Someone asked earlier what would have happened if the Bf 109F-1 had been available earlier in the Battle of Britain.
But here's a more pertinent question: what if the Luftwaffe recognized the need for longer-range fighter operations and got the Bf 109E-7 with its 300 liter drop tank into service by early July 1940, or had them installed as Rüstsätze by mechanics on the front line on earlier Bf 109E models? With the ability for longer combat operations over the UK, would that have resulted in a lot more Spitfire and Hurricane losses, which would have made the British winning the Battle of Britain a lot more precarious situation?
Interestingly, the very same subject i was thinking recently (drop tanks for Bf-109). Imo, drop tanks would have the following effect: from memory, historically the Bf-109 was critically hampered by lack of range, f.e. only being able to spend 10 minutes or so over London, the pilots having to "fight with an eye on the fuel gauge", thus inhibiting them from fighting to their fullest abilities, and also having to leave the bombers early in many cases, and again if i'm not mistaken, an important number of Bf-109s ended running out of fuel and having to crash land. Yet despite all this, Bf-109 has a superior kill ratio to Spitfire (not to mention Hurricane). BUT, with drop tanks, the Bf-109E could stay for much longer above southern England, and probably stay with the bombers all the way in attacks as far as London (over which they can spend perhaps 25-30 minutes, not just 10), meaning they will engage more british fighters and thus save that many bombers from being shot down. They could even cope better following Goring idiotic instructions like waiting the bombers in the air (thus consuming precious fuel), or sticking to them all the way to and from target without disengaging to pursue RAF fighters. The following numbers are completely speculative, but imo i would guess that because of more fuel the Bf-109E would be even more effective against RAF fighters over southern England as far as London, so let's say that for whichever number of Bf-109Es lost over a given period, perhaps 15-20% more RAF fighters are lost over the same period compared to OTL, while Luftwaffe loses 25-30% less bombers over the same timeframe. But if Hitler and Goring are as idiotic as OTL and they still stop the attacks against airfields then likely Luftwaffe still will not manage to obtain air superiority over the southern England, but overall RAF will be even more bloodied compared to OTL, perhaps taking a bit longer to regroup and come back in numbers against attacks such as the one on Sept. 15th against London- so perhaps Battle of Britain day would be Sept. 30 or something. But overall i say the RAF loses on average 20% more fighters and germans lose on average 30% less bombers compared to OTL if the Bf-109E had drop tanks, IF the airfield attacks are stopped as in OTL. If not, then Seelowe is likely implemented, how that will go is of course another matter.
As for the 109F-1, well the simple answer is that if somehow they would have started it's developing and especially testing in 1939, even a few gruppe of these for the battle (the DB-601N engine was already there, only need the F airframe), with drop tanks, will cause havoc among the RAF fighters, not even 100 octane fuel will give them a fair chance, sort of like the Focke-Wulf scourge of 1941-1942. The Miles M.24 and the M.20 might see service in this scenario (and be wiped out without further ado). Really bad summer for RAF that would be.