Concerned about Avid Ascend MPG

often, new tires require break in. also, the radius will be larger compared to worn tires, affecting your odometer.

 

This part is much less than people think. Remember that the contact path on the road doesn't have a radius. It is flat, so the normal circle equations don't work.

The more meaningful wheel measure is the length of the steel belts beneath the tread. These control the working rolling circumference more strongly than does the actual tread radius, and should not change much with tread wear.

A better check is to compare the RPM (revolutions per mile) spec on on new vs old tire data sheets. If these are different, the change will impact both odometer and speedometer calibrations, and thus MPG too.

 

what do we know about avid ascends?

 

For tire size 195/65R15:

Michelin Defender: RPM = 832, dia = 25.0"
Pirelli PZero: n/a this wheel size
Avid Ascend: RPM = 828, dia = 25.1"

The Ascend's RPM differs by 0.5%, but because it shifts both odometer (car goes farther than odo displays) and speedometer readings {testing at higher wind drag point) readings, net result will be about a 1% shift in apparent (not real) MPG.

This accounts for part of your MPG loss, but certainly not the bulk of it.

 

Also, make sure that you were comparing against earlier winter MPG, not some remembered MPG tests from last spring-summer-fall. Winter mileage is hampered by lower energy content of the seasonal fuel blends, and often by weather. Both should be improving about now.

 

My PiP had Avid Ascend TouringS tires. On a recent rainy and windy trip to North Carolina, I averaged a whisker over 50 mpg.

 

This was some years ago, but I had Yoko Avid Ascend on my HCH as summer tires. I was getting 48-52 mpg which was better than later changed Ecopia.

 

Here is an illustration of the difference. The Michelins with 25.0" tread diameter, compute out to a rolling diameter of just 24.24". The Avids have 25.1" tread diameter, but only 24.36" of rolling diameter. Between both sides, that is 3/4" of rubber, or 12/32nds" on each side, that doesn't contribute to the working rolling diameter.

12/32nds is slightly deeper than the original tread, so the true rolling circle surface is slightly below the actual tread. I.e. about where the steel belts are.

 

The original Yokohama tire that the Prius (2012) sported were developed especially for the their low rolling resistance. It’s partly responsible for the fuel mileage figures that Toyota was able to post. They were very light weight (18 lbs). The Avid Assends that OP put on his car are full 3 pounds heavier than the original stock tire, coming in at 21 lbs per tire. That mass requires more horse power to get the car rolling and up to speed. Hence, the fuel mileage is going to suffer.