[Triumph23]

I can't answer all your questions but I would say this. I have 8.5j wide 19" tyres at the front which are 235/35/19 in my base Giulia and there is plenty of room. My wheels are QF size and offset. I can go 245 if I wished. As long as you keep the rolling diameter of the 20" tyre the same as the 17" tyre, 225/50/17 which is what I had originally, I dont think you will have any issues with 8.5j assuming you keep the same offset as the QF. Others with more knowledge will be able to guide you better.

 

[Triumph23]

I don't have my car with me at present to measure but what you are trying to do is a "plus 3" sizing, going from 17" to 20" which is quite a lot.

For 19" , 245/35/19 is OEM size and same overall rolling diameter as 225/50/17. The aim is to keep rolling diameter always the same whilst increasing alloy wheel diameter. This is done by shorter sidewall tyres as you go to larger wheels. So if you go to 20" you chose a different tyre size to maintain the same overall rolling diameter. You want the difference to be less than 3%. See attached.
And also this calculator should help. Enter your current and new sizes plus offset, it will give you a visual and number comparison. Going to 20" will mean you will end up with very short sidewall and worse ride and if roads are bad, risk of wheel buckling if you hit potholes.

https://www.willtheyfit.com/

 

[Triumph23]

I have already checked all these things (I have used a tyre calculation tool also, see first post).
The biggest OEM tyre used front axle is 225/40 19" with overall rolling diameter of 663MM
245/35 20" overall rolling diameter is 680MM
235/35 20" overall rolling diameter is 673MM

Reference guide for car wheel and tire dimensions such as size, offset, PCD

Wheel fitment and tire size guide and knowledge base. QA forum about wheel specs for your vehicle

www.wheel-size.com

View attachment 96821


The main question is,
Do 245/35 20" tyres fit base giulia ?
Otherwise, Do 235/35 20" tyres fit base giulia?

@MikeFish fitted 20" wheels but that was in a QF. You may want to search for his posts to see what tyre size he used. I don't think there is any difference in suspension shape between base and QF. I am not 100% sure though.