rpm reading at a set speed ?

[mike price]

I am running 255/60R15 on the rear of my barracuda.  At 70 MPH the tach reads 3300 RPM is this high or is it correct ? can not find anyplace to help me answer this question.  Gears in the rear end are unknown assumed they were 3.23's .  if you can answer great or send me to a place to help me answer the question.

SHOULD HAVE SAID THE TRANNY IS A 727 IN THIS 70 BARRACUDA, IF IT MAKES A DIFFERENCE.

Thanks in advance Mike

[anlauto]

In my opinion that sounds high for 3:23's, but without knowing the rear axle gear ratio, you're pretty much in the dark..
...also, a factory tach is not known for it's accuracy

[Dakota]

Your calculated tire size is 27.05 inches in diameter.  Assuming your final transmission gear ratio is 1:1, a 3.91 differential gear would result in turning 3401 rpm at 70 mph which looks pretty close to what's listed in your post.

For a 3.23 differential gear, you'd be at 2810 with the same tire size at 70 mph.

I got this info from a "Tremec Tool Box " app for iPhones.   It's pretty handy.

[anlauto]

Hey that's pretty cool....He could have 3:55's, what would that result in ? around 3100 ?

[rikkitik]

Link to an online calculator

https://purperformance.com/p-29669-rpm-calculator.html

[HP2]

3:73 gears...3310 rpm.

[Dakota]

Hey that's pretty cool....He could have 3:55's, what would that result in ? around 3100 ?

3.55 ratio would be 3080 rpm @ 70 mph for this tire size.

[7212Mopar]

Jack up the rear of the car, trans in neutral and release parking brake. Put a tape on drive shaft as a reference mark. Rotate the tires to see the number of drive shaft rotation vs tire rotation and you can then find out your rear gear ratio. This is the easy way not to have to pull the center section. 3.23 and 3.55 are very close so you have to count carefully and look at driveshaft diameter and angle rotation. 0.23 is less than half while .55 is slightly more than half of a rotation.

Factory tach is usually off and higher due to age. If you have a timing light with RPM readout, you can compare the two readings.

[mike price]

Will jack up rear end and see what the count is and post the results. 

Thanks for the help so far..

[70vert]

Jack up the rear of the car, trans in neutral and release parking brake. Put a tape on drive shaft as a reference mark. Rotate the tires to see the number of drive shaft rotation vs tire rotation and you can then find out your rear gear ratio. This is the easy way not to have to pull the center section. 3.23 and 3.55 are very close so you have to count carefully and look at driveshaft diameter and angle rotation. 0.23 is less than half while .55 is slightly more than half of a rotation.

Factory tach is usually off and higher due to age. If you have a timing light with RPM readout, you can compare the two readings.

I just did this myself earlier this year, for similar reason. I got a 8.75 rear from a 70 R/T and was told it was a 3.23 year, but seemed to tach a bit higher than I expected. After checking as per above it seems it is a 3.55! I know it is not a huge difference so checked it multiple times to confirm. Not really what I had wanted but guess I'm OK with it for now.

[mike price]

After checking tire and drive shaft rotation 6 times.  it appears there are 3.23 gears in the rear end.

Guess the tachometer is off as suggested. 

Thanks for the help guys.

Mike