Most I got from a tank was 420 miles, I believe I averaged 34.5 mpg on that trip. (75 mph drafting behind my friend's 1-ton dually)

 

ahh that makes more sense. Smaller fuel tank. ah ha!

 

The best i ever got on the tank was 380miles and that was nothing but highway driving. Everytime i fill up my tank i reset the odometer so there is no wrong calculations. the weather has gotten a lot colder and when i have the heat on i have it on the window defogger and going to the feet so outside air is being heated. Is that what might be causing the drastic decrease?

 

Are your tire pressures correct? Low tire pressure could decrease mileage.

 

using your heat or a/c will decrease your fuel economy because your engine has to work slightly harder to work it.

but what i kinda dont get is..... if you get the most horsepower out of your engine when you use a cold air intake, would cold fuel make a difference?? i was just kinda thinking about this earlier...

 

Colder air has a denser oxygen content. (More oxygen available means more combustion possible, thus more power) CAI's also tend to have lower restrictions than a stock intake as they are designed for performance only rather than a balance of performance vs. noise vs. ease of assembly.

 

My 04 mazda 3i2.0L (60,000)(damn owner before me driving from missouri to cali and back three times) get no where near that gas miliage im lucky to get 250 miles before my low fuel light comes on! I have custom 2.5in cat back exhaust and and a CAI and these mods really dont mess up your Gas miliage that bad if anything they should help

 

Denser oxygen which is colder just provides a better combustion with less work hence better efficency which leads to better gas miliage. Think about it when the piston strikes cold dense air over warmer lose air which one is going to perform better? after all you all your doing is smashing air and gas together lol. But if you really want to get into thermodynamic physics which my bro just went through (god rest his soul) an internal combustion engine at is max is only 35% efficent it was some low percentage like that

 

I finally had my 2.0 on the highway last week. With the cruise control set @ 80 mph, I got 31 mpg.

My city driving mpg seems to vary a lot. It might have something to do with how often someone is in front of me at a red light. If I am behind someone, it can take up to 1/2 mile to reach 35 mph. If I am first at the light, it takes about 100 ft. ;-)

 

if you follow my link you can see the exact the capacity of the fuel tank for the 3 and the est fuel economy and turns out that you should be getting around 380 for the "i" and 360 for the "s" in auto and in city, what you need to consider is the weight thats in your car your car, tire pressure, weather conditions, driving habits inculding shift points, traffic, and if your heater and A/C is on, these are the main varibles when calculating your milage also assuming your mostly stock.

https://www.mazdaforum.com/m_22576/tm.htm