Air Density and Track altitude plays a huge role in track times. Especially in Naturally Aspirated (NA) engines - only to be exacerbated by vehicles whose primary speed component is their power-to-weight ratio. And by that, I mean, when you have a stock NA Mustang putting down 450 hp and contains that "brute force" speed, even bad air density isn't going to slow that thing down that much.
But the 2.5L (that goes for the Mazda3 and Mazda6), its key to speed is that we have light cars -- 3100 and 3300 respectively. Throw in 2500 ft track elevation and less than ideal air density, you'll get a terrible time.
I think what's important to remember is that Mazda engineered these cars on power-to-weight, not straight line performance. That's why they're so darned fun to drive. If the "butt-dyno" feels good, that's what it's all about.
In the event you're trying to capture those elusive C&D and MT published times, you'll want ideal conditions which typically have 3 components:
1) This one you have no control over: Track Elevation. You can't do anything about the sea level elevation of your track. Simply, the closer your track is to sea level, the more densely packed the air is. Think of Denver Sports teams - other teams always have trouble playing in Denver because the air is "so-thin". Hell, field goal kickers love it because the football slices right through the thin air. Cars on the other hand hate it, they want dense air.
2) This one you do have control over: Outside temperature. Pick a day to go to the track when the outside temperature is below 50 degrees (assuming you live in a part of the country where you can pull this off).
3) This one is ultimately an issue anywhere: Humidity. You want the humidity as low as possible. The more humidity, the more wet air your motor is dealing with. Your motor wants dry air.
If you're able to capture number 2 and 3 on an ideal day, take note of the barometer reading of the air pressure system you're dealing with. You would ideally like it to be 30.50+ inches of Mercury (inHg). Catch 2 and 3, along with a high barometer reading -- you high tail it to your track for test and tune or whatever they call it these days.
All that said, if free time just isn't on your side and you have to deal with whatever conditions are thrown at you, take note of your track altitude, outdoor temp, humidity, and barometer reading. This website will throw all of that at you - you just need to select the drag strip you're at.
Simply keep it on your phone and screen shot it when you're the next car in line. Then compare your timeslip to your screen shot and you'll get an assessment of how "accurate" that 1/4 time is. The higher the "density altitude" number on the website, the less accurate your time is. Ideally you want that number as close to 0 as possible.
But wait, there's more! You can actually correct your time with a Density Altitude Calculator, found here:
DA Calculator - Density Altitude Calculator - DragTimes.com
It asks for all of the relative data I listed above, which calculates the Density Altitude (DA). Then you stick in your time and trap speed and it corrects it for you. Poof. NOW you have an accurate 1/4 time when corrected for ideal conditions. Enjoy!