Can I put E85 in a regular car that takes 87?

No. E85 is only safe in a flex-fuel vehicle or a car specifically tuned for ethanol. A regular 87-octane car lacks the ethanol-rated fuel-system parts and the software to meter the ~30% extra fuel E85 needs, so it will run lean-mapped, throw codes, and can corrode fuel-system components over time. Check for a flex-fuel badge or yellow fuel cap first — if it isn't there, stick to gasoline.

Is E85 cheaper than 87 octane?

Per gallon, yes — E85 is commonly $0.40–$0.90 cheaper. But a gallon of E85 holds roughly a third less energy, so real-world MPG drops about 20–30%. On a cost-per-mile basis the savings largely wash out, and when the price gap is small, 87 usually comes out cheaper to drive on.

Does E85 have higher octane than 87?

Yes, considerably. E85's effective octane is around 100–105 thanks to ethanol's knock resistance, versus 87 for regular gasoline. That high octane is why tuners use E85 on boosted engines — but octane only helps engines built or tuned to exploit it. In a stock 87-octane car, E85's octane provides no benefit and the ethanol causes harm.

How do I know if my car is flex-fuel and can run E85?

Look for a yellow fuel cap or a yellow ring inside the fuel door, a "Flex Fuel" or "E85" badge, and confirmation in the owner's manual. Many flex-fuel vehicles also show the ethanol blend on the dash, and a specific VIN digit often denotes the flex-fuel engine. If nothing indicates flex-fuel, assume the car cannot run E85.

Will E85 make my car faster?

Only if the engine is tuned for it. On a flex-fuel or ethanol-tuned car — especially a turbo or supercharged build — E85's high octane and charge-cooling let a tuner run more timing and boost for real power gains. On a stock, non-flex-fuel car it does not add power and can cause damage. High octane resists knock; it doesn't add energy on its own.

Is it bad to leave E85 sitting in the tank?

It's not ideal. Ethanol absorbs water and degrades faster than gasoline, so E85 is a poor choice for seasonal or rarely driven vehicles. For long-term storage, gasoline with a fuel stabilizer holds up far better. If a flex-fuel car will sit, filling with regular gasoline before storage is the safer move.

E85 vs 87 Octane: Which Fuel for Your Car in 2026?

Written & researched by Kenji Mori Detailing & Paint Protection Editor

Updated June 2026

Affiliate Disclosure

Some links on this page are affiliate links. As an Amazon Associate, we earn from qualifying purchases — at no extra cost to you. We do not buy, test, or physically handle the products we cover; our rankings come from aggregated owner reviews, ratings and expert consensus. Commissions never change our verdicts. How we rank & full disclosure.

E85 (Flex Fuel)

A blend of roughly 51–83% ethanol with the balance gasoline, sold from yellow-handled pumps. Its high effective octane (~100–105) makes it a favorite for boosted builds, but only flex-fuel vehicles can burn it safely.

Best for: Flex-fuel vehicles and modified turbo/supercharged cars tuned to exploit ethanol's knock resistance

Typically $0.40–$0.90 per gallon cheaper than 87, but ~20–30% worse fuel economy offsets much of it

Very high effective octane (~100–105 RON-equivalent) resists knock, ideal for forced induction
Runs cooler and denser — ethanol's charge-cooling effect lets tuners chase real power gains
Usually the cheapest fuel at the pump per gallon
Burns cleaner with lower tailpipe CO and particulates, and it's largely domestically produced
A properly tuned E85 setup can add measurable horsepower over pump 91/93

Roughly 25–30% less energy per gallon, so real-world MPG drops noticeably
Only safe in flex-fuel or purpose-tuned vehicles — it can corrode fuel systems not rated for it
Pump ethanol content swings seasonally (E51 winter to E83 summer), complicating tunes
Absorbs water and degrades faster, so it's poor for long storage or seldom-driven cars

87 Octane (Regular Unleaded)

Standard regular-grade gasoline at 87 AKI (the (R+M)/2 number on the pump). It contains up to 10% ethanol (E10) and is what the vast majority of U.S. cars are engineered to run.

