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Best Axle ratio for soft conditions


TPH25ND
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I'm looking at buying a used Mack with MP8 and Mdrive. Probably a CXU613.  I'm looking at doing in state hauling up to 105,000 lbs, but also need the ability to bring a 90,000 lb load out of a soft field.  Most used trucks a see are 2.47 and 2.60 axle ratios and 3.08's.  I know the 2.47's and 2.60's are no good for what I want.  What ratio's would be the best for what I want to do?  Is 3.08 adequate or do I need to be at 3.25, 3.58 or more?  How about overdrive vs direct transmissions?  Most I see are overdrive.  The price range I'm looking at, doesn't allow me to look at the 13 or 14 gear mdrives, just the 12 speeds.  Any advice would be appreciated.

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18 hours ago, TPH25ND said:

I'm looking at buying a used Mack with MP8 and Mdrive. Probably a CXU613.  I'm looking at doing in state hauling up to 105,000 lbs, but also need the ability to bring a 90,000 lb load out of a soft field.  Most used trucks a see are 2.47 and 2.60 axle ratios and 3.08's.  I know the 2.47's and 2.60's are no good for what I want.  What ratio's would be the best for what I want to do?  Is 3.08 adequate or do I need to be at 3.25, 3.58 or more?  How about overdrive vs direct transmissions?  Most I see are overdrive.  The price range I'm looking at, doesn't allow me to look at the 13 or 14 gear mdrives, just the 12 speeds.  Any advice would be appreciated.

Pardon my ignorance - When you say soft field, do you mean unmaintained surface, loose dirt? mDrive OD has a starting gear ratio of 11.74:1 and can be used to haul 90,000 to 105,000 lbs with ratios around 2.79 - 3.08 for well maintained roads. The biggest issue you will face when in loose sand is not so much drivability but these relatively fast ratios will not have much mechanical advantage at the wheels. During wheel slip situation, there is more potential to have higher torque be sent into wheels and if a wheel-hop happens tremendous amount of torque spike goes through the system destroying axles, thru-shafts, axle-shafts, differential halves and drive-shafts. Improper use of diff. locks, power launch or rock-free function makes it even worse. A deeper ratio (3.56 and above) will help alleviate the situation but not fool-proof. 

Deeper ratio comes at the expense of your sweet spot at 65MPH being 1500RPM range and above leading to lesser fuel economy. Direct Drive will  seriously compromise top-speed though the starting gear ratio of 14.94:1 will help with startability in off-road conditions.

 

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