65 V8 with C4 - Correct RPMs?

limey66 · Jun 7, 2011

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 · Jun 7, 2011

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 · Jun 7, 2011

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 · Jun 7, 2011

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 · Jun 7, 2011

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 · Jun 15, 2011

Sounds like the tranny is locked in 2nd gear.

robbz28 · Jun 15, 2011

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 · Jul 3, 2011

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 · Jul 3, 2011

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 · Jul 3, 2011

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 · Jul 3, 2011

Rmoore45 said:
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 · Jul 4, 2011

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

2+2GT · Jul 4, 2011

Rmoore45 said:
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 · Jul 4, 2011

2+2GT said:
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 · Jul 5, 2011

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 · Jul 5, 2011

woodsnake said:
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

65 V8 with C4 - Correct RPMs?

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