Originally Posted by x'ed
im not expert, but i know in a stick shift, shifting to first at 20mph in anything short of a ferrari is bad news. My friends wrx refuses to go into 1st at anything about 5mph. In my car, downshift and 20mph will never happen. I remember putting my old grand cherokee 4.7 in first from about 20mph and it felt like i hit a brick wall. Its not about the motor being able to handle it, its about the transmission handeling it. But like i said, i don't really know much on the topic, that's just my guess.
In a manual transmission car, you can downshift into a gear as long as its not going to throw you past redline. In other words, if you can accelerate to 25mph in first gear, you can downshift to 1st gear once your speed goes below 25mph. Now of course you better not let the clutch out quickly or else you will "feel like you hit a brick wall". To prevent this, you have to tap the gas just before you downshift to match the RPM's to the gear and speed. If you don't have the RPM's matched perfect you're going to be going for a bucking ride. The idea of downshifting is to use the engine for its braking ability.