35mminpixels

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45mm topx

If you wish to save directly from AI at a different resolution, use Export instead of Save for Web, It allows you to choose between 72ppi, 150ppi, 300ppi or Other (this will allow you to add your own):

"When you save artwork in a bitmap format, such as JPEG, GIF, or PNG, Illustrator rasterizes the artwork at 72 pixels per inch."

80mm = 227px at 72 Pixel/Inch (the resolution at which Illustrator's Save for Web, saves) so I'm not sure where the problem is.

Mm to Pixels Converter

i'm making a template of a ticket of 80mm for a web app in Illustrator CC 2017, but now realize that when i go to export>save for web, the size of the artwork and content change to pixels, but not transform 80mm to the actual size in pixels instead the program just changed 80mm to 227px which is the same size on the inside ruler of illustrator.

Mm to pixels 300 DPI

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You can open your exported image in Photoshop, go to Image>Image Size and you will see the resolution of your AI exported image has a resolution of 72.

You mentioned in the comments but "really 80mm are 302.362204728px" and that is right and wrong at the same time. I have mentioned in my answer that save for web saves at a resolution of 72 Pixel/Inch. That means 80mm will have a value of 227px when exported.