Cruising RPMs

Austin · 10-28-2005, 12:55 AM

I'm ordering a Dynatrac Dana60 axle and new gears for the front diff - trying to decide between 4.56 and 4.10 gears.

What are your cruising RPMs in 5th gear (converter locked) on the highway at 60mph or at 70mph?

bestatchess · 10-28-2005, 06:07 AM

That sounds delightful! Where are you getting the gears for the front differential?

The Titan's tranny is:

1st 3.827
2nd 2.368
3rd 1.520
4th 1.000
5th 0.834

For Big Tow with stock gearing:

4th @ 65 mph = 2300 RPM (from Boat & Trailer 2004 towing vehicle article)

5th @ 65 mph = 2300 x 0.834 = 1918

5th at 60 mph = 1918 x 60/65 = 1770

5th at 70 mph = 1918 x 70/65 = 2066

If you have bigger than stock tires, you would have to multiply in that ratio also.

For example, with 37" tires at 70 mph = 2066 x 32.7/37 = 1826

With a 4.56 rear and 37" tires at 70 = 1826 x 4.56/3.357 = 2480

With a 4.1 rear and 37" tires at 70 = 1826 x 4.1/3.357 = 2230

Using the bigger tires gives a ratio of 32.7/37 = .883 is almost the same as the 5th gear overdrive of .834 so it is like you are already in 5th gear with stock tires when you are actually in 4th gear. (Maybe that is why you are asking about RPMS in 5th gear?)

To counter the effect of the 37" tires, you would need the reciprocal ratio of 37/32.7 = 1.13

4.1/3.357 = 1.22

4.56/3.357 = 1.358

So either axle ratio leaves you with an effective gearing lower than stock.

Your crawl ratio compared to OEM would be:

33.35 x 4.10/3.357 = 40.73 or

33.35 x 4.56/3.357 = 45.30

corrected for tires = 40.73 x .883 = 35.96 or 45.30 x .883 = 39.99

The 45.30 (if you used stock size tires) would be getting into Jeep Rubicon w/ auto tranny territory.

What would be really nice is a 4 to 1 transfer case!

Austin · 10-28-2005, 07:58 AM

Quote:

Originally Posted by bestatchess

For Big Tow with stock gearing:
4th @ 65 mph = 2300 RPM (from Boat & Trailer 2004 towing vehicle article)
5th @ 65 mph = 2300 x 0.834 = 1918
5th at 60 mph = 1918 x 60/65 = 1770
5th at 70 mph = 1918 x 70/65 = 2066

Thanks Best. I've already done all the math - just wanted to make sure my initial RPM assumption was correct.

I think I'll end up going with 4.10's. If I did 4.56's I'd be just over 2650rpm at 75mph, which would make it less of a highway rig. 4.10's drop that number down to around 2400rpm, which is probably going to make quite a difference in fuel economy ('cause, ya know, that's why I bought the Titan... to get good mileage...

).

Quote:

Originally Posted by bestatchess

That sounds delightful! Where are you getting the gears for the front differential?

Same place - Dynatrac. There is another company up here in my neck of the woods working on it (Randy's Ring & Pinion), but Dynatrac will likely get done with it first. They're saying they're pretty certain they can get a high pinion Dana 44 to fit the front case with only slight modifications. I'm jumpin' on it.

It'll be nice to have ARB's front & rear.

Quote:

Originally Posted by bestatchess

Using the bigger tires gives a ratio of 32.7/37 = .883 is almost the same as the 5th gear overdrive of .834 so it is like you are already in 5th gear with stock tires when you are actually in 4th gear. (Maybe that is why you are asking about RPMS in 5th gear?)

Obviously not why I'm asking - buy you're correct - in 4th gear, my truck acts as if it's in 5th gear with OEM sized tires. That makes for a really pissed truck when I actually let it shift into 5th gear - it just doesn't like it. I usually keep it in 4th, so it'll be nice to have 5 forward gears again.

Quote:

Originally Posted by bestatchess

What would be really nice is a 4 to 1 transfer case!

Atlast makes one - www.advanceadapters.com - but I don't think they're quite ready to offer it to Titans yet. They were a couple months away last time I talked to 'em.

TROGDOR · 10-28-2005, 08:36 AM

I'm a little confused - I've got BT and OR with the BFG stock OR tires ... my rpms at 70 mph are a shade over 2000 - say 2050 or less. How does that fit with this:

Quote:

Originally Posted by bestatchess
For Big Tow with stock gearing:
4th @ 65 mph = 2300 RPM (from Boat & Trailer 2004 towing vehicle article)
5th @ 65 mph = 2300 x 0.834 = 1918
5th at 60 mph = 1918 x 60/65 = 1770
5th at 70 mph = 1918 x 70/65 = 2066

Austin · 10-28-2005, 08:54 AM

TROGDOR - you say you have around 2050rpm at 70 in 5th.

The numbers Best gave from Boat & Trailer are 2066 at 70 in 5th.

I'd say your numbers are pretty much equal. Where's the confusion?

TROGDOR · 10-28-2005, 10:12 AM

I think I was confused as to how the 5th gearing at 65 was higher at 2300

ecbmxer · 10-28-2005, 10:28 AM

Man this is awesome! Austin your truck will be a beast with these gears and ARB's front and rear. Let everybody know approximately how much you pay for the gears cause I bet the Stillen-Charger people will be needing that info pretty soon.

kartman · 10-28-2005, 10:50 AM

Dynatrac makes nice stuff, good call.

Austin · 10-28-2005, 10:52 AM

Quote:

Originally Posted by TROGDOR

I think I was confused as to how the 5th gearing at 65 was higher at 2300

Take another look at Best's numbers.

4th @ 65 = 2300 rpm
5th @ 65 = 1918 rpm

Austin · 10-28-2005, 10:57 AM

Anybody else - Are you at about the same number that Best posted?

Austin · 10-28-2005, 10:58 AM

Correction - I only want 3.357 gear'd people to answer.

ptgarcia · 10-28-2005, 08:46 PM

4.10's should be just about right. You'll still be turning a higher rpm on the highway but the lower final drive will help get those lead weights turning

rtapia · 10-29-2005, 08:27 AM

I have a 4x4, BT, OR Titan and my numbers match those mentioned above.

Vyrus · 10-29-2005, 08:57 AM

65MpH in 5th - ~1900 RPM

70MpH in 5th - ~2000 RPM

Numbers seem to match up pretty well.

-Cyrus

Austin · 10-30-2005, 08:43 PM

Best - the 3.36 rear end is actually 3.357, or 47/14.

Do you know what the 2.94 rear end is?