It would be difficult to say with much accuracy how many points your credit score would drop due to a voluntary repossession. There are too many factors that affect the score: the depth of history (the number of years covered in the report), the number of items reported, the number and amount of open accounts as opposed to paid accounts, the number of inquiries over the past 90 days. Those are just a few of the factors that go into determining a credit score.
If everything else on the report is shown as paid as agreed, and there is a bulk of items reported, the score might drop 50 points or less; if there are few other items reported and any other derogative items are included it could be closer to 100 points. It sounds like you’ve worked hard to keep your credit in good shape, so I would imagine there are no other derogative items reported.
One mitigating circumstance would be the drastic change in your income subsequent to your purchase of the car. You could amend your credit report with an explanation of why you had to turn the car in (similar to what you have stated above). Anyone pulling your credit report then for any reason would be able to read your explanation. Just go to annualcreditreport.com and get a copy from each of the three major credit reporting bureaus (you get one free copy of each every year), then you can amend the report of your voluntary repo. Doing this may not increase your credit score, but it will be your explanation on record for as long as that item is shown.