How to Determine Rear Axle Ratio on Used Trucks
 

Hey guys (and gals). New here, first post. I'm about to become the proud owner of my first Dodge AND my first Cummins. My Dad has driven Dodge diesels for about 20 years now. I've always been very impressed with their towing capacity and fuel economy, so I've decided to take the plunge. Now the question.
I'll only pull a heavy load a few times a years, so I don't feel I need a low gear ratio such as a 4.10. Obviously I can't get a build sheet on every vehicle I'm interested in as I'm doing all of my truck shopping on-line and need to know how to determine the rear axle ration from the VIN. I will be buying a year model in the 2002-2006 range.
Thanks in advance for any help.
GG

http://www.lmctruck.com/icatalog/de/full.aspx?Page=7 . Goto this website it will explain the VIN. I don't think you can get axle ratio from the vin. My 07 is a 3.73 and at 2000rpms the speedo is at 72mph. I think the 4.10's are like 2200rpms at 70mph.

Call your local dealer and give them the vin, if it is a friendly dealer they wont have problem printing out a spec page. I also work at a dealer if you send me a message with the vin in it I can look up the specs as well.

If it's an auto and you take it for a test drive, at 72 MPH you should be turning 2000 RPM's if it has 3.73's

If it has 4.10's you'll be 200 - 300 RPM's higher.

The other option, if you can, jack one back tire off the ground and make a mark on the driveshaft. If you rotate the tire once and the driveshaft goes around 3 3/4 times, you have 3.73's. If it rotates slightly more than 4 full revolutions, it has 4.10's.

Another hint, if you look at a 2005.5 - 2006 with manual trans, they only came with 3.73 gears, BUT the manual is geared lower than the auto and will rev like 4.10 gears in 6th.

Best bet is go to the dealer with the vin's, thats what i had to do.

I think that's exactly right NickBeek. I have a 2006 Ram 4x4 Heavy Duty 6 speed manual. They told me it had a 3.73 gear, but engine RPM is about 2300 at 70 MPH. This had me confused so I did the test where you rotate one rear wheel while off the ground and compare turns of the drive shaft. Best I could tell is the gear is a 3.73. The doc for the truck just showed 10.5 and it took me some reading to find out that is the diameter of the ring gear.