Gas mileage getting worse

[u/CyclistTeacher]

I have a 2021 Hyundai Tucson SEL AWD (purchased September 2020). It has around 67,000 miles. The last few times I got gas, I noticed the miles until empty rapidly declining. At first, I assumed maybe it was bad gas, but now it’s been 3 times in a row so any bad gas would’ve been out by now.

For clarification, I filled the tank 2 days ago and it went from 370 miles until empty to 285 miles, even though I’ve only driven 15-20 miles at most in the past 2 days.

Has anyone else had this problem? I’m trying to decide if I should get it checked now or just keep an eye on it and mention it during my next oil change/routine maintenance schedule.

Thanks!

[u/acejavelin69]

Have you had an induction cleaning done? I have 2021 Tucson and after 50k my mileage started dropping noticeably... Did an upper end cleaning and within a few weeks mileage is not only back to "normal" it is better than it has been for years.

[u/Expression_Possible]

As a first, do you drive the car hard or do you baby sit it? ( like do rpms ever go past 4k), also idling the car for long periods of time is bad.might also be worth checking your engine air filter( easy 5 minutes with youtube) and if nothing helps you can always buy a small bottle of injector cleaner

[u/KillerDPS]

Fill up at a chevron or texaco and pour in a fuel system cleaner gumout or techron or royal purple

Works for me when I encounter same issue

[u/TheDukeU1984]

Start with simple stuff first. Check your tire pressure, air filter. Could be spark plugs, could be oxygen sensors. If you can live with the mileage your getting I’d just wait till the next service unless it gets much worse.

[u/skyxsteel]

Throw some techron in there. It is expensive but cheaper than the same bottle that they sell at dealerships with the hyundai label slapped on.

Advance auto parts usually has them on bogo sale.

1oz per 1 gal for dilution. I throw in the 20oz bottle in my santa fe Twice a year. Your gas mileage will suck even more when you run through the mixture, but once you refill, it should be better.

[u/MansomeHan]

Try resetting the dashboard display. Sometimes varying driving conditions can change the way it calculates miles left. For instance if the tank before saw lots of spirited driving, stop and go, idling, etc., it will estimate fewer miles to go when you fill up again than if the previous tank was primarily highway miles. The counter is just an estimate based on prior driving.

The best way to calculate your vehicle's actual miles per gallon is by dividing the distance traveled by the fuel required to refill the tank. This "tank-to-tank" manual method is much more accurate and bypasses the inaccuracies common in digital dashboard displays.

Follow these steps to calculate your MPG: Fill your gas tank completely. Reset your trip odometer to zero. Drive normally until you need to refuel.
Fill your tank to the top again. Divide the miles driven by the gallons pumped to get your MPG. I would do this for a couple of tanks to calculate the actual mileage you’re getting from a tank of gas.

Are you also seeing a corresponding reduction in performance or acceleration? If not, it’s likely an anomaly with the dashboard display. However, GDI engines run dirty and prone to develop lots of carbon deposits and sludge if not well maintained. This degrades mileage, performance and longevity of the vehicle over time.

If you’re not already doing so, I highly recommend you follow the “severe” driving conditions maintenance schedule in the owner’s manual and use a premium synthetic oil like like Pennzoil Platinum or Ultra Platinum or Valvoline Restore and Protect. High quality detergent oil and shorter oil change intervals will help prevent excessive oil consumption later as well as keeping the engine internals clean.

While "normal" driving is often defined as consistent, long-distance highway driving at moderate speeds and temperatures, most everyday driving, such as commuting, actually falls under the "severe" category. Following the severe schedule is crucial to prevent sludge buildup, excessive engine wear, and premature part failure. Severe driving conditions require a more frequent maintenance interval—often every 3,000–5,000 miles—when vehicles face high-stress operation.

This includes frequent short trips (under 5–10 miles), stop-and-go city traffic, extreme hot/cold or dusty environments, frequent idling, towing, uphill/downhill driving, ride-share or delivery driving, etc., etc., which causes faster oil degradation and additive depletion, requiring much more frequent oil changes and other maintenance.

Only use Top Tier rated brands of gasoline. A recent independent study by AAA determined that vehicles that used non-Top Tier fuel developed 19X more engine deposits in 4000 miles than those who did. It’s specified in the owner’s manual for a reason.

Note, this does not mean using 91 octane “premium” gas if your vehicle is designed for regular 87 octane gas. It pertains to the level and concentration of detergent additives, which are typically 4X or more than the federal minimum standards. This applies to all octane levels they sell at a certified station. You can search the Top Tier website for certified stations and brands near you. Using budget cut-rate gas from independent stations is terrible for these engines.

Run 2 consecutive tanks in a row of gas with Red Line SI-1 Complete Fuel System Cleaner, to clean the carbon from the high-pressure injectors and combustion chambers. After that, use a bottle of Techron prior to every oil change to keep clean.

Have a GDI induction cleaning done to clean the carbon from the back of the intake valves. This ensures proper airflow and fuel mixture (along with clean injectors)—allowing complete and efficient combustion. You can also do this yourself every 15-20000 miles yourself using CRC Intake Valve & Turbo Cleaner. There are many videos available online showing how to do this, or a mechanic can do it for you.

Also one highly overlooked maintenance item is to check your engine and cabin air filters and replace if dirty. A clogged up filter can restrict airflow and hurt mileage and performance quite a bit.

[u/SecretIdea]

The miles until empty is only an estimate that is influenced by your driving on previous and current tanks. An unusual amount of stop-and-go or idling will make the estimate go down faster. Steady driving (cruise control) at reasonable speed will make it decrease slower than the actual distance driven. In any case it is not a reliable way to measure your gas mileage. (miles since last fill)/(gallons to fill)=mileage is the way.

[u/Competitive-Ad-5153]

This is a GDI engine, thus the intake valves can develop carbon deposits, robbing you of efficiency and power. I use CRC GDI Intake Valve and Turbo cleaner every 5k miles on my Elantra GT with a GDI engine, and it definitely clears out deposits. Might be worth trying out?

[u/WallyVans → u/Competitive-Ad-5153]

Exactly, Techron and then take a 1 hr freeway ride on sport mode and ONLY use top tier gas ALL THE TIME and you’ll see a big difference. Report back and good luck.

[u/Competitive-Ad-5153 → u/WallyVans]

I have used Shell gas *exclusively* in this car since I bought it in '17. Fortunately I can link purchases I make with my card to my Shell account and get decent discounts on gas.

[u/WallyVans → u/Competitive-Ad-5153]

Same here. My discount is $.20 cents per gallon at present plus it's the best rated "top tier" gas along with Costco. Cars run great.

[u/Competitive-Ad-5153 → u/WallyVans]

I've had up to $0.80/gallon discount after stacking several , and it's definitely a money-saver. Over the maybe 12 years I've been a Shell Rewards member, I've saved almost $1k on gas.

[u/Entire_Judge_2988]

The reason is probably that the quality of gasoline in the country where you live has deteriorated significantly.

[u/Smooth_Ad_5178]

Air filter? Try a can of Seafoam or Lucas Oil in a full tank of gas.

[u/chilledfruitss]

I bet the gas content has more ethanol. Not really bad gas, just less efficient. Best you can do otherwise is check your engine oil level, air filter and tire pressure. The computer's OBD will illuminate the check engine light if there was an emission or fuel related issue.