You could change without the rental firm knowing, but it's very risky.
Suppose there is an accident while the other driver is driving.
If there are any witnesses to the accident (drivers of other vehicles, for example), the company will find out.
When considering this possibility, remember that endeavoring to drive very carefully doesn't help very much, because your car could be in an accident caused by another driver.
If that happens, any insurance will likely be invalid, leaving you responsible to pay for any damage.
There could of course be additional penalties for renting the car fraudulently.
The second driver fee is probably relatively small even considering the relatively remote possibility of the above.
travel.stackexchange.com/users/19400/phoog
@phoog's answer is very good, but I'm posting this answer to place more emphasis.
Do not even consider doing this.
Only the people who are named drivers of the vehicle are insured to driving it.
This isn't just a matter of you being liable to pay damages if you're in an accident (which could be a huge amount of money depending on what/who you damage/injure/kill).
In most jurisdictions driving without car insurance is a serious criminal offence which can see you fined, banned from driving, or even jailed.
There has been mention in the comments of relying on your own car insurance to let you drive other vehicles.
This is very unlikely to work, unless you have a particularly lenient policy document.
Most will place restrictions on driving in foreign countries and, more importantly, require you to have the consent of the owner of the third party vehicle.
In this scenario you would not have their consent.
travel.stackexchange.com/users/13208/JBentley
The legal and boring answer is: you can't .
Changing drivers will violate your contract (which is a civil violation) and violate the terms of your car insurance.
The real life answer is: no one will find out in 99% of the cases .
If the police stops you, they rarely verify the name on the rental contract and it's even more rare for them to care about a wrong name if the "official" driver is in the passenger seat.
Some might check insurance, but you could just buy some cheap insurance in your own name to show to the police.
Theoretically speaking you might get into an accident with another vehicle with plenty of witnesses, which no insurance will agree to cover, but that's just a risk you will have to take.
Estimate that risk and make a choice.
travel.stackexchange.com/users/9009/JonathanReez
I usually rent a car to travel with my family during weekends.
I would guess if someone else drove the car instead of me, the company would never know unless you have some accident.
And then you are in serious trouble, because insurance would not cover it - it's in the contract.
It is a bit like those european cities where you buy a public transit ticket, but there is usually no one on the bus or train to take it.
You keep it in your pocket.
Or you simply walk into the train and ride.
A lot of times this will work and you'll save some money, but if the ticket man comes, you are fined.
In the end it would be a matter of risk versus benefit.
My personal rule is the following:
Only ask for a second (unauthorized) driver to take the wheel if the risk of accident with the this driver is significantly lower than yours.
This usually means asking my wife to drive if I am too sleepy or tired - which is not uncommon.
But, of course I should not be doing this, because the extra fee is insignificant given the peace of mind.
Well, at least in Brazil.
travel.stackexchange.com/users/41540/heltonbiker
You'll have bigger problems then the car rental company, you wont be insured and will be committing an offense.
Besides the face that if you crash then you wont be insured.
travel.stackexchange.com/users/41646/Jake
A car rental company in Iceland charges extra for two people driving the car. I was wondering what would happen if we pay the fee for only one person, but have an alternate person drive. How will they come to know this? Not just asking to evade fee, but also need to know this for emergency.
Can we change drivers without the rental firm getting to know?
travel.stackexchange.com
<iceland>
travel.stackexchange.com/questions/65556
travel.stackexchange.com/users/41501/icelander
This would mean driving without insurance, which is illegal and would violate the contract and terms of your car insurance. You may be unlikely to be found out, but if you are caught doing this you could be fined, banned from driving, or even jailed, and you would not be protected in an accident. Allowing an unnamed person to drive a rental car is illegal. In the event of an accident, serious consequences may arise. However, in reality, nobody is unlikely to know the driver is unnamed. The individuals involved have to decide if the risk is worth it.
It is illegal to allow an unnamed person to drive a rental car and could result is serious issues if there was an accident. In reality, most of the time, nobody would know the friver was not on the contract. It is a risk that has to be weighred up before the decision is taken.