After reading so many threads on tires and fuel economy and nothing really seeming scientific or conclusive, i decided against LRR and instead to get the tires in my budget that had the performance I was looking for. Since inflation, tires have seemed to have gone up 50% where I am in California, so even a set of budget tires cost me $560 total. Because I could not find adequate information and most tire threads are full of anecdotes, I just decided to roll the dice and go with Kinergy ST H735, tires that no one really recommended for the Prius. Before I was getting about 40.5 mpg at 40psi. 42mpg if I drove conservatively, but the way I normally drive it averaged 40.5. I got the new tires and waited about 1500 miles of break in to start clocking the mpg and tested the psi for a few hundred miles, all driven the same. At the standard 36 psi I was getting a slightly disappointing, but expected 38 mpg. Lots of people said to expect a drop off if you didn't get LRR's so I was actually ok with this drop. Here's the weird part: I decided to bump the psi to 40 and the mpg went DOWN to 36mpg. I have no idea why this happened. The ride comfort was terrible too. Then I tried 38psi and it went up back to 40.5mpg. Anyways this is just another anecdote, but I figured I'd give my experience with the Kinergy ST H735, which I'm now very happy that I got no drop-off in mpg. 36psi = 38mpg 38psi = 40.5 mpg 40psi = 36 mpg ???? Thoughts? I can't explain the significant drop with higher tire pressure.
Well, I'll speculate. 1) The tire traction ratings may differ....even though brand-to-brand ratings are valid, changing brands brings a variable. If your original tires were rated 400, and the new tires at 200, well that could be. Typically, a lower rating causes decreased mileage. 2) Alignment has changed over time and you are dragging your front toe. Typically, a cause of decreased mileage. 3) Maybe it is tune-up time? Engine air filter. Typically, a cause of decreased mileage. 4) Seasonal gasoline formula changes. Typically, a cause of decreased mileage. 5) Tire pressure. Well bumping up the pressure usually increase miles per gallon, but increases wear on the center of the tire. So maybe, I am guessing, the higher pressure is increasing traction, noting # 1 above. Or the higher pressure is increasing the tire temperature over its performance operating range. Do note, with a tire brand change, a pyrometer is your friend. FWIW you can ballpark pressure by looking for a 10% rise in pressure from cold to hot. Looks like you found 38 psi as ideal. So anyway, all guess and no answer. My vote would be toward traction ratings and/or tire temps. Good hunting
