Fuel economy

From my father, who has just driven to Melbourne and back.

The car is a Honda Accord, or Prelude, or something like that.

Several years ago, I was sitting alongside a couple of retired mechanical engineers from XXXX (former employer) both of whom were espousing the use of high octane ULP and saying that they made up the extra cost by getting extra fuel economy and performance. One of them drove a Volvo which should have made me wary.

Last April [2006], I drove the Honda back from Melbourne on one tank of petrol. It was standard ULP and we covered 733km on 54 litres or 7.37 litres per 100 km or 38.28 miles per gallon while cruising at the speed limit with the air conditioning on and with automatic transmission etc.

So yesterday at Hamilton [Victoria], I thought that I would test the theory and see what improved fuel economy would be achieved with Super grade ULP.

Accordingly, I filled right up with 49 litres of the very best Shell high octane ULP and off we went to Horsham, then on to Warracknabeal and up to Brim. A look around various cemeteries finding graves of ancestors and lunch at Warracknabeal and then a steady drive home.

Now all of this was on good roads, almost flat country with no stop/start traffic, just the occasional stops for lunch and necessary toilet and coffee activities. No hill climbing and not really any heavy road traffic and at the end of the day we had covered 691 km and at the same Shell service station as last year, I put in 53 litres of standard ULP.

The super-high grade ULP gave a fuel economy of 7.67 litres / 100 km or 37.6 mpg.

That is, the higher grade fuel gave a very slightly worse economy than the standard fuel.

Ah well, bang goes another engineering urban myth.


High compression engines in some cars make better use of the premium fuel. If the car is designed properly and adjusts the spark timing automatically for the fuel. I have done a series of test over about six months for a Subaru Liberty Wagon 1998 model year. The fuel economy is measurably better over identical routes and driving styles. The difference in cost between the fuel negates the benefit however. I still use the premium because the acceleration is noticeably better and the engine doesn’t ping as much on steep hills. To each his own.

Comment by Jack | April 4th, 2007 9:48 am | Permalink

I almost continually monitored my car’s fuel economy for the 5 years I owned it. What can I say, I am a nerd :) . I covered a lot of kilometres in a week (about 500 – 600km) driving on the same roads under the same conditions, which meant I built up a statistically significant amount of data. The premium unleaded fuels always gave me a consistently measurable 0.2L/100km – 0.3L/100km improvement. This isn’t enough to totally offset the extra cost of the fuel however. I also knew my car altered it’s fuel and ignition maps when it detected higher-octane fuel in the tank and that seemed to be validated by my measurements.

Something that had a much bigger impact on the fuel used was the Air Conditioner. This would add 0.7L – 0.8L/100km to the fuel consumption.

Comment by Andrew | April 4th, 2007 11:32 am | Permalink

I also have recorded every fuel stop for 4 Hondas and a Suzuki over many years and have been unable to attribute any fuel saving to particular fuels. It is notable that the ethanol “enhanced” fuels (E10) do provide poorer economy by about 5% but notably better performance. I think this is to do with the extra oxygen in the alcohol.
Ralf Sarich’s Orbital Engineering in Perth have just concluded a study of a number of cars using E10 and E5 and don’t have very complementary results.
The study was financed by the Federal Gov.

Comment by Paul P | April 4th, 2007 11:11 pm | Permalink

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