I’m creating this thread based on a new discovery while trying to confirm speedometer accuracy as it relates to my other thread “Rear Differential in my 72 442.I took the suggestion from BackinTheGame (made in the other thread noted above) and downloaded DigiHUD and took the car out today to check the speedometer. It reads about 1 mph under the app speed. I also checked the rpm’s:



It’s a stock TH400 (other than a shift kit noted again below).

@ 55mph turned right at 3000 rpm

Regarding mention of a 3.91 below, this has not been confirmed yet. I’ve been working on calculating it in the other thread. I plan to check it by jacking the car up soon.

Sounds about right for a 3.91 I thought, based on an online calculator I used.

Previously I noted around 3700-3800 at 70 which also calculates about right with a 3.91.



So I decided to get on the interstate next. It clearly shifted twice going up the on-ramp and I merged over at about 55.



I decided to air it out, pulled the shifter back to 2nd and nailed it at the same time. Ran the car up to about 75 and shifted back to 3rd, letting off the gas and backing down to 70.



Held right at 70 and checked the rpm’s.



3200 @70 - down about 600 rpm’ from what it’s been doing consistently over the last few months since getting it running.



Note the transmission also has a shift kit. Shifts pretty firm and will chirp tires going to second.



I’ve noticed no transmission noises or obvious slipping and no other notable issues. Shifts are crisp from 1-2 and 2-3. Does not seem to overheat.



I’m thinking the torque converter may have been slipping previously (???) but when I shifted manually today it somehow got past that, if only temporarily.



I’m no mechanic. Does anyone have other thoughts about this. Could there be an issue with the shift kit rather than the torque converter? Obviously, I’ll have it checked but the shop I want to use at my brother’s recommendation isn’t open this weekend.



Thanks for any feedback anyone can give!

John

 

3200 @ 70 more than likely 3:42's, though as previously pointed out in your other thread, you are running undersized tires. Pull the diff. cover and confirm, rather than constantly guessing..

 

Thanks for replying. I plan to do that as mentioned above but my question here is why did my car go from turning 3700/3800 rpm at 70 mph to turning only 3200 rpm at 70 mph?.

I suspect a potential transmission issue and explain in detail above as to why. I’m not considering trying to figure out the rear diff ratio in this thread. Purely asking about the transmission. Thanks for any thought on that you may have.

 

A shift kit won’t affect anything other than the gear changes. What tachometer are you using to determine RPM?

 

Thanks. Did you see my question about the torque converter? It’s the factory tach.

I will say that by ear today I could hear the drop in the engine revs at 70/3200 rpm (app verified speed/my speedo is 1mph off)) as opposed to the 3700/3800 it’s been turning at the same speed for some time now.

 

What are you using for a tach? If it’s the factory tach, I’d be willing to bet that’s where the difference in cruise rpm is. The factory tachs are not accurate, or consistent.

Id suggest you stop guessing. Pull the diff cover and find the gear count so you know exactly what ratio you have. Once you have an accurate gear ratio, it’s simple math to calculate cruise rpm based on tire size.

 

Good morning, Matt. And thanks for the additional feedback. I’ll concede and agree that’s the only way to know for a fact what the rear axle ratio is and I’ll find out. I’m certainly not arguing against that and not trying to be obstinate.

The engine just sounded to my ear (ignoring what the tach indicates) “significantly less taxed” at 70 yesterday (speedo and app agreeing within 1 mph) after the hard acceleration/shifting than it has been sounding at that speed since getting it running recently after sitting for 15 years. It was like the transmission (torque converter?) decided to finally “get in line and act right.”

Anyway, maybe that’s not even in the realm of possibility - maybe I’ve imagined it but I feel like I’m pretty dialed in noticing things about the car. I’ll just get things checked out and go from there. I really do appreciate the feedback.

 

If you have the original 50 year old tach, send it out to be rebuilt & recalibrated.

 

Thanks for the suggestion. I’d like to do that soon. Do you have a recommendation for the best place for that?