joined: May 2014
Posts: 23
Likes: 1
From: Waynesville MO
worm gear question.
Once you've found the speed correction, divide the indicated speed by the actual speed, and then multiply that times the number of teeth in the driven speedo gear. That will be the new desired number of teeth.
For example, 70 indicated mph / 63 actual = 1.111;
1.111 x 18 teeth = 20 teeth needed for the driven gear in the trans.
For example, 70 indicated mph / 63 actual = 1.111;
1.111 x 18 teeth = 20 teeth needed for the driven gear in the trans.
Last edited by freddy4099; Jun 30, 2014 at 11:45 AM. Reason: more info
Safety Car
Joined: Oct 2004
Posts: 3,926
Likes: 739
From: Youngstown Ohio
This link will help....scroll to select your application..
http://www.transmissioncenter.org/GM...eter_Gears.htm
http://www.transmissioncenter.org/GM...eter_Gears.htm
