Are there any aftermarket parts to buy for our 2015 Tahoe that'll help it get better gas mileage?

 

Doubtful. When you bought a 4x4 you signed up for less gas milage.

Better mileage? Same as it is for every car:
1. Keep your foot out of the gas.
2. Synthetic oils and fluids.
3. Good gasoline, not the crap stuff that plugs up your injectors.
4. Tire pressures at or slightly above normal (not during the winter)
5. Dont haul anything you dont need.
6. Use cruise control when possible.
7. Keep your air filter clean
8. Use good spark plugs

We tend to think about gas prices so much, but you can save a ton of money by buying cheap car insurance, doing routine mainanance and driving safely. I see too many people drive like hell, then slam on the brakes for a light or sign. Brake jobs can be expensive, even if you do them yourselve. Me? I coast up to the sign making my brakes last longer then normal.