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Discussion Starter · #1 ·
My current setup on my 67 Chevelle is a PG with a 10 bolt 3.08 open rear gears with 205/70R/14 tires. I plan on switching to 245/60R/15 tires. Can someone tell me which gear I need to put in the tranny for an accurate speedo reading.

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Joe,

Your rpms will increase by about 120 and speedo will be off 3-4 mph.

Example:
205/70/R14 at 60 mph = 2455 rpm
245/60/R15 at 60 mph = 2337 rpm

Changing wheels/tires, your speedo would read 60 while actually going 63mph. (2455 rpm with the 245/60/R15 = 63mph.) You'd have to change back to a speedo gear for a 2.93 axle ratio to make it right but I'm sure that the next gear ratio GM offers is a 2.73.

You're not changing that drastically in overall diameter. Personally, I wouldn't change anything and just keep in mind that you're going a bit faster than your speedo indicates.

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Dale McIntosh
TC Gold #92
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www.dalesplace.com
 

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Discussion Starter · #4 ·
<BLOCKQUOTE>quote:</font><HR>Originally posted by Dale McIntosh:
Joe,

Your rpms will increase by about 120 and speedo will be off 3-4 mph.

Example:
205/70/R14 at 60 mph = 2455 rpm
245/60/R15 at 60 mph = 2337 rpm

Changing wheels/tires, your speedo would read 60 while actually going 63mph. (2455 rpm with the 245/60/R15 = 63mph.) You'd have to change back to a speedo gear for a 2.93 axle ratio to make it right but I'm sure that the next gear ratio GM offers is a 2.73.

You're not changing that drastically in overall diameter. Personally, I wouldn't change anything and just keep in mind that you're going a bit faster than your speedo indicates.

<HR></BLOCKQUOTE>
 

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Discussion Starter · #5 ·
Gene , Dale

Thanks for the quick replies. I wish I had a source like this ten years ago.


<BLOCKQUOTE>quote:</font><HR>Originally posted by Dale McIntosh:
Joe,

Your rpms will increase by about 120 and speedo will be off 3-4 mph.

Example:
205/70/R14 at 60 mph = 2455 rpm
245/60/R15 at 60 mph = 2337 rpm

Changing wheels/tires, your speedo would read 60 while actually going 63mph. (2455 rpm with the 245/60/R15 = 63mph.) You'd have to change back to a speedo gear for a 2.93 axle ratio to make it right but I'm sure that the next gear ratio GM offers is a 2.73.

You're not changing that drastically in overall diameter. Personally, I wouldn't change anything and just keep in mind that you're going a bit faster than your speedo indicates.

<HR></BLOCKQUOTE>



------------------
The never ending joy rides!!
96 Caprice Classic LT1
81 Malibu Classic
78 Cheyenne SWB HD pickup
69 350SS Camaro
67 Chevelle Convertible
 
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