Wow that would be so illegal her in NZ. Nice to know some countries give you the freedom to do stuff like that. Here a spot of rust will take your car off the road. A broken spedo would never be allowed. Even if replaced with GPS.
Basically, if your car came with a safety feature, it has to still be there and it has to work. Car had no ABS when new? Doesn't need it. But if it had it, then it has to operate.
As for rust, it needs to be close to structural points in order for your car to fail a WOF. If you have rust near a pillar or on your firewall, there's a good chance it's worse than it looks and a safety hazard. Rust is a cancer for cars, it might not look bad at first, but it will eat your car from the inside out.
Your car basically just has to be safe, that's what your WOF is for. You can modify your car to your hearts content, you just need to get an engineer to sign off on it to say it's safe. Want to stuck a RB25DET in your old ute? No worries, just get a cert.
Then you come to Australia, where each state has a plethora of "anti-hoon" laws, banning you from having both a pod filter and a straight pipe, and other arbitrary rules to stop you from modifying your car. It's ridiculous and also inconsistently enforced, nobody seems to know what the actual rules are here.
I am not sure what you can get away with for DIY replacements - perhaps a police officer might give you the benefit of the doubt if you were polite and explained it was temporary while you are waiting for the real part, but I really suspect this would force your car off of the road at the next annual safety check/inspection (MOT test in the UK)
Drive a safe speed for the given road conditions. Match speed of other drivers. Not hard to do.
You're welcome to match other drivers - enjoy the speeding tickets you'll both get.
Exceeding the speed limit is still illegal even in places where the lack of speedometer is not illegal --- they just don't care how you know your speed.
The brainwash is thorough in this one.
There is a good reason this is illegal in most of the world. Not sure why you don't put car safety as a priority? 40,000 people die every year in the US, and over 4 million are seriously injured.