Dodge Diesel - Diesel Truck Resource Forums - Gear ratio questionI have a 1996 Dodge Ram 2500HD SLT (2 wheel drive) with the Cummin

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Gear ratio questionI have a 1996 Dodge Ram 2500HD SLT (2 wheel drive) with the Cummin

I have a 1996 Dodge Ram 2500HD SLT (2 wheel drive) with the Cummins turbo diesel and 47RE automatic transmission with a DTT valve body. The engine has been modified to produce approximately 325 hp and 800 ft/lbs. Can you tell me at approximately what RPM does the torque converter lock up? I ask this because I am looking at changing out the differential gears from 410 to 354 and want to make sure that I am not turning too low of an RPM that can cause damage to the transmission or other drive line components.

Currently, I have replaced the 235/70 stock tires with 285/70 tires. With the 410 gears here are the RPMs I turn in OD with torque converter lockup:

1,500 rpm - 48 mph
1,750 rpm - 56 mph
2,000 rpm - 66 mph
2,250 rpm - 76 mph

I have been told that the gear replacement will drop my RPMs by about 400 RPM. Will that be too low of an operating RPM? I am looking to improve my fuel mileage on the highway at 60 mph and above.

In your honest opinion, would this be a beneficial or recommended swap?

Thank you for your time,
Mike

I run a nv5600 now...which is a slightly higher (lower numerically) od than the 4500....with 3.54 gears....and recently went to 19.5's and a 33.4 inch tall tire.

My mileage went up....My truck is highly modded thou. But the higher gear lowered the cruising boost....I cruise down the road (towing 6k or more) at 1600 to 1900.....and with the lower boost the afc is not adding as much fuel. I think Im going to ad a air valve and second boost gauge in the afc line to taylor the afc signal cruising.....

If you dont make enough power.....the higher gear could make it go the other way, i think, just depends on how you use your rig.

Mine is all stock except for 235-85 tires ( 33") and I run about 1750 at 60-62 MPH, which for my truck is optimal.

From what you posted, unless you are running very heavy in the hills, I think the 3.54's would be a good fit.

Since you are a D70, why not just get a whole rear instead of changing gears?