Fuel economy of oil not so simple?

[Mitch Alsup]

Originally Posted By: TheKracken
So, one thing I am still unsure of since Fuel economy is not based on the numbers I posted. Would a 5w20 or a 0w40 be better for fuel economy at start up? Which would be thinner?


With few (?any?) exceptions, any xxW-40 oil will be thicker than any yyW-20 oil at any temperature (seen inside an automotive engine), and thus consume more power getting pumped around the engine, thus yielding lower FE.

 

[Silk]

I achieved a 20% improvement in fuel economy just by dropping my speed....and got to work 3 minutes later. Of course we have to ignore such findings, the public won't approve.

I ran my Volvo on 15W-40 and 5W-40 with a rear main leak. After fixing the PCV the leak reduced to a drip, so decided to try 5W-30 - the rear axle was damp with oil, so decided it better get out of my engine fast! I had some Penrite 20W-50 for my bike, and topped it up with some Fuchs CFX 15W-40. On a recent trip to the city I got my best economy out of it yet. Normally it will be in the low 8 litres per 100km, on a good day dropping into the 7's. I got 7.1 litres per 100km, I've never seen it average that low before. That's 33 US mpg, or 40 mpg Imperial which I am more familiar with.

There could be other reasons, but I have got my best economy with the thickest oil i've used yet. Certainly not going back to 5W-40 !