Best for: Nearly every naturally aspirated daily driver whose owner's manual specifies 87 octane

The national baseline grade — regular is the cheapest gasoline, though pricier per gallon than E85

Correct fuel for the huge majority of cars — the manufacturer designed the engine around it
Roughly 25–30% more energy per gallon than E85, so better real-world MPG
Available at essentially every station in the country
Stable and storage-friendly with far less water-absorption drama than E85
No tune, no hardware changes, no compatibility worry — just fill and drive

Low knock resistance — inadequate for high-compression or boosted engines that need premium
Fewer clean-air and octane benefits than ethanol-rich blends
No performance headroom for tuners chasing power
Running it where the manual requires premium can trigger knock and pulled timing

How they score, side by side

Scores reflect aggregated owner reviews and expert consensus — not our own bench testing.

E85 (Flex Fuel) 87 Octane (Regular Unleaded) out of 10

Octane / knock resistance

10

5
Real-world fuel economy

4

8
Everyday compatibility

3

10
Performance headroom (tuned)

10

3
Cost per mile

7

7
Storage & convenience

4

9

Spec-by-spec comparison chart

Factor	E85 (Flex Fuel)	87 Octane (Regular Unleaded)
What it actually is	Blend of ~51–83% ethanol + gasoline	Regular gasoline, up to 10% ethanol (E10)
Pump octane rating	~100–105 effective (high knock resistance)	87 AKI, the (R+M)/2 baseline
Who can run it	Flex-fuel vehicles & ethanol-tuned cars only	Almost every gasoline vehicle
Energy per gallon	Lower — ~25–30% less than 87	Higher energy density
Real-world MPG impact	Drops ~20–30% vs gasoline	Baseline economy
Price per gallon	Usually $0.40–$0.90 cheaper	National regular baseline
Cost per mile	Often similar to 87 after MPG loss	Predictable baseline
Performance potential	High — big gains when tuned for it	None beyond stock
Fuel-system compatibility	Needs ethanol-rated lines, pump, seals	Universal on modern cars
Storage / seasonal notes	Absorbs water; blend varies by season	Stable, storage-friendly

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The three highest-rated options in this category, if you'd rather skip straight to a proven pick.

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Best for the widest OEM approval list and long drain intervals

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Rated for extended (up to 20k-mile) intervals
Strong LSPI and timing-chain-wear protection

Why buy it: the synthetic owners reach for most because it carries a huge approval list — the right pick for the majority of modern engines, provided the grade matches your manual.

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Best Value

Amazon Basics Full Synthetic 5W-30 (dexos1 Gen 2, 5 qt)

Best for a fully-approved synthetic for the lowest price per quart

API SN Plus / dexos1 Gen 2 licensed
Genuine full synthetic base
Lowest cost-per-quart of the group
Same spec compliance as name brands

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Best for engine cleanliness from a gas-to-liquid base oil

PurePlus gas-to-liquid base oil
API SP / ILSAC GF-6, dexos1 Gen 3
Excellent for keeping pistons clean
Backed by a mileage protection program

Why buy it: owners who care about keeping deposits off pistons like the ultra-clean GTL base — a genuinely premium synthetic that still meets mainstream specs.

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What E85 and 87 octane actually are

These get lumped together as "cheap fuel options" at the same station, but they are fundamentally different products aimed at different engines. Confusing them is one of the more expensive fuel mistakes a driver can make, so it's worth being precise about what each one is.

87 octane is regular-grade gasoline. The 87 is its anti-knock index — the (R+M)/2 average of the Research and Motor octane numbers, the yellow figure on the pump. In the U.S. that regular fuel is typically E10, meaning up to 10% ethanol blended into gasoline, which every modern car handles without a second thought. It's the baseline grade the overwhelming majority of vehicles were engineered to burn.

