On a 10" width rim the recommended tyre size is 265-275. 255 is stretched and 285 is slightly oversize. Check here for recommended tyre sizes:
Tire Width for a Wheel/Rim Size Chart
Mercedes runs different widths back to front, however these are carefully matched so that the diameters are almost identical. The Tiguan is designed to have all 4 wheels the same size. If you run a mismatched set front/back it will cause the wheels to rotate at different speeds and the ABS system to think that you have constant slippage. Also you will limit your ability to rotate tyres so I wouldn't recommend it for both these reasons.
Put the wheel/tyre options into an online wheel fitment calculator to check how the fitment looks compared to stock. Here's stock vs. 285/35R20 on 20x10+35:
https://www.willtheyfit.com/index.ph...=10&offset2=35
You'd have to compare diameters and speeds but I wouldn't recommend running anything that wasn't within ~0.5%
Personally I'd stick to 255/40 all around, however if you really wanted to push it then stick on 2 x 285/35R20 on 2 wheels. Put them on the front and run around the streets to see if you get clearance on full lock and over bumps etc. If they're fine stick 285's on the other 2 for he rears and job done. If they rub then stick the 285's on the back and put 255/40 on the front. They're the closest tyre size and if you're not lowered you're going to have the best chance of them fitting fine. I believe there's people running 275/35 but not sure if anyone is run 285's.
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