Took my 2014 Acura MDX in for scheduled oil change at 22,000 miles. For the first 22,000 miles I averaged 20.72 MPG, Immediately after the oil change the MPG dropped to 17.26, a 20% drop in MPG. Same driving habits, part city, part freeway. What could cause the significant drop in mileage after an oil change. Could the mechanic have left something disconnected, etc, etc. Puzzling. Bengk
Forgot to add, I have driven over a 1000 miles since the oil change, with the average of 17.26 MPG. bengk
What was the ambient temperature when you were averaging 20+ mpg, as compared to the ambient temperature now with lower mpg readings?
Is the Check Engine Light lit up?
Does your car call for full-synthetic motor oil?
If that is the case, are you sure that the mechanic used full-synthetic?
A 2014 should have a good warranty. Just tell the place you bought it your miles per gallon is lower now.
If the check engine light has not come on yet my best guesses would be change in fuel blends, weather changes or incorrect oil. Where did you have your service work done?
I think your car calls for using 0w-20 grade of oil…Check and see if there was a mix-up at the shop and they installed an incorrect grade like maybe 15w-40 or 10w-40…
How are you measuring your gas mileage?
Are you relying on the computer gizmo? Sometimes the mech would let the car idle for a while on the engine change bay and maybe also do a run for donuts and this could have dropped the mileage, but you mention 1000 miles and that should have improved by now.
Have you looked at your receipt and also popped the hood and make sure everything is okay. Look for loose tubes, air filter box that is open and listen for hissing sounds.
There’s a chance that they could have inadvertently reset your mpg readout when they were resetting your oil life monitor. It’s an easy honest mistake and I’ve done it myself.
I’m betting on the change in ambient temperature. Your last 1000 miles are during winter.
That can make a big difference.
Not using e85, are you?
Not using e85, are you?
I don’t think the 2014 Acura MDX comes as a Flex Fuel vehicle.
Not using e85, are you?I don’t think the 2014 Acura MDX comes as a Flex Fuel vehicle.
Doesn’t mean it can’t/didn’t happen. 
The comments made so far are good, but for a drop over 20% I’d look for something which might have been bumped or disconnected (by mistake) during the oil change. Check the engine bay, listen for air leaks, then do the “normal” stuff . . . make cxertain that you have the proper oil (0w20), proper air pressure in your tires, nothing dragging (brakes, especially parking). Gotta be something to cause that drop. Good luck! Rocketman
Doesn't mean it can't/didn't happen.
Then there could be a lot more problems then just a 22% decrease in gas mileage.
My annual drop from summer to winter is nearly 20%.
I agree with @melott. It’s most likely winter that’s doing it. A combination of cooler ambient temps and winter blend fuel. Car engines take longer to warm up, running rich until they do.
+3 for winter driving. Gasoline is reformulated for winter and it is common for gas mileage to drop around 10%.
+1 Rocketman. Maybe hey adjusted the air pressure in your tires with a defective gauge and now the tires are low.
20% is a pretty big drop. Could be winter effects, as mentioned above. If not that, the most common cause of mpg decrease – other than the owner’s measurement error – usually has something to do with the coolant temperature. Either it is too low, due to a faulty thermostat for example, or the coolant temp is ok, but the engine computer thinks it is too low, due to a faulty or unplugged engine coolant temp sensor.