I have now had three cars in three different countries, yet fuelly will only allow me to set a location/currency which is them applicable across my entire account. Looking back on old data for cars in other locations, the currency has been updated for each to reflect my current location, not the location where I was running each car.
A better solution would be to have a location, and therefore currency, set per vehicle.
When we evaluate potential features for the site a question we always ask ourselves is: would this change apply to most users of the site? A handful? One or two? We have limited resources, so we almost always have to choose features that will apply to most users. I think your situation is an outlier compared with the majority of our users.
Associating cars with a country rather than a user would be a significant structural change for the site. It would require weeks of work, and then longer for testing/fixing. The work involved wouldn't be worth the benefit to the few users who would eventually take advantage of it.
Keep in mind we're not the only fuel-tracking game in town. There might be other systems that already do this.