My mechanic (age 64) told me NO when I asked if using the mid grade gas would harm my 2007 Nissan Murano. He said that I might find a very small percent of loss in my mileage 1-2% but that the computers would adjust for any changes in the gas grade. My question now is, can I use regular gas? I shouldn’t have bought this car in May 2007 but it’s paid for so I feel like I’m pretty much stuck for a few years. Thanks.
Whether you can get by with less than 91 octane (premium) varies from one car model to another. It isn’t safe to generalize. See what your owner’s manual says about fuel grade.
If it says that you can use 87 octane (regular), it will be safe to use that or mid-grade (89 octane). It may recommend 91 octane for best performance.
If it specifies 91 octane only, you would be risking engine damage if you used anything less. In an emergency, you can get by with a partial tank of something less than premium as long as you drive gently and finish filling the tank with premium at the first opportunity.
According to the federal EPA site ( http://www.fueleconomy.gov/feg/noframes/23757.shtml ) your 2007 Murano V6 is supposed to use premium gas. You should be able to drop down to midgrade gas without harm, as your mechanic advised, but go no lower. Your engine would be unhappy with you.
Age of your mechanic may have something to do with his recommendation. Your 07 Murano has a 10.3:1 compression ratio and was designed to use premium efficiently. Our 05 Sable Duratec 30 is 10.5:1 and Ford recommends regular. Because the PCM can map to the appropriate advance given a lower octane fuel. If the PCM was dumb, detonation would occur. It will also map to premium but that isn’t cost effective. The owners manual likely states Recommended Premium and not Required Premium. The PCM will map to regular. After all, if the engine will take 10.3:1 compression will, it take 9.2:1. Try mid grade first. Takes one tank to reeducate the PCM. Preserve some engine performance and mpg with next lower grade.
You should use the grade of gasoline specified in your owner’s manual. If it says “Premium is required,” then you need to buy premium.
If it says, “Premium is recommended,” you might be able to get away with mid-grade. But why gamble? I suggest you buy the fuel recommended in the owner’s manual, just to be safe. You spent a lot of money. Don’t ruin the engine by using the wrong fuel.
The engine computer will, always(?) adjust, IF it can, and will never fail? If not, “Hello, new engine!”.
On the other hand, it’s impossible(?) for the engine to operate beyond the adjustable limits of the engine computer, or, is it?
If someone says, “Why! So and so did it.”, would that be carte blanc adequate permission? As Dirty Harry said, “Do you feel lucky?”
Not really a valid argument. The computer is constantly adjusting operating parameters regardless of the grade of gasoline, even premium. If it can handle regular as part of its operating parameters, it will continue to do so, carbon buildup in the cylinders notwithstanding. if the knock sensor fails, it will cause problems with premium also, or not at all.
Scenario: You’ve put in regular grade gasoline. You’re on the on-ramp to the freeway. Traffic is coming fast. You’ve got to get up to traffic speed (20 mph above posted limit) to merge into the lane. In anxiety, you floor it, never hearing the sounds from the engine. You’ve made it! You let off the gas, and the engine performance seems down. How many times have you had to floor it in similar circumstances?! Did some ill effects accumulate?
You take the car to able mechanic. Able mechanic examines the engine, and says, “Do you know that there are holes melted in your pistons from pre-ignition (detonation)? What octane fuel have you been using? The timing parameters of the engine were exceeded.” You, “But, I was told it would be alright, that the engine and computer could, and would (without fail), adjust to any demand. Was I told wrong by? Alas!”
Not all computers are created equal (actually it involves a lot more than the computer 
) Not all can adjust properly for the use of regular. That is why some owner’s manuals recommend premium (this car can adjust to a wide range, although it will reduce performance and economy, and some say the car requires premium, which means the computer has less range and the typical driver is likely to encounter conditions that may well result in engine damage over time.
It’s easy to think you always save money when downgrading gasoline, but that’s not always true.
For example. If gas costs $4.00/gallon, you have a 20 gallon tank, and your car gets 20 MPG, then it will cost you $80/tankful to drive 400 miles.
If you save 20 cents/gallon to use a downgraded gasoline, you’ll save $4/fillup at the pump.
However, if the “detuning” your car does to compensate for the lower grade fuel drops your MPG by 1 mile/gallon, you haven’t saved any money at all.
That’s because your tankful of gas will only get you 380 miles. You will need an extra gallon to drive that original 400 mile distance.
In the above scenario, you haven’t saved any money.
I’ll let others comment on the likelihood of a 1 MPG drop when using 87 octane (regular) in vehicles calling for higher grade fuels.