In 1971? I’d believe a car company (and the average customer) would find it unbelievable at how much of the cost would go into pollution controls and active seat belts and passive restraints.
But it would have another effect: In 1971 there was a lot of research going on into dead-end technologies. Just in engines there were: Wankel engines, external-combustion engines (stirlingengine.com/faq), gas-turbine engines. Just knowing that the engine of 2020 would still have pistons connected to a crankshaft would have redirected research funds.
This is a great thread, with a lot of avenues of thought possible.
I’m hearing the OP’s request as “Take an invention back in time to help humanity…” with an additional implied condition that the invention itself can stand alone and provide help beyond just the device itself.
For instance, if you were to take a modern automobile (or one up to about the 1970’s that is almost completely mechanical) with a complete toolkit in the trunk, then upon arriving, you wouldn’t just demonstrate your ability to drive around on existing roads until running out of fuel, you’d take it apart and show the refined materials involved, and draw sketches of the machines needed to build such devices, then the planning ability to put it all together in a consistent, repeatable manner. You’d need to carry with this invention the whole idea of assembly lines and supply chains, plus advanced materials like polymers, to boot.
A cell phone or GPS receiver would be pretty useless by itself, as your device represents just the tip of the iceberg of a much larger, more complex system.
Perhaps the best idea I’ve seen here is to take with you a disease-resistant, high-calorie, easy to grow plant such as a potato, to a place where it could best be used. A living thing that you can eat carries with it the ability to make more, and “stands alone” much more easily than a technological device removed from context.
How about bringing back with you a modern way of thinking? I believe technology and much of its ability to become widespread and continuously improved, got its start during the enlightenment. Then, social and economic conditions gave rise to a swelling tide of scientific curiosity, great advancements in math, philosophy, experimentation…something you need to get from where we were (pre 1600s) to where we are now (modern era). Could you carry with you an “infectious” philosophy that catches on before its time and converts everyone into a more modern framework? Maybe. It would have to be very carefully calibrated and timed.