If you use too little, it won't get your clothes as clean. If you use too much, your clothes will come out of the washer still with detergent in them or perhaps you'll have issues with too much suds leaking out of your washer (or at least out of the tank portion potentially into some of the electronic components.) There's probably a pretty wide margin of error, though, and you'd have to use a lot too little or a lot too much to see any noticeable difference, though. If you're happy with the results you're getting, keep doing what you're doing. If you feel like doing some experimentation with the amount of detergent you use, hell, everybody needs a hobby.

You almost can't use too little detergent, recommended amounts are actually too much and tend to leave residue. You only need 1-2TBSP to properly clean your clothes.

The detergent goblin takes them, as you haven't provided the right payment.

The only harm is using too much. My mother-in-law has ruined multiple washing machines over the years. She doesn't understand that modern high efficiency machines require very little detergent and proceeds to clog them up with too much.

I literally had to teach my wife how to do laundry correctly when we got married so she wouldn't ruin the washing machine.

Any effect should be immediately obvious, shouldn't it? If your clothes are still dirty after washing, that's something you can see/smell/feel. Anything else that your average detergent claims to do is luxury.

The first technolgy connections video with the words washing machine in it.

How much you need depends entirely on the hardness of your water.

If half a serving is enough to get your clothes smelling clean, then you're using enough.

For washing clothes, remarkably little soap is needed to clean ordinary soiling. Washing machines are very efficient. Using too much, besides simply being a waste, can accelerate the degradation of some fabrics, shortening the life of your clothes, and on the extreme end it results in overproduction of suds. Most washing machines from the last 30ish years will detect this and stop their cycle until the suds have died down, so it also wastes time.

Too little and your clothes will be exposed to more friction, causing things to get pulled and rubbed more.

My newer LG with a mobile app once scolded me for using too much.

Typically one trusts the instructions, but it does depend on the kind and quality of detergent, the quality of the machine, and the amount and types of clothes. I myself just entered an era of having no working machines for two days (on my third and am surviving on a pink hoodie over purple plaid shirt, jeans, and temple garment bra/panties which is my "last life") because the slots are broken on both home and public machines and do a half-effective job.

Theres no way to figure out what will happen or what the optimum amount of detergent is because it can depend on a lot of things like the fabric (type, color), type of stain (oil, dirt, paint, ink etc), detergent formulation, washing machine, water (minerals/tds and temp) etc. The worst that can happen is permanent stains on your clothes but that can sometimes happen even if you use the right amount. You clothes might also not smell as strongly of detergent but that doesnt mean they arent clean.

I always use less detergent and add some vinegar.

Switch to powder, your wallet will thank you.

whenever I see westoids say to use X amount, I always divide by 4, or 7