I just have a little over 1000 miles on them now but, they have been great. Noise level is very low, wet traction has been excellent, at 32psi front and 30psi in the rear, the ride is much smoother than the Goodyears and I am extremely happy with them.
I do not know if my mileage went down or up as I really did not pay much attention to the mileage on the Goodyears. With the Venza being brand new, I do not think you get your best mileage on a car until you have at least 5k to 10K miles on it. I can tell you that on a 75 mile trip at a near constant 70mph, the computer said I was getting a little more than 27mpg. My around town mileage is showing an average of 22.5mpg. This is with about 1800 miles on the Venza.
I think the 255x50x20 size tires are just slightly taller than the originals and only turn about 8 revolutions less in a mile than the 245x50x20's. Speedometer changes will be neglible.
They look great and were a change-for-the-better to my Venza.