How much does oil viscosity actually impact MPG?

If you want to find quantifiable evidence a Dyno test is needed. Suggest a difference in HP at your cruising RPM at highway speeds. Or just go for max HP difference. Percent HP difference might just be efficiency gained.
 
If you want to find quantifiable evidence a Dyno test is needed. Suggest a difference in HP at your cruising RPM at highway speeds. Or just go for max HP difference. Percent HP difference might just be efficiency gained.
But not just any old dynamometer. The tests are standardized ASTM tests including D8226 and D7589 part of the Sequence VI.
 
Look on the sidewall of your tires.

If they read 51 PSI Max, start at 35 and work your way up in 2 PSI steps until they get squirmy.
I have 44 in my rear tires, and 37 up front…

Tire pressure is a compromise between fuel efficiency, tire life, and safety/controllability.

I highly suspect you are well biased away from safety/controllability at the inflation pressures you are recommending.

But you do you.
 
From one of my previous posts, here are a few variables other than oil grade that influence fuel economy. Unless you control and standardize all of them, you cannot isolate the tiny effect of changing the oil grade in your personal vehicle by simply monitoring your MPG.
  • Tire pressure
  • Tire alignment
  • Engine temperature
  • Acceleration/braking pattern
  • Percent short trips
  • Idling time
  • Gasoline BTU variations
  • Engine condition/maintenance
  • Road type & conditions
  • Aerodynamic drag
  • Air conditioning usage
  • Seasonal gas formulations
  • Air temperature
  • Air humidity
  • Gasoline temperature
  • Tire type
  • Consistency of fill
  • Altitude changes
  • Vehicle weight
  • Computer MPG accuracy
  • Octane rating/accuracy
  • Hilly terrain
  • Air filter condition
  • Traffic jams
 
I've been experimenting for years with oil and i have to say that i've never been able to see any difference in fuel economy between oil grades and types. In fact my best tanks were on 15W40 and my worst ones on 5W30. Currently experimenting with higher tire pressure and this on the other hand can make a tremendous difference. I don't recommend it but i'm currently at 48 psi rear and 39 psi front (above Mercedes spec) and i've gone from 8.5L/100 km to about 7.5L/100 km so far. I use various hypermilling techniques and never drive above 60 mph. All highway, no short trips.
 
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