I will be putting new wheels/tires on shortly and are getting a bit over-analytical about tire size and outside diameter and wanted to do a reality check to see if it will actually make a noticeable difference.

For example, lets compare 255/35-20 and 245/40-20s (tires will most likely go on a 8.5" rim):
The OD is 0.7" larger on the 245/40s (and almost the same amount larger than stock)
Now, I don't care about the speedo (the weight should be similar, so ignore that as well), but will the subsequent change in overall gearing actually feel noticeable?

I like the added sidewall of the 245/40s and would prefer to run them, but not at the expense of acceleration or feeling.

This get further convoluted when you look at 245/40-19s and whether or not the 0.5 smaller OD might actually help acceleration (like a gear change).

Anyone have any experience running different OD tires on their car with any real-world feedback (not the typical theoretical response of "it should make a difference" -- I already know that)?

 

Adding 0.7" to the diameter is slightly less than a 3% reduction in "effective gear ratio." This would be the same as changing 3.55 gears to 3.45 which is so negligible that you will not really feel the difference, nor would a timeslip at the dragstrip show a difference (all else held equal, such as traction & weight).

 

You wont notice a difference.

 

The difference between my snow tires and stock tires is less than 1% and I don't notice any difference at all.

 

Stock wheel size: 235mm 55% 17"
235mm / 25.4 inches/mm = 9.252"
Diameter = 9.252" * 55% * 2 + 17" = 27.177"

Option A size: 255mm 35% 20"
255mm / 25.4 inches/mm = 10.039"
Diameter = 10.039" * 35% * 2 + 20" = 27.027"

Option B size: 245mm 40% 20"
245mm / 25.4 inches/mm = 9.646"
Diameter = 9.646" * 40% * 2 + 20" = 27.717"

Option C size: 245mm 40% 19"
245mm / 25.4 inches/mm = 9.646"
Diameter = 9.646" * 40% * 2 + 19" = 26.717"


With Option A @ 70 MPH indicated, speed = 70 MPH * 27.027" / 27.177" = 69.6 MPH actual
With Option B @ 70 MPH indicated, speed = 70 MPH * 27.717" / 27.177" = 71.4 MPH actual
With Option C @ 70 MPH indicated, speed = 70 MPH * 26.717" / 27.177" = 68.8 MPH actual