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2005 chevy trailblazer_ls
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Well I searched on a few websites and I found a complete rear end with 4.10 Gears. It also already has the G80 locker. It's from a 2005 TB EXT. However.. The only problem is that it is located in Va. So needless to say I wont be getting this one. But I just wanted to make sure this was possible. I know I would have to have the TB tuned for 4.10 gears. This would be the only thing I'd have to do right? Of course getting the new rear end under the truck and getting it connected to the brake lines and getting the suspension connected too. I wouldn't have to do any transmission work would I?

 

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No, you won't need to do any work to your transmission.

The nice thing about buying a whole rear axle is not having to deal with differential gear alignment and shimming. That can be a pain in the rear. Because every gear is slightly different, they have to be positioned with shims to have a perfect contact patch. That can be expensive. While trading out a rear end isn't easy, it's fairly 'bolt-on.'

Plus, if you get the EXT rear end, you'll have the option of throwing in an ARB or other selectable locker own the road.
 

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I've got 3.73 gears right now. Would the 4.10 gears make towing easier on the truck? How about mpg in just regular driving conditions?
It will increase your final drive ratio. Thus increasing torque to your wheels. So, launch and towing capacities will be improved. Gas mileage will theoretically take a negative hit because your RPMs will be in a higher, less efficient band.
 

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is he going to have a problem if the front is not the 4.10 also?
being that he is only 2wd no he wont.. if he had 4x4 then yes the front gears would have to be changed to 4.10 otherwise when he kicks in 4x4 he would shear his drivetrain somewhere along the way...
 

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If you are running taller tires, the 4.10 will behave just like a 3.73 with standard-sized rubber. Making the tires taller will effect the final gear ratio as much as changing gears.

Here's a scanned in chart that lists approximate engine rpm for different tire and gear sizes. (Edit -- drew a new and improved chart) The table below shows engine RPM in a 1:1 drive ratio (not overdrive) at 65 mph. The green is about factory stock RPM level. Yellow is under-geared, which should give better economy while the red is over-geared, which should give better performance. Going much above the red or below the yellow will drastically reduce performance.

To use this table to factor gear and tire sizes, start by deciding the RPM that you wish for the engine to turn at the preset 65 MPH, then the tire size you wish to run, then figure out gearing (or gearing, then tire size). Note that our overdrive will slow all these RPMs by about 25%, so take off 25% from the RPM number in the table for your actual engine RPM at 65. Also consider if that RPM means that the engine will be lugging at speeds under 55.

As a for instance, let's say that a guy with 3.42 gears is running a tire 33" tall. His engine at 65 will be turning 2263 RPM in "3" (not OD) and in overdrive, it will be 1775 RPM. At 55 this will be 1500 -- pretty low, and actually lugging the engine. (formula for factoring speed difference is RPM over speed (1775 over 65) times RPM over calculated speed (X - [RPM to discover] over 55). Cross multiply known RPM by desired MPH and divide by actual MPH, which will give the new RPM for the calculated speed.

It would look like this:

1775 X
65 55 (1775 X 55) divided by 65 = X

 
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