For the kind of power our 289 makes, we're able to get by with an 8-inch Ford rear end. Because we've gone with overdrive, we contacted Randy's Ring & Pinion for a Limited-Slip differential with 3.55:1 cogs, which puts the 289 at around 2,000 rpm at 70 mph in overdrive. If you go with 3.00:1 or 3.25:1 gears, this drops the 289 below its power band. If you're going with straight drive - a C4 or Top-Loader three- or four-speed transmission - axle ratio needs to be 2.79:1, 2.80:1, or 3.00:1 for optimum street performance and efficiency. If you want more pep, go with 3.00:1 or 3.25:1.