I will be traveling by car on July 6 through the 13th to Mexico. My visa with a I 551 stamp was issued on July 2. I was admitted into the country on 29 July. I’m confused on which is my actual expiration date. Because I’m cutting it real close
Your US immigration visa endorsed by an entry stamp becomes an evidence of your LPR status and is valid as such for a year from entry (when you acquired the status). So for example: if you were admitted into the country first on July 29th 2020, your visa with the entry stamp can be used as an evidence of your LPR status until July 28th 2021. Source: USCIS.
That said, given you entered the country in July, it must have been almost a year since and you should have gotten your actual I-551 (green card) long time ago. You should check with the USCIS why you haven't and issue a replacement. You can also go to a USCIS field office to get a temporary stamp in your passport that would serve as a I-551 evidence until that replacement arrives.