As long as the Car is not moving a Photo Tachometer can do the job and they are faily cheap to buy and you can use it on anything (other vehicles) that rotates you can get the reflective Tape to stick to and have clear shot at with the beam of light.

But, that won't work if someone to drive the Car and check the rpms while doing that.

When an item is rented there is the rental cost and shipping back and forth. There is an Insurance cost if the item being shipped back is expensive enough for that.

We have more then a few People on the forum that dabble with electrical stuff. Perhaps you could sell the Plans or a kit for someone to assemble.

__________________
84 300D, 82 Volvo 244Gl Diesel