Chris, sounds like your syncro's have failed. They are replaceable and it is a big job. All the clutch does is enable the vehicle to go in to and out of gear smoothly. That is the basic explanation. Are you saying when you reach 170/kph you can't put it in to reverse (joke) it would kill you and your car if you did. Or do you mean the car has 170km on the clock? All your clutch will do is slip, so when you accelerate it you will hear a lot of engine revving with no speed increase. Or taking off on a hill, will be a nightmare, car will roll backwards with lots of revving. If it is your throwout bearing, you will just hear a lot of dry whirring noises coming from the centre of the car. To test if it is the clutch. Find a small hill, pull up and put on your handbrake. Then engage your clutch but no accelerator, clutch only allow it to idle up the hill. If the car does it but pops out of gear, your syncros are likely shot. If the car slips and either goes nowhere or rolls back, your clutch it shot.