Vadim. You covered the subject well. The T425 with a 3:07 FD, 14 inch tires, and a 4,000 lbs car was a good combination. GM had the peak HP for the 427, and 455 reached in the 3,600 to 4,200 RPM range. So higher RPM's in the 4200 range, and above isn't an issue. We have 16 inch tires, and 12,000 lbs as you covered. And a different cam profile. That's another can of worms. Till about 2003 we only had the 3:07, and 3:21. I think there was a 3:42 from Cinnabar? So the Power Drive was a way to get the RPM's up to get out of luging our engines years ago. But as you mentioned. The PD is adding more load on the T425. Then Jim K at Applied GMC came out with some great FD ratio choices up to a 4:10 ratio. These ratios ended the luging. And allow the engine/trans to work more efficiently. Some say the PD,or a lower FD ratio are the same. No. Not at all. Is the PD bad? No. It's better then nothing. In your situation. I'd leave things as they are till you have trans issues. Then change the FD ratio. I'm thinking of going to 4:10 from my 3:70. Towing, or not. I'd recommend 3:70. When I'm pulling my enclosed trailer with a car in it. A 78 Royale 403, and the 3:70 FD. My GVW is about 21,000lbs. As a side note. My mileage went up with the 3:70. As to newer engines at 1500 RPM's at 65 MPH. Their design is another kind of animal. Bob Dunahugh Member GMCMI.
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