Tire size vs Speedometer reading

I drive a '21 I4 SR rwd that came factory with 245/75r16 toyo open country A31s. I'm convinced that the speedo etc. Is spec'd for 265-75 and that this is why there isn't a way to calibrate for tire size, I always seem to be at the theoretical under in terms of speed and my MPGs are consistently over the epa ratings (my lifetime average is 24.66 over just north of 22k miles across the US) at a point where I feel that the increase in tire size would bring it back down to the ~21-22 highway.

Could it be the weight difference from the lack of front drive?

I live in ECT and I'm more of a Jeremy Clarkson than a James May behind the wheel.


What are your thoughts, has anyone else experienced a similar notion?

 

I didn't notice much difference when I went from 265/70 to 265/75/16. Obviously gas mileage went down a bit, due to the extra weight, but isn't noticeable to me. Same with performance.

I check my speedo, when I see those radar speed limit signs, and they are bang on.

*But, the V6 may be different than the I4

 

I have LT265/75r16s on mine. The speedometer agrees with the radar readouts that I come across. I noticed the same with my '09 4-banger, with the same tires.

 

Speedo in my truck is spot on with 265/75s. I checked it with my gps.