The obvious answer to the title is that 3rd down conversion rate should only look at what a team actually did on 3rd down. But nobody actually cares about a team's 3rd down conversion rate. What people are really talking about when looking at that stat is how often can an offense keep the drive alive for another set of downs.
Below is my attempt at ranking every team by their adjusted 3rd down conversion rate. All I did was simply add a team's 4th down conversions to their 3rd down conversions, then recalculated their 3rd down conversion rate.
| Team | 3D% | 3D% Rank | Adj3D% | Adj3D% Rank |
|---|---|---|---|---|
| Philadelphia Eagles | 50.0% | 1 | 60.3% | 1 |
| Buffalo Bills | 49.2% | 2 | 53.3% | 2 |
| Dallas Cowboys | 47.2% | 3 | 52.8% | 3 |
| Green Bay Packers | 42.7% | 8 | 50.8% | 4 |
| Los Angeles Chargers | 42.4% | 9 | 50.4% | 5 |
| Minnesota Vikings | 41.5% | 11 | 49.6% | 6 |
| Detroit Lions | 39.7% | 15 | 49.2% | 7 |
| San Francisco 49ers | 44.8% | 5 | 49.0% | 8 |
| Chicago Bears | 43.6% | 7 | 48.9% | 9 |
| Kansas City Chiefs | 45.5% | 4 | 48.2% | 10 |
| Los Angeles Rams | 42.1% | 10 | 47.6% | 11 |
| Baltimore Ravens | 44.1% | 6 | 46.5% | 12 |
| Atlanta Falcons | 41.4% | 12 | 45.7% | 13 |
| Carolina Panthers | 37.7% | 19 | 45.4% | 14 |
| Houston Texans | 41.1% | 13 | 45.2% | 15 |
| Miami Dolphins | 39.6% | 16 | 44.8% | 16 |
| Indianapolis Colts | 39.2% | 18 | 44.8% | 17 |
| Tampa Bay Buccaneers | 39.5% | 17 | 44.4% | 18 |
| Denver Broncos | 40.5% | 14 | 43.2% | 19 |
| Washington Commanders | 37.2% | 21 | 42.6% | 20 |
| New Orleans Saints | 36.5% | 22 | 41.9% | 21 |
| Las Vegas Raiders | 33.1% | 29 | 41.5% | 22 |
| Cincinnati Bengals | 37.3% | 20 | 40.7% | 23 |
| Arizona Cardinals | 35.7% | 24 | 39.7% | 24 |
| Jacksonville Jaguars | 35.3% | 25 | 38.7% | 25 |
| Pittsburgh Steelers | 36.1% | 23 | 38.5% | 26 |
| Cleveland Browns | 33.8% | 27 | 38.2% | 27 |
| New England Patriots | 34.1% | 26 | 37.9% | 28 |
| New York Giants | 30.5% | 31 | 37.7% | 29 |
| Tennessee Titans | 33.3% | 28 | 37.7% | 30 |
| Seattle Seahawks | 31.5% | 30 | 35.2% | 31 |
| New York Jets | 25.0% | 32 | 30.0% | 32 |
Not too big of a surprise at the top and bottom. The Eagles, Bills, and Cowboys are great at getting a fresh set of downs and the Jets are awful at doing so.
But there are some interesting results in the middle. The Lions for example are only 15th in 3rd down conversion rate, but when you account for how many times they are then able to convert on 4th down, they are 7th in keeping drives alive.
The Chiefs on the other hand are 4th in pure 3rd down conversions, but have only converted 3 4th down attempts all season dropping them to 10th in adjusted 3rd down rate.
By this logic, just count every first down gained regardless of what down it was before.
4th downs tend to be attempted either in desperation or in high probability to convert situations. Both cases skew the numbers in a misleading way. 4th down conversion attempts also offer a relatively low sample size. You shouldn't get a ton of praise for converting that 4th and 1 or 2. Every team should be able to very consistently gain 2 yards. And failing to convert 4th and long in the final 2 minutes when you're losing shouldn't be held against you.
Barring an end of half FG attempt, teams go for it on 3rd down 100% of the time. For obvious reason. And for 99% of the game, teams operate as if they have 3 downs to gain 10 yards, not 4. If you consistently convert 3rd downs because you consistently have a shorter way to go, that is a very good sign for your offense. If you are constantly facing 3rd and longs, and struggle converting because of it, that is a very bad sign for the offense.
Tldr, 3rd downs, thanks to overall strategy/situation as well as sample size are much more indicative of a good offense than 4th downs.