# Gas Mileage Drop **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 11, 2022, 11:56pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793 "2022-07-11T23:56:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 11, 2022, 11:56pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/1 "2022-07-11T23:56:15Z")
I have a 2012 Toyota Rav4 with less than 100K miles. I’ve had it for 6 years and have reliably gotten 25 MPG. In the last few months the mileage is down to 21.5. I change the oil regularly, and changed the air filter, even though the old one looked clean. I set the trip odometer every gas fillup and calculate the mileage out of habit. Any suggestions about what might be causing the drop in fuel efficiency? StG ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [July 11, 2022, 11:59pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/2 "2022-07-11T23:59:15Z")
Hmm. I usually find a significant mileage drop in the winter, but it pops back up in the summer. Are you tires properly inflated? (Though I think they’d have be severely deflated for a 3.5 mpg drop – maybe someone has a calculator to help me out here.) Using the same gas? Not a gas that is (cheaper but) up to 15% ethanol? (I noticed a significant drop two weeks ago when I decided to save 10 cents a gallon on that gas). ETA: Oh, **kenobi\_65** just beat me to it, but AC can eat into mileage a good bit. ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [July 11, 2022, 11:59pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/3 "2022-07-11T23:59:26Z")
Two quick thoughts, both of which are probably not right, but I have to ask: - Is your driving still a similar mix of highway/city driving, compared to the past? - Are you running the AC more right now? Edit: this site suggests several other possibilities (fuel filter, fuel injectors, oxygen sensor, spark plugs, etc.) > **[10 Reasons Why You Might Be Getting Poor Gas Mileage - San Francisco...](https://www.mysfcarguys.com/blog/10-reasons-why-you-might-be-getting-poor-gas-mileage)** > > Are you not getting the gas mileage you desire? San Francisco Automotive Solutions examines the common causes for poor fuel economy. ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [July 12, 2022, 12:36am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/4 "2022-07-12T00:36:17Z")
I’d be thinking it may be time for new plugs or plug wires. Whats the owner manual say? I’ve had them last over 150k miles, but not 10 years. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [July 12, 2022, 12:48am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/5 "2022-07-12T00:48:43Z")
> [@pulykamell](#): > > Are you tires properly inflated? (Though I think they’d have be severely deflated for a 3.5 mpg drop – maybe someone has a calculator to help me out here.) Hmm, this was my first instinct. Most automatic tire pressure sensors I’ve dealt with will let the pressure get around 25% low before it goes off. Even if it doesn’t explain all of it, this could explain the majority of it. Plus, if they’re low, they’re wearing funny. Check your tire pressure first. ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 12:54am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/6 "2022-07-12T00:54:45Z")
The manual says to replace spark plugs at 120K miles or 12 years. But that’s certainly doable. My driving hasn’t changed, except to be greatly reduced since Covid. I have started buying my gas at Costco, but that’s supposed to be a pretty high-quality gas, I think. Now that I think about it, I did replace all four tires a few months ago. I hadn’t thought about a correlation. StG ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [July 12, 2022, 1:04am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/7 "2022-07-12T01:04:24Z")
> [@kenobi\_65](#): > > Is your driving still a similar mix of highway/city driving, compared to the past? > [@StGermain](#): > > My driving hasn’t changed, except to be greatly reduced since Covid. To me, “reduced” probably means more city and less highway/rural driving … could that be? And yes, tires, AC and spark/injector issues are other possible causes. Had a tune up recently? ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 1:07am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/8 "2022-07-12T01:07:12Z")
Well, I live in the country, so mileage-wise, many of my miles are spent getting to the city. I’ve never had a tune up. Like, never, on any car. I thought they were sort of obsolete. StG ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [July 12, 2022, 1:28am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/9 "2022-07-12T01:28:00Z")
> [@StGermain](#): > > Well, I live in the country, so mileage-wise, many of my miles are spent getting to the city. I’ve never had a tune up. Like, never, on any car. I thought they were sort of obsolete. Not _as_ necessary as they used to be, but things like spark plugs, fuel filters, etc. do get worn out or clogged. I have a 10 year old car; I just had a bunch of that stuff done this spring (as well as some needed suspension work). New plugs and filters made a real difference on its performance and mileage. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [July 12, 2022, 2:30am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/10 "2022-07-12T02:30:42Z")
> [@StGermain](#): > > I think. Now that I think about it, I did replace all four tires a few months ago. I hadn’t thought about a correlation. Hey, the tire installers all seem to inflate my tires a bit below what I prefer them to be. If you haven’t checked the pressure recently, I would advise it. It’s almost free, and it might be the cause. Tires usually lose 1-2 PSI a month, to begin with. Most folks run their tires under-inflated, as a result. Check the recommended PSI for your car and see how close you are to it. The other options, clogged injectors, plugs, filters, etc. would normally result in a drive-ability problem eventually. But you may not push your car to the point where you would notice them. I’d start with the tire pressure, and then work from there. On the third hand, did you happen to go from a tire that advertised being fuel efficient to one that wasn’t so fuel efficient when you changed tires? ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 2:33am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/11 "2022-07-12T02:33:33Z")
I had low-end Michelins on it when I bought it, replaced it with some other tire whose name I don’t recall. I never thought about gas mileage when tire shopping. I certainly can check the pressure, though. The TPMS sensors aren’t working, and haven’t for the lats year. StG ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [July 12, 2022, 3:17am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/12 "2022-07-12T03:17:28Z")
