Axle Ratio

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#11

08-05-2010, 08:20 AM

Aufgeblassen

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Join Date: Dec 2008

Posts: 168

Quote:

Originally Posted by Jackrabbit379

A friend of mine has a 359 with 3.55 rears. 475 CAT, with a 15 behind it. It's cool driving about 72 and the motor is running about 1600.
Feels like you are going down a country road in a Cadillac.

My new truck should run about 1550 RPM @ 72 MPH (not that I EVER run at that fuel wasting speed!!!). :smokin:

#12

08-11-2010, 12:09 PM

Aufgeblassen

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Posts: 168

Update:

I got my new (actually a 2006) Volvo 780, and the 3.42 axle ratio vs. the 3.58 of my old truck lowers my RPMs from 1340 or so to an even 1300 RPM @ 60 MPH. :smokin:

#13

08-12-2010, 04:19 AM

Aufgeblassen

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Quote:

Originally Posted by Mackman

Get real trucks with 3.90s or 4.10s :thumbsup:

I know you guys are talking OTR

Yeah, real dumb trucks!

No, I run regional/Southeast only, and am home EVERY weekend. Pretty much just Florida & Georgia.

#14

08-13-2010, 09:13 AM

terrylamar

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Join Date: Apr 2006

Location: Austin, TX

Posts: 1,567

Quote:

Originally Posted by Shnerdly

Actually, it's the opposite. The larger the number, the higher the RPMs of the engine will be.

Copperhead has it right. The way it is usually talked about makes it kind of an odd concept.

When people say they are lowering their gear ration, the are actually going to a gear set that has a higher number but the reference to a lower gear ratio means a lower top speed.

If you have 3.42 gear ratio, your drive shaft will turn 3.42 revolutions to one revolution of the wheel. If you have a 3.58 gear ratio your drive shaft will turn 3.58 revolutions for one revolution of the wheel giving you a lower top speed and slightly higher RPM if all other factors are the same.

The other factors are tire size being 22.5 or 24.5 and low profile or not. Also the transmission matters. Some are direct drive meaning the output to the drive shaft is equal to engine RPM's in high gear. Some are overdrive meaning the drive shaft is going slightly faster then engine RPM's in high gear. There are even some underdrive transmissions out there though they are not very common.

Yes, Yes, I understand this concept, my brain was in the wrong gear!

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#15

08-15-2010, 01:41 PM

Steel Horse Cowboy

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Join Date: Jul 2010

Location: NW Indiana

Posts: 778

Quote:

Originally Posted by Mackman

Get real trucks with 3.90s or 4.10s :thumbsup:

I know you guys are talking OTR but if it makes you feel any better I got 3.90s with a 13speed. Turn around 1550-1600 rpm at 60

My 99 FLD has 3.90's, a direct 10 speed, and 22.5LP's and at 1550 im going 65mph ???

Guess your final drive must be higher than mine.

Anyways, I keep it about 1400 and go 60mph in the right lane and have avg'd 8.2mpg for the last 2 years (pulling light dry bulk)

#16

08-15-2010, 01:44 PM

Steel Horse Cowboy

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Join Date: Jul 2010

Location: NW Indiana

Posts: 778

Quote:

Originally Posted by Aufgeblassen

Yeah, real dumb trucks!

No, I run regional/Southeast only, and am home EVERY weekend. Pretty much just Florida & Georgia.

Not true....... I just sold one of my 2005 IH that had 3.23 rears with a 435 C-15 and at 75mph it was at 1450rpm. Got 6.3mpg....... I'm doing the same run in my FLD now and getting over 8mpg with the 3.90's

I work 4 days a week, Chicago to Corpus Christi and back.