Higher than normal RPM on highway, poor gas mileage
I have a 2007 Honda Civic LX 1.8L 5-speed automatic with 184,443 miles. I've maintained the car very well including changing the engine oil regularly and the transmission fluid/filter twice. The car drives fine but I used to get 30 mpg and now I'm only getting 20 mpg or less. The only problem I've noticed while driving is that the RPM at 60 mph is 2000 whereas it used to be 1700. The fuel injectors and throttle have been clean. I changed the spark plugs (OEM). The tires are in good condition and at proper inflation. I don't think the brakes are sticking. Local mechanic suggested overdrive might not be working. Check engine light is not on. Transmission shop did a free inspection and test drive and said they couldn't find anything obviously wrong, but suggested a mechanical inspection for about $600. I'm at my wits end with this. Any suggestions? Thanks.
Edit. Forgot to mention that about 70% of my driving is on the freeway.
Do you feel it shifting into 5th gear? Is the torque converter lockup working? Did the transmission shop record data for analysis during the test drive?
If your torque converter lockup isn't working your engine RPMs will increase due to slippage in the converter. One quick way to tell is when at speed if you step on the gas the engine RPM will increase before road speed increases if the lockup isn't working. If the lockup is working then engine RPM will be locked to road speed as it would be with a standard transmission.
Really what you need is to have a transmission pro record and analyze data from the transmission to see what's going on.
Thanks again for the reply Chuck. I tried stepping on the gas while at speed. I did not notice any delay. I tried it at various speeds. I honestly have not noticed any change in how my car drives. I'll see if I can find a transmission specialist with the capability to do what you recommended. Thanks.
Thanks for replying. I honestly can't tell if it's shifting into 5th or not. I don't know what a torque converter lockup is. How can you tell if it's working or not? The transmission shop did not mention anything about recording data during the test drive.