E85 is a different animal. Despite the name, it isn't a fixed 85% ethanol — under ASTM specs it ranges from roughly 51% ethanol in the depths of winter up to about 83% in summer, with the balance being gasoline. It's dispensed from clearly marked yellow-handled pumps, often labeled "Flex Fuel." Because ethanol resists knock far better than gasoline, E85's effective octane lands around 100–105, well above even premium pump gas. That knock resistance is the entire reason performance builders care about it.

The consensus among mechanics is the cleanest way to frame it: 87 is a grade of gasoline; E85 is a substantially different fuel. You can move freely between 87, 89 and 93 in most cars because they're all gasoline of differing octane. You cannot casually swap gasoline for E85 unless the vehicle was specifically built or tuned to run it.

E85 vs 87 Octane — being worked on, seen from behind

Can your car even run E85?

This is the first and most important question — and for most drivers, the answer is no. E85 is only safe in a flex-fuel vehicle (FFV) or a car that has been deliberately converted and tuned to run ethanol. Feeding E85 into an engine not designed for it is not a minor octane upgrade; it's a fuel the hardware and software weren't built to handle.

Here's how to know what you have:

Look for flex-fuel badging. FFVs often wear a yellow fuel cap or a ring inside the fuel door, a "Flex Fuel" or "E85" badge on the tailgate, and a note in the owner's manual. Many FFVs also read the blend on the dash.
Check the VIN and manual. The owner's manual states plainly whether E85 is approved. On many GM, Ford and Chrysler models a specific VIN digit denotes the flex-fuel engine.
Assume "no" if nothing says "yes." If there's no badge, no yellow cap, and the manual is silent, the car is not a flex-fuel vehicle and should never see E85 from the factory.

Why does it matter so much? A true FFV has ethanol-compatible fuel lines, a hardened fuel pump, ethanol-safe seals and injectors, and — critically — engine software that reads the ethanol content and re-tunes fueling and timing on the fly. Ethanol is both corrosive to some ordinary rubber and metal components and demands roughly 30% more fuel volume for the same air. A non-FFV lacks the injector headroom and the software to compensate, so it will run dangerously lean-mapped, throw codes, and over time can corrode fuel-system parts that were never rated for high ethanol. The tuned-car exception exists — plenty of enthusiasts run E85 on a standalone or piggyback tune with upgraded injectors and pump — but that's a deliberate build, not a swap you make at the pump.

Octane, knock and why tuners love E85

For the performance crowd, E85 isn't about saving money — it's about octane and charge cooling. Its effective 100–105 octane makes it extraordinarily resistant to knock, the uncontrolled detonation that forces an ECU to pull timing and rob power. On a boosted engine that headroom is gold: a tuner can run more timing advance and more boost on E85 than on 91 or 93 pump gas, and the gains are real, frequently in the double-digit horsepower range on the same hardware.

Ethanol brings a second advantage the octane number alone doesn't capture: charge cooling. Ethanol has a high heat of vaporization, so as it atomizes it pulls a lot of heat out of the intake charge. A cooler, denser charge is more knock-resistant still and packs slightly more oxygen per cylinder fill. That's why turbo owners often describe an E85 tune as feeling like free intercooling. Ethanol also carries oxygen in the fuel molecule itself, which is part of why it needs more volume — and part of why a properly built E85 setup makes strong, clean power.

The catch is that all of this requires the fueling to keep up. Because E85 needs roughly 30% more fuel by volume, an ethanol tune usually means larger injectors and often a higher-flow fuel pump. Do it right and E85 is one of the cheapest routes to serious power. Do it half-way — big ethanol content, stock fueling — and you run lean where it hurts most. This is squarely a modified-car topic; a stock FFV simply enjoys the octane without chasing the last horsepower.

E85 vs 87 Octane — the full job in progress

MPG, price and the real cost per mile

Here's where the "E85 is cheaper" logic gets complicated, and it's the number drivers most often get wrong. E85 almost always costs less per gallon — commonly $0.40 to $0.90 under regular — but a gallon of E85 contains meaningfully less energy than a gallon of gasoline. Ethanol packs roughly a third less energy by volume, so a full tank of E85 carries you fewer miles.

