Phoenix8990

I have a 69 Cutlass with a 455, TH 400, 3.23 posi rear. At 60 mph I am doing 3000 rpm. From the charts I have read I should be doing 2500 rpm. I have factored in the correct tire size P215 70 R14. 25.85" dia. Since the speedo is not correct I use the Waze app on my phone and assume it is pretty accurate. Is this a torque converter stall issue. Seeing as a PO has done a lot of work on the car I would think there is a good chance he changed the converter as well. If he didn't and it the one from the TH350 would that create this issue? I believe they are interchangeable but the originally set-up most likely was a lower rear end. Any ideas are appreciated.
thanks,
Steve

Fun71

I had similar results with the theoretical RPM not matching up with the actual RPM. When I measured the height of the tires on my car I found they were shorter than what the internet said they were. When I used the actual tire height, the calculated RPM was much closer and I found the converter slippage was around 300 RPM.

joe_padavano

As noted, tires deflect under load, which changes the effective radius. The rolling radius is what matters, not the calculated one. Truck tire manufacturers publish rolling radius (or revs per mile, which is the same thing) but that info is not typically available for car tires. Keep in mind that radial tire sidewalls deflect more than those of bias ply tires, so even if you select a radial that matches the original outside diameter of an older car's bias ply tires, the rolling radius will be less and thus your rear axle will see more revs per mile than with the original tires.

As for torque converter, all converters slip unless it is a lockup converter. The higher the stall speed, the more it slips. This is why I maintain that the "right" converter stall speed for a car is the lowest speed that lets you launch the car successfully. There are countless threads asking about using a higher stall converter because some magazine article says that you should. Don't believe it.

olds 307 and 403

Unless you have a lock up converter, you will be 250 to 300 rpm above what most calculators say. I also found BFG's about 1/2" shorter than their claimed height.

Phoenix8990

It seems many people think a 3.23 rear is a pretty good all around rear. But if I am doing 3000 rpm at 60 mph as opposed to doing 70 it feels like I'm pushing things if I'm on the highway much and mpg will be pathetic. I've read about the overdrive trans but don't want to put that kind of money into my car and some people seem to have problems getting them set-up right. Either way I guess it is what it is and I might as well accept that I'll probably stay off the highway. Which seems ridiculous. But if I have a converter with a lock it would bring the rpm down? I also want to check my tach to make sure it isn't off some.
Thank you to all who take the time to respond it really helps,

Steve

Fun71

Well your short tires are contributing to the higher RPM. Put taller tires and the RPM will drop.

joe_padavano

Unfortunately, not as much as you might think. The stock tire diameter was about 26.5". The RPM drop is directly proportional to the diameter increase. Go from 25.5 to 26.5 and you drop about 120 RPM at 60 MPH (3,000 to about 2,880).

OLDSter Ralph

One thing to keep in mind.....tire diameters are measured with no load. Tire pressure and load can decrease the radius from axle center line to the ground. This changes the overall gear ratio and can affect RPM's. Bigger tires and higher air pressure does make a difference.

Phoenix8990

Hadn't given pressure much thought, I'll have to check it out, maybe tires as well over the winter.

thanks

joe_padavano

Still not that much of a difference. The pressure is what it needs to be. You can't arbitrarily pump the tires up to 40 psi. I run 32-35 in all my cars. And yes, tire sidewalls deflect, as I noted in post #3. The fact remains that the sidewall deflection is comparable for taller and shorter tires. It's still the relative difference in diameter that matters, and the difference is small. The OP has 25.8" tall tires. Change to 225/70-14s and you drop RPMs by about 3%. Make the change to 275/60-15s on the back at 28" diameter and you drop RPMs by a whole 8%.

OLDSter Ralph

Joe P, I missed post #3​​​​​​, probably due to my "notchy" mobile broadband signal.
You expressed it more eloquently than I did. I run 35 PSI in all my vehicles, but many don't. I don't recommend running tires at pressures above stated maximums on the tire.