3.23 gears, 70mph, 3400rpm? Does this sound right?

[NateCharger]

My 71' R/T with the 727 auto pulls 3400 rpm going 70mph (gps confirmed) with 3.23 Suregrip (swapped from Imperial w/disc brakes) rear axle. Can this be right???

 

[Badvert65]

Well, it's correct if your rear tires are 22 inches in diameter. What are your rear tires measuring?

 

[PurpleBeeper]

My 71' R/T with the 727 auto pulls 3400 rpm going 70mph (gps confirmed) with 3.23 Suregrip (swapped from Imperial w/disc brakes) rear axle. Can this be right???

Without pulling out my calculator, that sounds about right

 

[terzmo]

My 354 ratio turn that at 70 with 26 inch tires

- - - Updated - - -

I put in 245 by 60 by 15 for your tires and at 3400 you should be doing about 83mph
Here is a great link
http://vexer.com/automotive-tools/speed-rpm-calculator

 

[rakort]

My 71' R/T with the 727 auto pulls 3400 rpm going 70mph (gps confirmed) with 3.23 Suregrip (swapped from Imperial w/disc brakes) rear axle. Can this be right???

I could whip up a spread sheet real quick to confirm, but would need to know OD of tires and 3rd gear ratio of transmission

Sent from my SM-G900P using Tapatalk

 

[NateCharger]

The tires are 245/60/15

 

[terzmo]

see the link....says 83 mph at 3400

 

[SlinktRR]

Converter slip? The calculator with 26.6" tire and 3.23 is 3000 rpm at 70mph. What stall converter do you have.

 

[MT_Mopar]

Mine is at 3200 rpm @ 70-72 with a 255/60/15 with a 3:73 rear gear.

Without calculating yours seems on the too high side of things for a 3:23 gear set.

 

[Badvert65]

Your tires calculate to 26.6 inch diameter. And this gives the following results from WallaceRacing.com:
3.23 gear-> 85.75mph
3.54 gear-> 78.24mph
3.73 gear-> 74.26mph
So, if you are certain of the gear ratio (and speed), you must be experiencing a huge amount of transmission slip.

 

[440+6]

Or your tach is reading high, I know mine does.

 

[Badvert65]

Or your tach is reading high, I know mine does.

Good point.

 

[hunt2elk]

Your tach will read off it is a factory setup and you converted the car to electronic ignition.

 

[NateCharger]

That is the one thing I haven't confirmed, is the tach. And the speedo "bounces" 10mph higher than the GPS speed. Anybody know why the speedo "bounces"?

 

[A383Wing]

3rd gear ratio of our auto transmissions is always 1:1....we do not have overdrives in our cars

 

[Moparsmitty]

Your combo is similar to mine and at 70 mph I'm turning just under 3K RPM so I'm guessing that the tach is way off.

 

[monaco66coupe]

My stock 69' 340 Dart with 3:23 and E-70x14 runs 3,000 at 70 MPH.

 

[bee71]

My stock tach reads 1k higher as it is old and the speedo bounces at 55-65. Pretty much the norm for aged gauges. Doing a full resto on the car, not sure if I am going to go electronic on the VL and tach or just try to re-calibrate the tach.

 

[Sixpactogo]

That is the one thing I haven't confirmed, is the tach. And the speedo "bounces" 10mph higher than the GPS speed. Anybody know why the speedo "bounces"?

I would bet your tach is the culprit on the RPM issue. The speedo bounce is one of two things. 1- The cable is dry or kinked. 2- The speedo head is dry and needs to be cleaned and serviced. I would bet on #2

 

[petieg383]

Are you actually using the factory in-dash tach or an aftermarket tach? If its an aftermarket, you need to make sure it's set to work with an 8cyl engine, not a 4 or 6. That would throw it off too.