Here's a question for all you savvy people out there. A coworker just told me he had a car that required high octane fuel (not a Volvo), but he was able to mess with the timing to be able to put in regular grade fuel. This would help with the knocking, but would it be detrimental to anything else? I'm not trying to race the brick, just get around town. Would this even be worth looking into or would my fuel economy go to crap just for messing with things?

Thanks!!


2000 v70, 98k miles.
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'86 245, 188K

The car has been designed for a certain octane (check your owners manual -- as I don't know if you're in the USA or somewhere else, e.g. Canada), and believe me, the manufacturer wants to optimize mileage (for advertising, for gov't regs, etc.) as much as you do.

First, your engine may not even let you change the timing.
Some older cars do, and run well like that -- I had a Mercedes with a switch under the hood that retarded the timing for operation in 3rd-world countries with lousy gas; and I still have a Gelaendewagen that readily allows for retarding the timing in such similar circumstances; but you can really feel the loss of power, and see the loss in mileage.
But most car nowadays (including Volvos) have the computer set the timing instead of just allowing you to rotate the distributor (which is what your friend probably did).

And, despite that you can't, be satisfied, because retarding the timing (if you could do it) will result in lower gas mileage, among other things (some of which have already been pointed out in other messages).

Example: Let's hypothesize a ten cent difference (what it is here) -- for a $3 gallon price, $0.10 is merely ~3%. But if you lose, let's say, merely 3 mpg on a car that was giving you 25 mpg, that's an increase of ~12% in your consumption gasoline. So you'll really be buying 12% more gas at a 3% reduction in price. Still interested?

And remember, this is NOT the case of someone using premium in a car designed for regular: "you're wasting your money!" (I'm hearing this warning/advice all over the media) -- your car is designed for premium, so that "advice" is not appropriate for your case.

Do you or anyone know what octane is recommended for a 94 850? I consistently buy the cheapest 87 I can find and have no problems. The car isn't fast, but it's not a turbo and I don't expect it to be. I don't doubt the benefits for a newer V70, but am wondering what's best for my somewhat older car?

My last car was a 95 850 and during a cross country drive I experimented with higher octane gas. In the midwest (flat) I actually got decreased gas mileage (27 vs 28) with the mid grade stuff vs the low stuff. The conditions were consistent: entirely highway driving on flat ground running the tank down to a couple gallons before refilling. Haven't done the same with my newer (to me) '94 manual.

re: "...Do you or anyone know what octane is recommended for a 94 850?..."

I don't have the owners manual for that model/year. But besides the owners manual, manufacturers (e.g., Volvo) usually are careful to also include stickers (on either or both the fuel gauge on the dash as well as near the gas filler, on or behind the door) to warn that premium (93, in this country, viz. RON+MON/2) must be used if that is the case. If there are no such warning stickers, it's likely (though not definite) that 89-91 (mid-grade) is just fine. And I doubt that any decent European car should use 87, IMHO.

In my case, the cost differential between regular and premium is about 10 cents per litre (1.05 to 1.15), while I'm guessing that fuel economy improves about 8%. I do check the price differential each time I fill up though, and if it was consistently less than 8% I would buy premium exclusively.

re: "...cost differential between regular and premium is about 10 cents per litre...."

I didn't know that it's about $0.40 per gallon difference for you. Here (in N.J., U.S.), for Exxon-Mobil (Esso in Canada?) it's only $0.10 difference between premium and mid-grade (and another $0.10 between mid-grade and low or regular). Your $0.40 differential changes the calculations, as you noted.

Andy's comments about the engine having to run richer are something to think about, but here's my understanding, and experience.

Volvo "optimizes" our cars for premium fuel. However, running regular is ok. It will not knock or ping. You will notice more subdued engine response, and fuel economy will decline. However, for routine around-town driving, the performance trade-off may be fine. And according to my unofficial checks, the fuel economy loss is somewhat less than the price difference in the fuel.

In any event, I would recommend major brand fuels known to contain fuel system cleaners. I avoided fuel injector cleaning for 300,000 miles on a previous Volvo by running Esso fuel 95% of the time.

I don't agree with many of the other messages -- not about the octane, but the explaination. You can't change the engine timing. But the engine management system can and does -- in milliseconds, based on what the knock sensors tell it is happening.

The real problem with running less than recommended octane fuel is that it forces the engine management system to avoid knock (again that's why it reads the knock sensor inputs) and it will run the engine richer which uses more fuel and raises the operating temp of the catalysts, risking cat damage (or shortening life of the cats) over time.

Andy is absolutely right. In my message below, I was referring to changing timing in older cars that had no knock sensors or had fixed timing that you or a mechanic adjusted by hand. With the Volvo, you need to use the fuel that the manufacturer specified for your model.

Sorry Justin, it just won't work. The timing can be adjusted a bit to retard preignition, but the tradeoff is lower power and poorer fuel mileage. Essentially, the type of fuel required by your engine is a function of its design and construction: the compression ratio, injection, combustion chamber shape, cam shafts, etc. Use the grade of gasoline that your owner's manual specifies, and use a premium brand (Shell, Chevron, etc.) not a discount brand. If your car calls for 91 octane, you can alternate refills with 89 and 93 - the average will be about right. You can save a bit of gas by keeping your tires at the high end of their inflation range. Also, synthetic motor oil may help a little. The big factor, of course, is driving habits, but you already have an economical car compared to the typical lumbering SUV. Good luck, we're all wondering nowadays about how to save fuel.

Personally, I wouldn't mess with it. The timing is set to do does what its designed to do. It's not adjustable anyway. Keep your car tuned up, run premium fuel, keep your tires properly inflated and stay off the go fast pedal. Your resulting mileage should be pretty decent.

Thanks!! That's kind of what I figured anyway. Just thought it was a novel idea and wondered if it would even work.
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'86 245, 188K