The test, launched by comparison site Top 10 Broadband, allows users to zoom in on their postcode area to see what speed their neighbours net runs at.
It will be a wake-up call for internet service providers, thinks Alex Buttle, marketing director at Top 10 Broadband.
"One person at 1Mbps [megabit per second] could be next door to someone receiving 8.5Mbps," he says.
"We think some of this may be due to outdated technology some providers use in their local exchanges, as well as the fact that some providers use traffic shaping or throttling at peak times while others do not," he added.
The service, dubbed StreetStats, collects speed test data from users to build an interactive map.
More than 170,000 speed test results have so far been added to the map and the firm hopes to have two million by the end of the year.