Aggregated owner reports and EPA flex-fuel ratings consistently show a real-world MPG drop of about 20–30% on E85 versus gasoline. That means the cheaper pump price is largely — sometimes entirely — eaten by the extra fuel you burn. The honest way to compare is cost per mile, not cost per gallon: if E85 is 25% cheaper per gallon but your MPG falls 27%, you're actually spending slightly more to cover the same distance. When E85 is dramatically discounted it can edge ahead on cost per mile; when the spread is small, 87 usually wins the economics outright.

So for a flex-fuel driver choosing purely on running cost, E85 rarely delivers big savings — it roughly breaks even, and its appeal is more about domestic sourcing and cleaner emissions than your wallet. For the tuned-car owner, cost per mile is beside the point; they're buying octane and power, and the fuel-economy hit is an accepted trade. And for everyone in a normal 87-octane car, the comparison is moot — E85 was never an option in the first place.

Common myths and expensive mistakes

"E85 is just cheap high-octane, so it'll boost any car." No. In a non-flex-fuel car E85 isn't a bargain premium — it's the wrong fuel. The engine can't meter the extra fuel it demands, so it runs lean-mapped, throws codes, and can corrode fuel-system parts over time. High octane only helps engines built to use it.

"My car takes premium, so E85 with its 100+ octane is even better." Premium requirement is about octane, not ethanol tolerance. A premium-required car that isn't flex-fuel still can't handle E85's ethanol content. Give it the 91/93 the manual asks for, not E85.

"87 versus 89 versus 93 — might as well try E85 too." Those three are all gasoline; sliding between them is harmless. E85 is a different fuel entirely and doesn't belong on that ladder for a non-FFV. The gasoline grades are interchangeable; E85 is not.

"Running 93 instead of 87 will make my regular-fuel car faster." Generally no. If your engine is engineered for 87, higher-octane gas won't add power — octane only resists knock, and a car that doesn't knock on 87 gains nothing from premium. Buy the grade the manual specifies and save the difference.

"I can leave E85 in the tank all winter." Ethanol readily absorbs water and degrades faster than gasoline, so E85 is a poor choice for seasonal or seldom-driven vehicles. For long storage, gasoline with a stabilizer is the safer bet.

Which should you put in the tank?

There's no universal winner here, because it isn't really a head-to-head. The right fuel is dictated by your specific vehicle, and for most people the decision is already made by the manufacturer.

Run 87 if your owner's manual specifies 87 (or "regular") and the car isn't flex-fuel — which describes the large majority of vehicles on the road. It's the fuel the engine was designed around, it delivers the best real-world economy, and it's available everywhere. Don't pay for premium it doesn't need, and never reach for the yellow pump.

Run E85 if you have a genuine flex-fuel vehicle — then it's a legitimate choice, cleaner-burning and often cheaper per gallon, with the honest caveat that your MPG will drop and the savings roughly wash out. Or if you have a car specifically built and tuned for ethanol, E85 is one of the best-value power fuels available, provided your injectors and pump are sized for it.

If your manual requires premium (91 or 93) and the car isn't flex-fuel, buy that grade — not 87, and not E85. The single most reliable rule across every case is the same: put in exactly what the manufacturer specified, confirm flex-fuel status before you ever consider E85, and let the badge on your fuel door — not the price on the sign — make the call.

E85 vs 87 Octane

Affiliate Disclosure

E85 (Flex Fuel)

87 Octane (Regular Unleaded)

How they score, side by side

Spec-by-spec comparison chart

Our Top 3 Picks

What E85 and 87 octane actually are

Can your car even run E85?

Octane, knock and why tuners love E85

MPG, price and the real cost per mile

Common myths and expensive mistakes

Which should you put in the tank?

Recommended Products

Frequently Asked