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Old 12-29-2012, 09:11 AM
vintage42's Avatar
vintage42 vintage42 is online now
BMW CCA 1405
Location: Louisville, KY
Join Date: Jun 2007
Posts: 3,810
Mein Auto: 1997 BMW Z3 1.9 5MT
Originally Posted by rudyrov View Post
... you are saying that you get better mileage if you use premium gas? That does make sense, especially with the gas they feed people in some states...
The second sentence does not make sense ;-) because premium is going to be the same in any state.

If an engine can take advantage of 93 octane, it will perform better than on lower octane. Performance includes more power from the same amount of gas, which translates to mileage. But the increase in mileage is very small.

Many engines, like my 2006 Scion xB, cannot benefit from anything more than 87 octane. Many other engines are rated at mid-octane 89, and some of those can either tolerate 87 or take advantage of 91/93, or do both.

It all depends on the engine's ECU and how much it is designed to retard or advance the ignition timing based on the inputs it receives from the knock sensor.

Whatever the case for a particular car, the Owners Manual will say. And our 1997 Z3 1.9L Owners Manual says that the engine can run alright on regular 87, is "rated" on premium 91, and that super premium 93 "will produce slightly higher levels of fuel economy and performance."

It is nice to have an engine that can run on any gas. If you want economy of cost and don't need the max power, use 87. If you want max power and don't mind paying an extra 20 cents per gallon, use 93.

Edit: I did some rough math comparing 87 and 93, assuming 93 was 20 cents and 1 mpg more. Ten gallons of 93 will go 10 more miles at a cost of $2.00 more. So it's about a wash on cost, while the 93 will make max power and fun.

Last edited by vintage42; 12-29-2012 at 09:38 AM.
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