RPM at 60mph?

70’ b-body, 440 with an 833 (1967), 3:55 and a 28” tall tire. The online calculators show I should be turning 2500-2600 rpm at 60mph, I’m turning 3000rpm. I’ve confirmed the cars speed with a gps so not sure what’s up with my combination. What rpm are you folks turning using a similar set up?

That's the RPM for a 4.10 with a 28" tire.

My Challenger with a 383/727/3.55 runs 3000rpm @ 60mph. I have 255/60/15 rear tires, not sure the diameter. I thought it kind of high myself. Not sure if the tach is not reading correctly, or torque converter slipping some? I verified my gears when I had the case out, thinking the gears may have been changed, but they were correct to the tag.

My Savoy with 4.10 gears and an 833 4 speed and 28" tires runs 3300 at 60 MPH

Gus

4g needs a 5g to keep the Rs down !

255/60 is a 27" diameter, giving 2660 RPM with no slip.

Verify the axle ratio if you didn't install them yourself. 275/60/15 tires?

If the axle and tire diameter check out, it's time to check your tach calibration.

Kevin

That’s sure what it sounds like 300rag! I assembled the 3:55 pig myself but the shop installed it, think I will jack it up and spin the wheel for a rough check. They could have put in the wrong one😳!

Correct Kevin, 275/60 15 tires. Tach calibration is a good call, freshly restored dash/gauges but it sure sounds like it’s buzzing high.

I checked yesterday to make sure where it is with my combo.

'68 Barracuda convertible, 408 Magnum based EFI's motor ((382hp & 525 ft lbs of torque at the wheels)), Stage 2 Extreme Automatic's 2004R transmission with a 3800 rpm lock-up torque converter and 3.91's turning 28" tall 275/60-15 Nitto Drag Radials. It spins 1950 rpm at 60 mph and gets 17+ mpg with the top down.

The 2004R is the best transmission I've ever had the pleasure of owning. With the lower 1st gear ratio, it's the equivalent of having 4.30 gear ratio out of the hole. The car only spins for 10 feet and then dead hooks before it goes through the rest of the gears. It gets up to 60 mph pretty quick too.

Man, I love that Cuda

I was turning 3100 rpm at 60mph with 3.91's, 26's and A833, so, I think you are in the ballpark.

Life is better these days with overdrive and 3.23's: 1700 rpm and + 4 mpg. No loss of acceleration either. Not sure why a 426 needs Slant Six gears to perform?
.

Because the most affordable way to put an OD manual trans behind a Hemi is the A833. All other options are multiples in additional cost.

3000 RPM with 3.55 gears and a 28 inch rear tire is a little over 70 mph. So one of your numbers is wrong. Not sure which but something in your equation is wrong. You were in fourth gear correct? 3000 rpm at 60 mph with a 28 inch tire requires more than a 4.10 gear. I have 4.56 gears in my Duster and it turns 3000 RPM at 60 mph with a 31 inch rear tire.

I was talking about the "need" to have 4.10's with a Hemi.

Thanks Andy, here’s the update. Haven’t been driving much lately but was finally able to get out in the car. Tried the gps again, turns out at 60mph gps shows 65mph so speedo is under reading by 5mph. When I slow to 55mph (60mph gps) my revs are at 2800rpm, seems a little better. I’m using the yellow 33 tooth speedo gear which I thought was correct for 3:55 gears, may have to change that. Next thing to check would be the accuracy of the tach, anyone have any idea how to I would go about that??

The best way would be to use a square wave generator. Sometimes technical colleges have them for electrical classes. If you are handy electrically, there are gobs of articles on the web how to build one using an op-amp but I'd see if I could borrow or use one before I resorted to that.

3000 rpm with 4.10 & 31 inch tall tires is 60 MPH.