Eurosport

Part
01
of two
Part
01

European Cyclists

While there is no recent pre-compiled source that fully answers your question, we have compiled and triangulated the following from the available public sources: 18.8% of Europeans consider themselves non-professional cyclists, up from 17% in 2005. This includes 13.8% of Germans over the age of 14, 10.3% of adults in Spain, 13.3% in France, and 13.9% of adults in the UK.

METHODOLOGY

Based on the research criteria, we understand "non-professional cyclist" to not refer to the number of Europeans who regularly use a bicycle as a mode of transport (e.g., to work), but those who ride for leisure, either in amateur cyclist events or on their own. Due to the paucity of recent data (24 months or younger), we were forced to rely on older material for much of our information. We will identify these sources in our deep dive below.

EUROPEAN AMATEUR CYCLING

According to a 2005 study, 17% of the population of the European Union members states age 15 and above participated in recreational cycling. However, the actual participation varies heavily by country: "Cycling is by far the most practiced physical activity in the Netherlands (53%), whereas only 2% and 3% participate in the sport in Greece and Portugal respectively."
Due to the age of the above source, we will attempt to corroborate it with a triangulation: A more recent source shows that there are 16.5 million bikes owned in the Netherlands (covering 99.1% of the population), with 1.3 million sold each year. This gives each bike an average lifespan of 12.7 years (16.5 / 1.3). In Europe as a whole, there were 263.8 million bikes sold in the 13 years between 2004 and 2016. Since the population of Europe is 743.1 million, we can estimate the total percentage of bike owners in Europe to be 35.5% (263.8 / 743.1). If, as in the Netherlands, 53% of those who own bikes use them recreationally, this suggests that overall participation in recreational biking is about 18.8% (35.5% x 53%) in Europe.
This is both very close to and slightly above the 17% reported in the 2005 study. Since as we will show below, cycling as a leisure activity has been on a slight rise in at least some areas of Europe, we have high confidence that 18.8% of Europeans consider themselves non-professional cyclists.

AMATEUR CYCLING BY COUNTRY

In 2013, 9.92 million Germans age 14 and up engaged in recreational cycling several times a week. (More recent information was not available in the public abstract.) Since Germany's population in 2013 was 80.65 million, and 88.95% are over age 14, this is equivalent to 13.8% of the population (9.92 / (80.65 x 88.95%)).
In Spain, 38.7% of the population claims to cycle recreationally, but only 10.3% participate on a weekly basis. However, this is up from 6.7% who biked weekly in 2010.
In France, in 2005, 13.3% of the population over age 15 participated in biking at least once a year. Despite a thorough search, we were unable to find a more recent statistic in any English-language source. Based on the rest of our study, we believe this to still be accurate within a couple of percentage points.
In 2005, about 9% of the UK's population participated in recreational cycling, a number that has risen to 13.9% today.

CONCLUSION

Based on the most recent available studies, we triangulate that 18.8% of Europeans consider themselves non-professional cyclists, up from 17% in 2005. This includes 13.8% of Germans over the age of 14, 10.3% of adults in Spain, as much as 13.3% of the French, and 13.9% of adults in the UK.

Part
02
of two
Part
02

Sports Fan Overlap

While there is no pre-existing information to fully answer your question, we've used the available data to pull together key findings: In Europe as a whole, soccer is by far the most popular of the four sports in question, followed by tennis in 3rd place, cycling in 5th place, and golf in 10th place. However, the popularity of each of these sports varies greatly by country and even over relatively short periods of time. Consequently, it appears that no marketing report at the regional or even the individual country level has attempted to collate overlapping sports fandoms.
Below you'll find an outline of our research methodology to better understand why information you've requested is publicly unavailable, as well as a deep dive into our findings.
METHODOLOGY
After a thorough search found no publicly-published polls that had been conducted across Europe that either provided the answer or would allow us to reasonably triangulate the overlap between the European fans of cycling, tennis, golf, and football. This search included both official European statistics sites like Eurostat, [8] Nielsen's free ratings reports, and articles in the popular media. We also expanded our search beyond our normal criteria of only using sources of up to two years old, but this still did not yield the necessary data.
Finally, we attempted to search for the requested information at the nation level. While it would be beyond the scope of a single Wonder request to research every European country individually, if we could find it for the major markets (e.g., the UK, France, Spain, Germany, etc.), perhaps this would yield enough data to at least triangulate a rough estimation. However, we were unable to find an analysis for the overlap between sports fandoms even for the UK; if such a report exists for any of the other countries, we were unable to find an English translation.
It is telling that, based on the table of contents, even an expensive marketing report on sports in the UK does not appear to have the requested information. Based on this, we believe that the available survey data has never been compiled to discover quantitatively the overlap between different sports fandoms; if it has, it has not been released in the public sphere.
Normally, we would attempt to triangulate the answer from the available information. However, the quantitative data is simply too scarce at the level of Europe as a whole to do more than take a wild guess.
USEFUL FINDINGS
In Europe as a whole, football (soccer) is by far the most popular sport, followed by tennis (3rd place), and cycling (5th place), trailed more distantly by golf (10th place). However, the exact order varies by country: In Italy, cycling (5th place) is more popular than tennis (6th). Conversely, in France, tennis (2nd place) is only slightly less popular than soccer, and golf does not even appear in the top ten most popular sports. Unfortunately, we were unable to determine the number of fans in each country or region from public sources.
There are surprising incongruities between different sources. One source notes popularity of the Tour de France, but cycling only appears in 5th place among French sports interests in another. For that matter, a 2016 survey indicates that rugby has taken football's top slot in the French heart, with 39% preferring it to football's 29%. While rugby is, of course, out of scope for the question, it shows how quickly a country's population can shift their interest in sports (a fact noted by the above Rugby World article.)
For another example, golf is the 4th most popular sport in the UK, but also widely considered to be the most boring. This incongruity is likely explained by how many Brits (22%) have grown bored with traditional sports, while 46% are excited about new sports. In fact, one-third believe that football will lose its dominance in the coming decade.
Based on the above, we have two hypotheses: First, given the differences in favorite sports between different European countries, attempting to take a continent-wide survey about sports fandoms would likely be vague and even misleading. Since this would be counterproductive to any business interest, there is little motivation to undertake the expense of surveying a large enough cross-section of Europeans. Second, we hypothesize that the rapidly shifting interest in types of sports (often caused by temporary factors like a team having sudden success on the world stage), surveys attempting to correlate overlapping interest in different sports even at the country level might find themselves made obsolete too quickly in comparison to the expense of compiling them.
The available hard numbers are very high level: According to World Atlas, football is of course the global favorite, with 4 billion fans. Tennis is likewise enjoyed around the world, with perhaps 1 billion fans. Golf is enjoyed by 450 million fans primarily in Western Europe, East Asia, and North America. Cycling is not in the top ten worldwide and so World Atlas does not provide an estimate of its number of fans or reach, nor does it break down the number of fans by region. As a result of this and not being able to find a survey of only European sports fans (unfortunately, English-language sources seem fixated on comparing the US with the world rather than on Europe), we cannot provide a recent count or percentage of European fans by sport.
CONCLUSION
Despite a lack of quantitative data due to the lack of homogeneity of Europe's nations and rapidly shifting interests among today's sports fans, we have established that of the four sports in question, football is by far the most popular, followed by tennis, cycling, and then golf. Worldwide, football is estimated to have 4 billion fans and tennis 1 billion fans, both spread throughout the world. Golf has 450 million fans, primarily in Western Europe, East Asia, and North America. We were unable to determine the number of cycling fans, either in Europe or worldwide.

Sources
Sources