I’ve got a 1994 Nissan Sentra w/ 201,200 miles. I live in Minnesota, and since it got really cold (about beginning of December), my gas mileage has really dropped off. I drive about 30 miles to work each way, almost entirely at highway speeds. I normally average about 38-40 mpg (about 370 miles per fill-up), but since the winter started, my mileage has dropped to around 32-33 mpg (only about 310 miles per fill-up). Could the drop-off be due to the cold? I have 5W-30 oil in it, which is recommended by Nissan. It’s a 5-speed and when it is very cold, the transmission is like molasses until it warms up, so I wonder if that is reducing the mileage. I just replaced the gear oil in the transmission at the end of October, too. However, I have never replaced the clutch. I bought the car with about 96,800 miles and the original owner said he replaced the clutch about a year before I bought it. I don’t feel it slipping, but could it be slipping every so slightly that it is reducing my gas mileage, but not enough for me to noice while driving? Thanks!
It’s the cold. Everyone’s gas mileage goes down in really cold weather.
If the clutch were slipping you’d notice it when accelerating and shifting. Clutches don’t usually slip “slightly” at highway speed.
Check the air pressure in your tires, too. As air temperature drops so does pressure, and the tires could be under-inflated, which reduces gas mileage. Inflate the tires to the pressure listed on the sticker on the door opening. Check the pressure before driving the car (tires cold).
A partially stuck open thermostat can cause low fuel mileage. If the engine coolant doesn’t get up to full operational temperature, the computer never goes into the closed loop, so the engine runs rich all the time. Or if a defective coolant temperature sensor for the computer is sending a signal that the coolant doesn’t get up to operating temperature when it does, this can cause low fuel mileage.
Your mileage drop is entirely normal. We also live in a cold area and have a Nissan and the same thing happens. It takes a long time for the whole running gear to heat up, in addition to the engine. If you lived in an apartment, parked in the basement, and parked inside at work, you mileage would be closer to your summer mileage.
As others point out, make sure the engine reaches proper operating temperature, and your tires are sufficiently inflated. I would also check the air filter; you could have some frost formation on there and get as a result insufficient air.