I meticulously keep track of mph. Since November 2016, the mph has dropped from around 30-33 to 25-27. I had changed oil and oil-filter on 01-Nov-2016, and the usual checks that auto-maintenance shops do. I am sure, at that time, all the fluids were topped off, brakes checked, and tire pressures checked and adjusted. What are the possible reasons for this drop?
You mean MPG - miles per gallon not MPH miles per hour?
Bad thermostat or bad temp sensor can cause a MPG drop without throwing a check engine light or even making the car run poorly. The mileage just drop quite a bit because the engine computer either thinks it is too cold - temp sender and sends extra fuel or it sends the extra fuel because the engine hasn’t warmed up yet - thermostat. It is easy enough to check.
Watch the coolant temp gauge to see if it seems to take a long time to warm up or never actually gets to the middle of the gauge range. Also check the actual coolant temp with a non-contacting IR reader. Point it at the upper radiator hose or better yet a metal coolant pipe. The reading should be close to 200 degrees warmed up.
When was the last time it have a tune-up?
How many miles on the car?
What method are you using to figure out the mpg?
When your MPH drops, engage the throttle. 
3 Likes
Shadowfax, every time I fill gas, I note the odometer mileage. I always fill the tank. The receipt gives me the gallons filled. I divide this into the miles difference between the current odometer miles and the previous fill odometer miles. That gives me the mph for that fill. I have been doing it since I bought the car in 2005. I also have the average mph (total miles driven divided by total gallons filled), but that may be relevant to the recent drop in the mph.
2 Likes
I meant to type “that may not be relevant…”
Ok, well, you’re doing it right which means you actually have experienced a mileage drop. I’d start down the list of what @Mustangman said.
I will repeat the question that Mike asked, namely…When did this car last have a tune-up?
Nowadays, a tune-up is essentially just replacing the spark plugs (and–most likely–the engine air filter, and possibly the PCV valve) but if this maintenance is not done on schedule, it can adversely affect your gas mileage.
And, I will ask the question that nobody else has asked, so far:
Is the Check Engine Light lit up?
2 Likes
MikeInNH, spark plugs were replaced @ 120,000 miles. Currently, the car has logged 133,000 miles. I give the car for routine maintenance every 5,000 miles as prescribed in the scheduled maintenance sheet.
Is the Check Engine Light illuminated?
VDCdriver, spark plugs were replaced @ 120,000 miles. The car currently has logged in 133,000 miles.
There is no “Check Engine” indication. The engine air-filer last changed was 17-Jul-15, the same time when spark plugs were replaced. I will check it; may be this is one of the reasons for the drop in mpg.
Good catch, Mustangman. I meant mpg.
I will try your suggestions as well as check engine air-filter that VDCdriver suggested.
1 Like
It’s at the age and mileage for a “lazy” thermostat.
I changed the thermostat (& radiator cap too) on my 2006 Matrix last year at its second coolant change.
Use only one from Toyota.
A dragging brake is another possibility.
After cruising a few miles with minimal brake usage check to see if one wheel is warmer than the others.
[quote=“tiru_narayan, post:12, topic:105246, full:true”]
VDCdriver, spark plugs were replaced @ 120,000 miles. The car currently has logged in 133,000 miles.
There is no “Check Engine” indication. The engine air-filer last changed was 17-Jul-15, the same time when spark plugs were replaced. I will check it; may be this is one of the reasons for the drop in mpg.
[/quote]Replace it if it’s dirty, but a dirty air filter will have almost no impact on fuel economy.
As others have stated.
If there’s a sudden drop in fuel mileage, the first two things to have checked are, a stuck open thermostat, or a coolant temp sensor for the computer sending an erroneous signal to the computer.
Tester
I lost some mpg switching from Michelin to bridgestone, instantly, new tires?
Are the new spark plugs OEM replacements?
If you bought Champions from WalMart, or some fancy plug with extra electrodes or magic powers, that could explain your loss of mileage.
1 Like
Barkydog, the last tires-change was on 11/28/2008 @ 51,900 miles. Still going strong with plenty of tread left. I get them checked and rotated checked every 5,000 miles.
the_same_mountainbik, the spark-plugs were replaced @120,000 miles on 17-Jul-2015. The car had been giving expected mpg up until nov-2016, about a year and a half and about 20,000 miles later.
I had bought the spark-plugs from Advance Auto Parts. It was platinum-tipped (or some such mileage-extension thing). I believe the OEM also was platinum-tipped.