For you Techy's out there
Thread Starter
Melting Slicks
Joined: Mar 2005
Posts: 2,187
Likes: 7
From: Chetwynd B.C.
For you Techy's out there
What are my rear end gears? Here are the facts as I have them.
1968 coupe with 4 speed. I'm running a 454 NOM and 255 60R 15 tires ( 26" tall, and 84.5" circ.) I get 3.75 rotations on the drive shaft to one revolution of the rear tires. I've checked the scale and this should show 3.70 gears. But I am getting 3200 RPM at 60mph ( will have to double check this but seems to me that's what it is)... is this right with 3.70's?? This seems like it should be 4.10 or 4.11's shouldn't it? Or am I out to lunch :o
thanx for any input,
shmoky
1968 coupe with 4 speed. I'm running a 454 NOM and 255 60R 15 tires ( 26" tall, and 84.5" circ.) I get 3.75 rotations on the drive shaft to one revolution of the rear tires. I've checked the scale and this should show 3.70 gears. But I am getting 3200 RPM at 60mph ( will have to double check this but seems to me that's what it is)... is this right with 3.70's?? This seems like it should be 4.10 or 4.11's shouldn't it? Or am I out to lunch :o
thanx for any input,
shmoky
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
These tachs and speedos are not all that great for accuracy when new....+- 5% is easy so at 60 a variance of 3 mph...and then add in for the tach....easy to cover a rear diffy change of only 10%....
gotta figger tire diameter for another variance independant of car instruments....
gotta figger tire diameter for another variance independant of car instruments....
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
There are online calculators all over the place. You plug in your variables (tire size, speed, tach, assumed gear ratio (I'm thinking you have a 3.90 gear) and it will give you whatever result you're looking for. Your tire is way short, but you probably already know that.
Burning Brakes
Joined: Feb 2005
Posts: 1,235
Likes: 0
From: Mesa AZ
60 mph at 3200 rpm with 26" tall tires and a 1:1 final trans gear ratio is 4.11 gears
http://www.slowgt.com/Calc1.htm#RPMInGear
http://www.slowgt.com/Calc1.htm#RPMInGear
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
With 3.73's and a 26" tall rear tire in a Camaro (BFG 235/60/15), I run 3100 RPM at 60, with a good Autometer Tach and the original speedo has been tested with radar at 50 MPH,..it's dead on.
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Drifting
Joined: Jul 2007
Posts: 1,712
Likes: 0
From: Tampa FL
If you're getting 3.75 when you rotate the tire, then that (probably 3.73) is your gear ratio, no question about it. Doesn't matter what your RPM's are or what any chart or calculator says it should be.
If you think your RPM's are too high or too low, maybe you don't have the stock transmission. I'm putting 3.70 gears in mine now (coming from 2.76) but I'm also changing out the tranny to one with an overdrive so I don't have crazy rpm's on the Interstate.
If you think your RPM's are too high or too low, maybe you don't have the stock transmission. I'm putting 3.70 gears in mine now (coming from 2.76) but I'm also changing out the tranny to one with an overdrive so I don't have crazy rpm's on the Interstate.
Race Director
Joined: Nov 1999
Posts: 10,643
Likes: 8
From: Foxfield CO 1970 Convertible
In '68, the standard tire was a bias ply and about 27 inches tall. Your tires are 27 inches tall. At 3200rpm, you should be going 60 and your rear diff ratio is 4.11. Sounds like you are right on...
Speed calculator
What's the "scale"? Have you checked the stamps on the bottom of the diff to see what the indicated ratio is?
Speed calculator
What's the "scale"? Have you checked the stamps on the bottom of the diff to see what the indicated ratio is?