Putting regular gas instead of premium
The fuel door on my 2015 Mercedes Benz C300 says minimum octane rating 91. Can you fill it with regular gas 87? Will it affect the fuel economy in anyway?
Follow the fuel door/owner's manual.
It seems that your car requires the premium gas. I do thin it is a smart idea to use regular gas.
Regular gas, super and premium have different octane level. (87 regular, 89 super, 91 or 93 for premium). Roughly speaking, the higher the octane in the fuel, the more compression it needs in order to burn. The problem is if your engine works on 91 and you use 87, there is a chance of detonation, where the fuel could pre-ignite by compression alone (no spark), therefore burning before the piston reached it's normal spot. An off-rythm combustion like this puts a lot of stress on the engine parts and obviously is far from ideal.
I would recommend staying with the 91 but if you go 87 pay close attention that your engine runs smoothly. Pre-ignition is a possibility, but it depends on the engine.
After doing a quick search, every single site I quickly read through said they strongly recommend using premium.
Your driving a Mercedes I'm assuming for the luxury and performance, why wouldn't you just use the premium and enjoy?
Your vehicle's combined MPG rating is 27:
15,000 miles a year divided by 27mpg equals 555 gallons of fuel.
If premium is 30 cents more than regular you are looking at a difference of only $166 for 91+ vs. the 87 octane fuels.
Sure it'll run on that you'll just lose a little bit of power that's all and maybe a tiny bit of gas and maybe a tiny bit of gas mileage but you probably wouldn't notice anything