> [@StGermain](#): > > I’ve never had a tune up. Like, never, on any car. I thought they were sort of obsolete. I guess I’m really talking about a service, which would include a tune up. Aside from anything else there’s probably an available update to your car’s computer software that might help. I don’t think you have yet addressed the AC question; if what I hear about the northern summer is true where you are, you’re almost certainly running it more often. ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 3:43am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/13 "2022-07-12T03:43:15Z")
I’m in Tennessee. Definitely using A/C, but always do in the summer and never noticed a big drop in fuel efficiency. StG ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [July 12, 2022, 4:13am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/14 "2022-07-12T04:13:21Z")
> [@StGermain](#): > > I had low-end Michelins on it when I bought it, replaced it with some other tire whose name I don’t recall. Hmm, even low-end Michelins can be a fuel efficient tire (they have 3 models in the top ten of one efficiency review). They’re a brand that usually gets expensive when performance (maximum grip or maximum lifetime) is the goal. The brand of the low-end tire is printed on the sidewall if you want to look up reviews, but I don’t know of any convenient way to compare MPG ratings of particular tire models. In the end, softer sidewalls provide a gentler ride, but also are less responsive to steering inputs, have greater rolling resistance and therefore less fuel efficiency. Also, I don’t know of any cost/efficiency calculator on the price of the tires vs. fuel consumed. The model you bought may work out once fuel costs and tire life are taken into consideration. > [@StGermain](#): > > The TPMS sensors aren’t working, and haven’t for the lats year. Ooof, get that fixed. In the end, it’s a safety issue. You may be running your tires way below the recommended pressure (they can prematurely wear and potentially fail). Those monitors are the most wonderful mandated safety regulation other than seat belts since my birth. Don’t ignore them. ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [July 12, 2022, 4:32am UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/15 "2022-07-12T04:32:12Z")
> [@StGermain](#): > > I’ve never had a tune up. Like, never, on any car. I thought they were sort of obsolete. Jeez! Change your spark plugs. Get a spray can of MAF cleaner and clean your MAF sensor and intake. And if you haven’t like just changed your air filter recently, change it again. Fuel filter ever been changed at all? We don’t need to mess with adusting the points or timing anymore, new cars don’t need that. Tune ups are simpler now but they are not obsolete. ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 1:43pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/16 "2022-07-12T13:43:19Z")
I don’t even know what MAF is. I don’t suppose the fuel filter’s been changed in the 40K miles since I bought it - I couldn’t say before.The spark plugs are iridium, according to the manual, and don’t need replacing frequently. StG ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [July 12, 2022, 1:50pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/17 "2022-07-12T13:50:33Z")
> [@StGermain](#): > > The spark plugs are iridium, according to the manual, and don’t need replacing frequently. As this auto care web page notes: > [@](#): > > If your spark plugs are on their last leg, they’ll take it out on your gas mileage. Worn out spark plugs can increase your vehicle’s fuel consumption because they won’t effectively burn the fuel that goes into the engine, meaning you’ll be shelling out more money on fill-ups. > > “A misfiring spark plug can reduce fuel efficiency by as much as 30%,” reports the [National Institute for Automotive Service Excellence](http://www.ase.com/News-Events/Publications/Car-Care-Articles/It-s-Easy-Being--Green-.aspx). > > In general, keeping your car tuned up can work wonders for your fuel efficiency. > > “Fixing a car that is noticeably out of tune or has [failed an emissions test](https://blog.firestonecompleteautocare.com/maintenance/vehicle-failed-emissions-testing/) can improve its gas mileage by an average of 4%,” reports [FuelEconomy.gov](http://FuelEconomy.gov). They also say: > [@](#): > > Luckily, spark plugs don’t wear out very quickly. You can typically get 80,000 miles on them before they need replacing. But if you notice any of these symptoms, it’s time to get your spark plugs checked out with an [engine tune up](http://www.firestonecompleteautocare.com/maintain/tune-up/). You’ve driven 40K miles on your current spark plugs, and you don’t know how many miles were put on them by your car’s previous owner. > **[Bad Spark Plug Symptoms | Firestone Complete Auto Care](https://www.firestonecompleteautocare.com/blog/maintenance/when-to-change-spark-plugs/)** > > Engine idling roughly? Changing spark plugs could be the solution. Learn when to change spark plugs by watching for these bad spark plug symptoms, plus why its important to keep this part of your car in great working order. Trust Firestone Complete... ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 1:51pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/18 "2022-07-12T13:51:35Z")
But the manual says 120K miles before a spark plug change! StG ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [July 12, 2022, 1:58pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/19 "2022-07-12T13:58:11Z")
> [@StGermain](#): > > But the manual says 120K miles before a spark plug change! That doesn’t necessarily mean that the plugs are _guaranteed_ to not fail or need servicing before then. Parts sometimes wear out faster than the recommended service schedule. Anyway, it might well not be the spark plugs. But, as I and others have noted, there are a bunch of things that could need servicing on your car, because it _is_ older, and because you don’t have your car tuned up; other than the tire pressure, you seem to be wanting to dismiss most of those suggestions. Have a mechanic you trust give your car a once-over, if you’re concerned about the mileage drop. ---
**Author:** ![StGermain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stgermain/32/2868_2.png) [@StGermain](https://boards.straightdope.com/u/StGermain)\ **Post date:** [July 12, 2022, 2:07pm UTC](https://boards.straightdope.com/t/gas-mileage-drop/967793/20 "2022-07-12T14:07:17Z")
No, I’m willing to have a tune up done. As I said, my mistake was in thinking they weren’t a thing anymore. I do routine oil/filter (air, oil and fuel) changes on my mower, and I just bought the stuff to do my Kubota tractor, although that back fuel filter looks like it’ll be hard to get to.I don’t work on my car, though - I take it in to the oil change place and get it serviced. I change the air and cabin filters myself, because they charge like 4 times the filter cost to swap those out. StG [Next page](https://boards.straightdope.com/t/gas-mileage-drop/967793.md?page=2)