RPM too high at cruising speed

[Hilljack68]

My Coronet has a 340/904 and a 3.55 rear end. All three have been recently rebuilt. The engine is strong as a bull and the tranny shifts just fine. But... It seems I'm revving too high at cruising altitude. Unfortunately, I don't have a tach, so I can't give you numbers. I had the exact same setup in a '69 Charger years ago, and it was very driveable at highway speeds.

A very reputable rear end guy in the area did the 8 3/4, so I don't suspect that. Is there a chance there may be something amiss in the tranny? I don't want to go with an overdrive unit if there's something that needs addressed in the 904. Any thoughts? Thanks.

 

[69 Sleeper Bee]

Your going to need to hook up a tach to be sure and go from there.

 

[Mike67]

Taller tires, slower cruise, different gear.

 

[khryslerkid]

. I had the exact same setup in a '69 Charger years ago, and it was very driveable at highway speeds.

340/904 in a '69 Charger. (Or small block ?)

You do feel it shift into third gear? 3.55 will increase the rpm noticeably. Are you sure your '69 had 3.55s ?

 

[HEMI-ITIS]

Verify the present gear by jacking the the tires off the ground, See how many times the driveshaft turns with one revolution of the wheel. 3:91 the DS will turn almost 4 times, 4:10 would turn a little more than 4 times.

 

[Hilljack68]

You do feel it shift into third gear? 3.55 will increase the rpm noticeably. Are you sure your '69 had 3.55s ?

Well, I believe they were 3.55s (it was almost 30 years ago). Yeah, it does hit all three gears - nice sharp shifts.

 

[Hilljack68]

Verify the present gear by jacking the the tires off the ground, See how many times the driveshaft turns with one revolution of the wheel

Yeah, that's on the menu for this week, but I'm pretty sure they're 3.55s. The Charger... not so sure.

 

[WileE]

As stated, you'll need to use a tach, and establish your actual gear ratio. Only easy quick fix would be larger diameter tires.

 

[Hilljack68]

So, when all's said and done, there's no way it could be a tranny issue?

 

[khryslerkid]

So, when all's said and done, there's no way it could be a tranny issue?

If under load in high gear and the rpm increases then yes it could be slipping.

 

[1 Wild R/T]

What mufflers are you running? What mufflers did you have on the Charger?
What Air Cleaner? What fan setup?

Often it's not so much the RPM's it's the noise you perceive.... A 340 is perfectly happy spinning 4000 rpm's which when using a 26.5" tire is 89 mph..

 

[diesel_lv]

Here's my favorite rpm/speed calculator. But you definitely need a tach to verify rpm, even if it's just laying in the seat next to you while you diagnose it.

https://www.reillymotorsports.com/speed-tire-rpm-calculator.html

 

[4mulas]

You’re revving too high but you do t know what rpm;s you’re doing?

Need more info.

 

[Kern Dog]

Earlier this year, I swapped out the automatic in my Charger for a Tremec 5 speed overdrive. Here is the difference in cruising rpms. I also have 3.55 gears.
First is with a regular 3 speed 727 automatic that has the same ratios as the 904:

2800 rpms at 60.
3000 rpms at 65.
3200 rpms at 70.
The above was the rpms with the automatic in drive, 3rd gear at speed.

I went out a bit further and measured the rpms again but in 5th, a .64 overdrive. This takes the 3.55 to a 2.27 final drive. The rpms are approximate but pretty close.
1650 rpms @60.
1800 rpms @65.
1975-2000 rpms @ 70.
Yeah....what a HUGE change!

The .64 overdrive drops the rpms by 1/3. That makes a dramatic difference in how smooth the car runs and feels at freeway speeds.

 

[hunt2elk]

What size rear tires are you running?

 

[493 Mike]

You’re Erving too high but you do t know what rpm;s you’re doing?

Need more info.

Man I like the "Erving"! Can I use that?
Mike

 

[Hilljack68]

What mufflers are you running? What mufflers did you have on the Charger?
What Air Cleaner? What fan setup?

Flowmaster 40s. Not sure on the Charger, but they exited in front of the rear wheel, so they were loud. Air filter?.. not sure why that would matter, but a 14x3 cotton gauze.

What size rear tires are you running?

255/60/15 27" tall

 

[Mike67]

Just a long shot but have you recently changed your converter? Do you know what its stall is?

 

[747mopar]

With 305/35/19's I used to run 3,500 rpm on the freeway with 3:55's, it sounded like it was honing along pretty good. Flowmasters and side exit = noise lol.

 

[1 Wild R/T]

Flowmaster 40s. Not sure on the Charger, but they exited in front of the rear wheel, so they were loud. Air filter?.. not sure why that would matter, but a 14x3 cotton gauze.

Because an open/unsilenced air filter makes a lot of noise.....

Direct drive fans make allot of noise too.... As do Flowmasters....