ive herd much controversy on the subject, from as little as 17% loss to as much as 31%, depending on the transmission...
i know the loss will vary from car to car, but if you can, tell me what you know. thanks

Thats pretty right actually maybe a lil higher up to 33%. It more depends on your sprung weight on the wheels and to a larger extent the diff itself. A ford 9" diff requires round 50 horsepower just to turn it... add to that a set of 20" chrome rims... starting to get the picture?

so the loss would be minimal in such a small/light car such as a datsun 1200, right?

hmmm maybe but then theres a traction problem, its hard to find the right balance

Now that is the key... My dad always lectured me on it that a fast car will be built around 3 principals:

1- Balance
2- Weight
3- Power

If those 3 things arent juggled properly, it wont turn, handle or go as fast as it should. Its this simple formula from 1-3 (1 being the most important) that i have always modded my cars around. :)

Too much power is usually a disadvantage in most motorsports. I know that wen i used to rally my old torana that i had MORE horsepower with the quad downdraught webbers with trumpets than i did with the 4 barrel holley i ended up using. But the difference was i had more torque with a holley and it was more TRACTABLE.

But yes the loss is minimal in a datsun but it is also relative to the power output of the engine.

no point having power if you cant get it to the ground to use it (drift cars excepted but for most applications especially the road)

simply put, unless you have an uncontrolable need to abuse your car merilessly, (As some of us do) then going overkill in your gearbox and diff choice will make you slower. I've known a few blokes who have raced with 1200's only using the four speed box. And as it is possible to race without abusing the driveline, they last suprisingly well.