Basically, because I'm building the car the way I want to build it, and for it to fit in with my life, where I live, and the practicality of where/when/how I will be driving - and not to fit in with what other people want for their cars. It doesn't have to make sense to anyone else but me. It may be the only car I ever build, so I dont care about trivial things like trans tunnels, wheel tubs, etc. There's more mods than that going on, believe me! The only original component of the car when Im finished will be 90% of the body panels. The rest, inside, under and all around, will be new or altered. What I want is a motor that will rip the tires off in several of the early gears, plenty of available power when shifting down to 4th gear to overtake multiple cars (4-8 cars) on single lane highways in a few seconds, and a top gear that will then allow for a good cruising speed at lower revs to save on fuel for longer trips (and from my lead foot when overtaking). So I've got the engine and power, I've got the gearbox to cope with the power and provides the double-overdrive I prefer for longer distance fuel economy (5th can be used as the primary overdrive for turning/twisting roads, while 6th is good for freeways/double lane highways). So, the right diff ratio will be important to achieve all of that - KNOWING - that anything I do will be a compromise due to the engine power - which normally isn't used as a daily driver, or is primarily a city-based vehicle and therefore making a true overdrive gear irrelevant to them. From all input I have received, no particular ratio will suit perfectly for any scenario, so a compromise situation looks best around the 3.9 to 4.11 ratios to achieve closest to what I want, depending on the final tyre and wheel choice I make. Thanks to everyone's help, I now know the contributing factors in evaluating the best diff ratio for me (and a 3.08 was waaaayyyy out!) and what I want, and where I will need to make the necessary compromises to satisfy me (probably highway revs, but not too much), while leaving plenty of usable power for, occasionally, being a little bit silly and having a bit of fun.