Is this a bad idea?

72Duster440 said

Wouldn't the end result still be the same? The 3 speed and 4 speed's final ratio is 1:1, so the rpms wouldn't be any different when in top gear. All you would be gaining is an extra gear 'in the middle'. Unless I read your post wrong...

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The automatic trans will consume more energy (fuel) due to converter slippage. Lock up transmissions have an edge over non lock up transmissions and manuals have an edge over LU converters. The power is more directly connected. This changes as the transmissions become more modern. But the old school transmissions that we use, the above applies.