The best time in year to go to Sweden and Finland is from late May to
early September, because this are normally the warmest month. From October to April there is winter,
at least in the northern parts of this countries, with ice and snow. If
you go early in the year you could have bad luck (like I had) and get a
little snow in the north, but you've got the advantage that there are
still no mosquitoes, not even next to the lakes.
Road conditions: The E-roads are often built as motorways
and they are as boring as motorways. The other roads are in good
conditions and they are not seldom better than in Germany. Only very small
roads are gravel-/mud-roads (depends on the weather) and they are
often dead-end-roads (up to 50km long).
Getting fuel: This is not a problem, not if you've got
a credit card (best Visa). At many fuel-stations there is no desk were
you have to pay, but there are automats which will (hopefully) accept
your card or sometimes they also accept notes/bills. The prices are
about 1,15 Euro per liter in Sweden and 1,07 Euro in Finland (may 2002).