About to order a 2017 RX 450h for my wife. Mileage and smoothness of ride are a priority for her and I understand that 18" wheels/tires will improve both. There are a number of features in the Luxury Package that we'd like, but don't know how much the 20" wheels/tires that come with it will affect mpg and and comfort. Any insights would be appreciated.

 

My RX came from th factory with the 18s. I just recently upgraded to the 20s. With the 18s I was consistently getting over 30MPG. My average for the first 5k miles was 32.7. After a few days with the 20s I'm only averaging in the upper 20s. The ride is definitely a little stiffer with the 20 s but it's not too bad. I absolutely hated the way the 18s looked.

 

The circumference of the 20 inches is 11.11% greater than the 18 inch tires. I wonder if the odometer does not register the correct mileage because of the difference in circumference. Perhaps if you take your mileage and multiply by 1.1111 you would get your actual mileage and then can calculate your mpg.

 

the 18 and 20 are almost exactly the same size, 12mm difference in circumference. so the entire way around the tire. not 11% grater
revolutions per mile (km)668.32 (415.28)671.65 (417.35)3.33 (2.07) 0.5% so about 3 revolutions more per mile, thats about as close as it gets.
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https://www.tacomaworld.com/tirecalc...5r20-235-65r18

 

Your link is interesting; however,

The formula for calculating the circumference of a circle is pi (3.1416) * the diameter
20 inch diameter * 3.1416 = 62.832 inches
18 inch Diameter * 3.1416 = 56.5488 inches
Difference is 6.2832 inches
6.2832/56.5488 = 11.1111
Therefore, the circumference of the 20 inch wheel is 11.11% larger
Simple math

 

you need to account for the tire sidewall. the 18 has a larger tire sidewall than the 20"
simple as that
(25 year ase master mechanic)

sites and conversions are used all the time for changing tire size and wheel size. this is not a 20" circle vs a 18" circle, if it was a train or trolley car maybe, But since we drive on tires, and tire size and sidewall comes in tons of different sizes, the assembly diameter and revolution per mile is used.

 

My Caviar Black RX350 came with the 18s. I found someone selling the F-Sport 20" wheels so I bought them and sold my 18s. There was a difference in ride quality. 18s felt plush and floaty whereas the 20s transmitted more road noise but eliminated the floatiness. I liked the way the 20s looked especially on a black car, but preferred the ride of the 18s. I think the 18s look fine on silver and white RXs, but on the other colors, they look really cheap.

That all said, I traded in the 350 for a 450h with the premium package which includes the 20" wheels. The ride is really good. Much better than my 350 with 18s. Might be the added 500 pounds of the hybrid system. But yeah i definitely notice a bigger difference in the 350 vs 450h ride quality rather than 18" vs. 20"

 

You are correct about math. but you are calculating the metal part of the disc not the outer part of the wheels.
Size measures the inside part of the rubber.

 

i must be reading this wrong, lol
metal disk, outer wheel, inside rubber ???
please look up to my 2 posts, it clearly and simply explains how an 18 and 20" tire and wheel combination can have the same rolling diameter.

the 18 and 20 are almost exactly the same size, 12mm difference in circumference. so the entire way around the tire. not 11% grater
revolutions per mile (km)668.32 (415.28)671.65 (417.35)3.33 (2.07) 0.5% so about 3 revolutions more per mile, thats about as close as it gets.
.
you need to account for the tire sidewall. the 18 has a larger tire sidewall than the 20"
simple as that
(25 year ase master mechanic)

sites and conversions are used all the time for changing tire size and wheel size. this is not a 20" circle vs a 18" circle, if it was a train or trolley car maybe, But since we drive on tires, and tire size and sidewall comes in tons of different sizes, the assembly diameter and revolution per mile is used.

 

By the way I was not replying to your post.
I was pointing out that another poster did correct math calculation for the rims and it is exactly 20" circle vs a 18" circle.
I felt that your post didn't clarify enough that person was looking at the rims

First tire measures 235/65/18 exactly 30" in diameter
Second tire 235/55/20 exactly 30.2" in diameter

As you can see sidewall on 20" tires is smaller but diameter is 0.7% larger than original.

 

Indeed the difference is small. Thanks for the math. Perfect!