To be honest, I am not sure I want to take the car with me any more. I've been trying to calculate various options, and it's just so stupidly expensive this year...
If I compare the "commute" (read: boring) part of the trip, let's say Helsinki to Frankfurt and back, counting fuel and sleeping (so a bunk on a ferry), but not food:
1. Own car with direct ferry to Germany:
Ferry Helsinki - Rostock/Travem?nde - Helsinki = 778 ?
Driving to Frankfurt and back ~1200km = 100l of fuel = 170 ?
Total: 948 ?
2. Own car with ferry to Sweden then to Germany:
Ferry Helsinki - Stockholm - Helsinki = 230 ?
Driving from Stockholm to Malm? and back ~1200km = 100l of fuel = 170 ?
Ferry Malm? - Travem?nde - Malm? = 336 ?
Driving to Frankfurt and back ~1200km = 100l of fuel = 170 ? (Here one could drive via Denmark, but consider extra hotel night + tolls + extra kms, and you'll find that it's not cheaper)
Total: 906 ?
3. Flying to Frankfurt and renting a car for the rest of trip (19-30.7):
HEL-FRA-HEL flights = 280 ?
Cheap sportscar or cheap cabrio as just checked from sixt = 500 ?
Total: 780 ?
I do want to drive on the trip, so I'm not considering a passenger seat here. Also, I would obviously prefer driving my car over a rental, but a) did that last year, b) even when not considering other overheads in addition to fuel, possible(probable) breakdowns and other risks, the stupid rental is still much cheaper than direct ferry, and marginally cheaper than Swedish option. And the 3rd option is not even optimized, I bet there are cheaper flights and cheaper rentals too.
EDIT: Corrected the mistakes in ferry prices, and took into account the discounts for return trip.