I've got nothing to do with the RSS feed, but I'm using FireFox and I never have had to switch between US/METRIC when using one computer. If I log in from another computer, the site defaults to US. I'd suggest maybe you check your anti-virus software, your browser settings, and validate that your system isn't dropping the cookie for this site.
I just set up a metric RSS feed. If you have Metric selected, you get links to the metric RSS feed now on vehicle profile pages. If you want to link to the metric feed directly, it's available at:
[car profile url]/log/metric-rss
When you switch to Metric, we set a cookie called showMetric that never expires. You should get the site in Metric as long as you use the same computer/browser combo from then on. If you are using the same computer and browser each time, you might check your security settings and make sure you're accepting long-term cookies from Fuelly.