Why is it almost impossible to put the car in first gear at high speeds?

1. Why doesn't the transmission allow me to put the car in any gear I want (e.g. first) at any speed I want?

2. Is the transmission (including the synchronous gears) disconnected from the flywheel when the clutch is pressed in?

• I assume they are disconnected because I can feel the synchronous engaging when I lift my foot off.
3. If they are in-fact connected, why do downshifting methods (e.g double clutching) exist then?

There are physical limits to sychronizers.

When you upshift the synchronizer slows the clutch down which is relatively easy. When you down shift the synchronizer has to speed the clutch up which is much harder. Going down just a single gear is not that bad. When you're on the highway, shifting into first is not possible because you're asking the clutch to speed up to well over 10k rpm. That kind of speed is difficult to achieve if not impossible from just a little puny synchronizer. Further the clutch may come apart from that kind of speed.

You miss understand how a synchronizer works.

The power train is broken up into 3 pieces; (engine up to and including the flywheel), (the clutch - input shaft - counter shaft - constant mesh gears) and (synchronizer hub-shift sleeve). When you push the clutch pedal the flywheel and clutch disc are disconnected. The sifter is connected to the shift sleeve. When you shift the shift sleeve is slid over by the shifter. The sift sleeve pushes on the struts which in turn push on the blocker ring. The blocker ring pushes agains the synchronizer cone and through friction causes them to match speed. When the speed is the same the shift ring slides into place and engages the dog teeth locking the transmission into gear.

The friction between the blocker ring and synchronizer cone is how a synchronizer works. That friction is enough to change the speed somewhat but not a lot.

• But that does not answer my question, should not the synchronizer gears be disconnected from the drive shaft when the clutch is engaged? Commented Oct 23, 2016 at 2:18
• @method i updated my answer to include how a synchronizer works. Commented Oct 23, 2016 at 2:50
• Thank you very much for that, but what is making the constant mesh gear rotating if the transmission is disconnected? Is that part also why it is not engaging into the gear? sorry if my questions sound stupid Commented Oct 23, 2016 at 2:59
• @method The clutch disk, input shaft, counter shaft, and constant mesh gears rotate as one assembly. The transmission then taps off power from the selected gear. The speed of this assembly needs to match the speed of the output shaft before the gear will engage. Commented Oct 23, 2016 at 3:27

Well you can definitely shift to first gear even when cruising at over 100 mph. But the consequences can be disastrous. Firstly your wheel rpm and the engine rpm are same(1:1 ratio) ,considering it as top gear. Now when you are trying to put the gear into first, the gearbox input shaft rpm will rise to a very high level and when u engage the clutch back there would be excessive friction wear and the engine tries to provide resistance to the drive-line. Because of this your car may skid out of control. Also by doing so, u may end up reducing the life of the clutch facing to a great extent and sometimes it may cause complete failure of the clutch friction plate.