Gear or calibration
Thread Starter
Burning Brakes
Joined: Aug 2011
Posts: 829
Likes: 288
Gear or calibration
It was easy enough to swap in a 140 MPH speedo in place of the stock 85 MPH speedo. Took a quick run to make sure it was working. Seems it is running between 7 to 10 MPH lower then what it should be as compared to a GPS. Was right on before the swap. Question is, does the new speedo need to be calibrated or do I need to change the gear in the tranny?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
It was easy enough to swap in a 140 MPH speedo in place of the stock 85 MPH speedo. Took a quick run to make sure it was working. Seems it is running between 7 to 10 MPH lower then what it should be as compared to a GPS. Was right on before the swap. Question is, does the new speedo need to be calibrated or do I need to change the gear in the tranny?
The new speedometer should be calibrated correctly. If you need to change the gears in the transmission you'll need to know how far your tires travel in one revolution. Put a gob of grease onto the tread then roll it far enough to leave two grease spots on your driveway. Give me that distance and I'll tell you which gears you need in the transmission (assuming you still have the stock 2.87 rear gears).
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
Different years offered different axle ratios. The speedos are all set to the same standard rpm/speed . You're close so you will only need to go up or down one ratio. I wish I could help you with the trans gear but they are color coded. It may be easier to see what color you have now and then look up the next color depend on which way your speedo is running .
Many Chevelle forums or suppliers have the gear color charts. Mostly because Chevelle came with 6 different axle ratios and it's a common problem with those guys. The trans gears are the same.
Many Chevelle forums or suppliers have the gear color charts. Mostly because Chevelle came with 6 different axle ratios and it's a common problem with those guys. The trans gears are the same.
Last edited by hugie82; May 31, 2015 at 04:17 PM.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
When I changed my differential from 3.73 to 3.36 a guy on Ebay selling speedometer gears advised me what combination of speedometer gears to use based on my tire size and gear ratio. I used his suggestion and my speedometer is right on.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
GM speedometers are calibrated for 1000 revolutions per mile which is 1000rpm @ 60mph. So, one of your speedometers is out of calibration. Doesn't mean you can't try a new driven gear and see if you can get it correct.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
Are you sure? 1000rpm @ 60 mph? Does that include all different gear ratios?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
The 1000 rpm @ 60 mph is an industry standard and always has been. It requires different combinations of gears in the transmission to make the speedometer spin that speed at 60 rpm.
Last edited by toobroketoretire; May 31, 2015 at 06:26 PM.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
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