Speedometer Gears in Thransmission
Thread Starter
Instructor
Joined: Dec 1999
Posts: 195
Likes: 1
From: Van Buren AR
Speedometer Gears in Thransmission
Well I have the Torq Thrust rear wheels on order. The front ones are already here. I bought 17" wheels and I'm putting 25.9" tall tires on them. My car is a 78 Pace Car so I have the close ratio four speed. The stock tires were 27" tall. The rear end ratio is 373. What gears do I need to make my speedometer read correctly? Thanks for your help.
Melting Slicks
Joined: Apr 2000
Posts: 3,286
Likes: 8
From: San Jose California
Re: Speedometer Gears in Thransmission (Arkansas Flash)
John,
Your friendly Chevy dealer sells speedo drive gears that are easily inserted in the tail end of the tranny. They seem to come in 1 tooth increments. They are only a couple bucks each. I believe you need to add a tooth when you use smaller tires. A 69 Camaro with 3.73 gears had a smaller diameter tire than the Vette and still used the same tranny. This might be a possible combination.
Yet to put it simply, if you have a 5% change in your tire diameter, you need a 5% difference in the number of your speedo gear teeth to keep everything accurate. I used my GPS to callibrate my speedo to actual. If you have access to one, do this before you buy a gear since you may have been already out of wack and didn't know it. Whatever your current speedo error is as a percentage of actual is the same percentage of teeth you need to change on the gear.
Chuck
[Modified by Chuck Harmon, 10:49 AM 8/9/2002]
Your friendly Chevy dealer sells speedo drive gears that are easily inserted in the tail end of the tranny. They seem to come in 1 tooth increments. They are only a couple bucks each. I believe you need to add a tooth when you use smaller tires. A 69 Camaro with 3.73 gears had a smaller diameter tire than the Vette and still used the same tranny. This might be a possible combination.
Yet to put it simply, if you have a 5% change in your tire diameter, you need a 5% difference in the number of your speedo gear teeth to keep everything accurate. I used my GPS to callibrate my speedo to actual. If you have access to one, do this before you buy a gear since you may have been already out of wack and didn't know it. Whatever your current speedo error is as a percentage of actual is the same percentage of teeth you need to change on the gear.
Chuck
[Modified by Chuck Harmon, 10:49 AM 8/9/2002]
