65 V8 with C4 - Correct RPMs?

[limey66]

I haven't been on here for a while, but a friend has a problem and I thought "I know who can help..."

He has a 65 289 with a three speed C4 auto box - with a 2.8 rear end (he thinks).

These are the RPMs that he is seeing:

1000 rpm = 20 MPH
1500 rpm = 30 MPH
2000 rpm = 40 MPH
3000 rpm = 60 MPH

Looks very linear to me, I would expect to see a difference in the RPM / Speed ratio as the box shifts through the gears.

Do these seem reasonable numbers to you guys or is the box not shifting properly?

Thanks fellas.

 

[Coyote5.0]

I couldn't tell you whether it is shifting right or not from that, but something doesn't seem right about those numbers. A car with a torque converter won't have a perfectly linear relationship (particularly at low speeds), even if the numbers are all for the same gear. Unless your friend somehow has a locking torque converter, it must either be a manual trans, or the numbers you gave are wrong.

 

[2+2GT]

OK, first question I always ask is what is he using for "Drive"?

The 64-66 shift pattern is:

P
R
N
•
O
L

If he's using the "•", that's rain & snow, eliminating 1st gear. The normal drive position is O.

As for trans function:

 

[limey66]

Thanks guys, that's what I thought, as the torque converter does it's work it will go from 100% slip to almost 1:1 so the numbers don't seem right.

I'll ask him to do more checks, but you can't get much wrong with speed and rpm readings...

 

[Hwyman]

Sure seems like something isn't right. 3000 @ 60 is high. The last car I had with numbers like that had 3.91 gears.

 

[68RCodeConv]

Sounds like the tranny is locked in 2nd gear.

 

[robbz28]

Is your friend certain he has the 2.79 gear ratio rear? The tag may say 2.79, but the gears could have been changed at some point in time....just a thought.

 

[sdmustangrider]

Just to bring this thread back from 2-3 weeks ago, that 3K rpm at 60 mph is VERY odd because I just put 3.40 gears in and it's turning at 3K at 60 mph. He could well be running the same gears as I am.

 

[Rmoore45]

I figure I'll add in my .02.
I have a '67 with a C4 and 2.79s, 1K rpm and 20 mph isn't quite right and 3K@60 is definitely wrong. I'm doing about 2500 rpm@ 60. The only number that seems reasonable is 2K@40 because I'm doing about 1850@40. His tach might be off or maybe mine is off.

sdmustangrider: How do you like the 3.40s? Good acceleration? I'm thinking about changing rearend gears.

 

[zookeeper]

I have 3.40's and a C4 ( just about to do a T5 swap later this summer) and think they are the perfect gears for both city and freeway. Much better than the 2.79's I had before.

 

[sdmustangrider]

I figure I'll add in my .02.
I have a '67 with a C4 and 2.79s, 1K rpm and 20 mph isn't quite right and 3K@60 is definitely wrong. I'm doing about 2500 rpm@ 60. The only number that seems reasonable is 2K@40 because I'm doing about 1850@40. His tach might be off or maybe mine is off.

sdmustangrider: How do you like the 3.40s? Good acceleration? I'm thinking about changing rearend gears.

The 3.40's are GREAT!! Can't stop stepping on the gas pedal today. haha. Added a trac loc with it and definitely can feel a whole lot of improvement. Really worth getting the 2.80 swapped out.

 

[Rmoore45]

Cool. I'm in the middle of a T-5 swap right now, so after that its probably going to be gears.

 

[2+2GT]

Cool. I'm in the middle of a T-5 swap right now, so after that its probably going to be gears.

If you have the usual 2.78, 5th gear will be just about useless.

 

[zookeeper]

If you have the usual 2.78, 5th gear will be just about useless.

How do you figure that? My '88 GT has a T5 and 2.79 gears and 5th is fine for cruising the freeways at 70+ without killing the motor. Is it a stump puller? No, but it is far from useless.

 

[woodsnake]

Rmoore45. what cam are you planning on running? That will have a lot of impact on your rear gearing. I have the Edelbrock Performer RPM, .496/.520, and with the T5, my cruising rpm's at 70 are 1800 RPM. Nice, but I have to down shift to get back on cam, to pass anybody. #.73's are very soon in my future.....

 

[Rmoore45]

Rmoore45. what cam are you planning on running? That will have a lot of impact on your rear gearing. I have the Edelbrock Performer RPM, .496/.520, and with the T5, my cruising rpm's at 70 are 1800 RPM. Nice, but I have to down shift to get back on cam, to pass anybody. #.73's are very soon in my future.....

I'm not planning on any cam swap. I think that the engine has a performance cam, but I'm not sure. Once I get the T-5 in, then I will think about other stuff. Probably going to be a borgeson conversion because my steering wheel has a good 1/8th play in it and its adjusted as much as it can be. Ok, I believe that's enough thread jacking :OT:

-Ranger