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I see that a specific Ducato can load up to 1400-1500 kg to reach the maximum allowed gross weight of 3500 kg.

I guess that's a limitation imposed by the law (a special license is needed above 3500 kg gross weight), but the van could load more.

What could be the tolerance to overload, assuming very careful driving and a road without big potholes?

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tl dr: Actually, it's not something which is imposed by the law, it's something which is there by design. Don't do it as it's not safe.

There's several things which goes into this:

  • Power: You have to have the power to get your vehicle moving. If it is underpowered for the load, you can run into issues merging into traffic or climbing a hill (yah, I'm including torque in with power, please forgive the faux pas). More weight requires more power/torque.
  • Braking: You need to be able to stop. If you've overloaded your vehicle, your stopping distance will be greatly increased. More weight requires more braking ability.
  • Suspension: By overloading your vehicle you run the risk of breaking something in the suspension (or permanently damaging it). It doesn't matter if you run over potholes, small dips and bumps can cause you issues. More weight requires better springs, bushings, axles, tires, & bearings. (I'm sure there's other things which I've not mentioned.)

All of these factors are designed into the vehicle, and from that design you get the maximum weight you should be carrying or towing. While you may be able to go over the given amount, there's not going to be a built in overload factor from the factory. It's not how they work. Even if a manufacturer would include a safety margin, 1) they aren't going to advertise it, 2) every manufacturer is going to be different. GM might include a 5% overage while Chrysler may go for 10% (numbers not actual, just something I made up).

In other words, you can most likely get away with overloading your vehicle to an extent. The specific vehicle will make a difference as to how much that might be. Also, you do this at your own risk. That's risk to your vehicle, your cargo, yourself, and those around you.

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    Also, sometimes brake/fuel lines and electrical wires are poorly placed, and can be crushed if the vehicle is overloaded. – the_storyteller Feb 15 at 18:27
  • I honestly thought the vehicle would be stronger by design and limited only on paper to comply with regulations. But still you are right, how much is unknown. – FarO Feb 15 at 19:43
  • @FarO if it could safely carry more weight, it would be sold for that purpose. Of course Fiat didn't pick a load number out of the air, because as you say 3500kg takes you into a different vehicle taxation and driver licencing category, but having picked the number, that's what it would be designed for. – alephzero Feb 15 at 21:41

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