Can someone measure the outer diameter on their front tires (225 40 18 and 235 40 18) inflated at 32 cold psi plus? Please state the condition of your tires. To measure I would put something flat on top of the tire and measure from the ground up. I need this to isolate the effect of sidewall bowing on a smaller rim size tire like 215 55 16 (front diameter measuring 25.25 inches off the car vs. 24.5 on the car) versus going plus size with stiffer sidewalls. Thanks.

 

This measurement will actually vary from tire to tire. Some tire models have stiffer sidewalls than others, even though the tires are the same measured size. You would probably get a different measurement front to rear, since there is more weight placed on the front tire.

 

With your measurements, include the following info:
- Tire brand
- Mileage
- Size (225 40 or 235 40 18)
- Front or Rear
Thanks.

 

Are you concerned about final effective wheel diameter for the speedometer calibration? It's insignificant for that.

If you want to isolate the sidewall bowing, just take the 215 wheel and measure it. Then use the jack to lift the weight off the wheel and measure it again.

Dave

 

I did that on the OEM size tire. Look at my first post. The difference in diameter on my front 215 55 16 was .75 inch. What I really want is to compare a 225 40 18 and 235 40 18 under load and inflated to 32 cold psi plus to my oem. Thanks.

 

Can someone just go to his/her car and take the measurement for me? PLEASE!

 

I'm coming up with 24 5/8" on the car--235/40-18.

However, I'm not confident that if I were to take the rim off, it would measure to 25.4" like every 235/40-18 tires shows as their specs. Therefore I conclude either my measurement is off, or I am not measuring the same thing.

Another thought is that with the 235's, I have an 8" rim, not a 6.5" rim like the factory's. Still, I think whatever the tires give is not significant as far as determining the correct +1 and +2. It's simple--235/45-17 for +1, 235/40-18 for +2. What that other guy did 215-17 is bizarre, I guess he wanted the wheels to look really small at the expense of any benefit in performance or the speedo being correct.

imho this measurement is neither reliable nor valid. You can be assured that 235/40-18 is the correct replacement for a 215/55-16, not a 225. When I go through radar my speedo matches the police "your speed" sign.

 

Thanks Frank. Anyone with 225 40 18?