Which speedometer gear
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Joined: Aug 2004
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From: Lakeland Florida
Which speedometer gear
On a 1973 with a Turbo Hydramatic (I believe its a 350) with, the best I can tell, around a 400 gear ratio. Which gear would I use to correct the speedometer Thanks.
Le Mans Master
Joined: Jul 2006
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From: charlotte north carolina
you have to know the exact diff ratio to get the correct gear. support the rear of the car on jacks, place a piece of tape on the pinion u-joint and rear tire with marker lines. rotate the tire an even number of turns, say 2 turns, while counting the number of rotations the pinion makes. that will give you the ratio. if you can't see the mark on the pinion, just trrun the wheel one more turn. dr rebuild has a chart that shows what gear is needed. i doubt that you ratio is 4, unless you have a drag racer. probably 3.23 or 3.36.
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From: Santo TX
Another way to do it is to find a stretch of highway with mile markers. Run the car at a consistent 60 mph and time yourself between mile posts. At 60 mph you will cover a mile in 60 seconds. Time it a couple of times to make sure you are accurate. If your time is under 60 seconds then you are going faster than your speedo reads so you need a gear with less teeth. If your time is slower than 60 then your speedo is reading high and you need a gear with more teeth. Find the percentage difference in your time versus 60 seconds (divide your time by 60). Now remove your gear and count the number of teeth on it. Multiply your tooth count by your percentage and that will get you the correct tooth count gear. It may not calculate dead on but a speedo is considered accurate if it reads within 5% of the actual speed. Oh, don't forget to properly inflate your tires before you take your timing runs.
Example: measured time of 55 seconds over 1 mile with speedo reading 60 mph. 55/60 = 0.91666
current speedo gear has 22 teeth. 22x0.91666=20.1666
so you need a 20 tooth gear to correct the speedo.
Example: measured time of 55 seconds over 1 mile with speedo reading 60 mph. 55/60 = 0.91666
current speedo gear has 22 teeth. 22x0.91666=20.1666
so you need a 20 tooth gear to correct the speedo.
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Joined: Aug 2004
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From: Lakeland Florida
[/SIZE][/FONT]I did jack it up. Made a mark on the drive shaft and on the tire. The drive shaft turned 4 revolutions for each round the tire made. I am not sure what Chevrolet made close to that gear ratio, however that would get me somewhere close on the speedometer.
Le Mans Master
Joined: Jul 2006
Posts: 8,117
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From: charlotte north carolina
i believe that the vettes had either a 3.7 or a 4.11 ratio offered, but only with a manual trans. is your speedo reading fast or slow? you'll have to pull the driven gear out of the trans and check the color. if it's indicating fast, get a gear with more teeth to slow the cable. i still can't believe that you have that steep of a diff. from the factory the turbo's had a 3.08 or a 3.36. has someone changed the diff gears?
one thing to keep in mind, is that it may not be an incorrect speedo gear that causing the error. the speedo itself may be out of adjustment. there is a calibration mechanism in the speedo itself that adjusts the tension on the needle. i believe that it changes the tension on the spring that pushes the needle counter-clockwise back to zero. maybe someone that has done this will chime-in.
one thing to keep in mind, is that it may not be an incorrect speedo gear that causing the error. the speedo itself may be out of adjustment. there is a calibration mechanism in the speedo itself that adjusts the tension on the needle. i believe that it changes the tension on the spring that pushes the needle counter-clockwise back to zero. maybe someone that has done this will chime-in.
Drifting
Joined: Sep 2006
Posts: 1,259
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From: MS
[/SIZE][/FONT]I did jack it up. Made a mark on the drive shaft and on the tire. The drive shaft turned 4 revolutions
for each round the tire made. I am not sure what Chevrolet made close to that gear ratio, however that would get
me somewhere close on the speedometer.
for each round the tire made. I am not sure what Chevrolet made close to that gear ratio, however that would get
me somewhere close on the speedometer.
the driveshaft revolutions for 39 or 41 turns. The L48 had a 3.08 standard and 3.36 optional, the L82 had a 3.55 standard
and 3.36 optional and without A/C, you could also get a 3.70 with the L82. The automatic transmission used on all 68-75
Corvettes was the Turbo 400. The 3.90 is an aftermarket ratio, not sure if that will make finding the correct speedo gear harder.
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