Wheel sizes on cars

[Terry Toenges]

I find myself in need of enlightenment. I lost some of the wheels from my HO John Bull cars. I would like to buy some wheels. I want to know what size to buy.
Here's my issue. I used my digital dial caliper and measured the Bull cars' diameter and they .406". For reference, I also measured standard Bachmann HO freight car wheels. They are .0456" in diameter.
I used this conversion chart to determine what real life size I needed.
https://justin.vashonsd.org/Resources/ScaleConversionCalculator/ScaleConversionCalculator.html
Convert from Scale to Real-life
(For the Bull wheels)
Scale Factor - 87:1
Scale Size - .406 inches
       =    
Real Size - 35.32 inches
-------------------------------------
(For the standard wheels)
Scale Factor - 87:1
Scale Size - .456 inches
       =    
Real Size - 39.67 inches
------------------------------------
That comes out to about 35" for the Bull wheels and about 40" for the standard wheels.
The HO wheels Bachmann sells 33" and 36" wheels. The Bull wheels are smaller than the standard wheels.
The standard wheels would be 40" according the my measurements.
What am I doing wrong in the calculation?
I did make sure I zeroed the caliper before I measured.
---------------------------
The Bull wheels


Standard wheels

   

[WoundedBear]

I don't think you include the flange. Just measure the centre area of the tread.

Sid

[Terry Toenges]

Thank you Sid. I just learned something new.
I never had the occasion to measure the wheels before. When I see 33" wheels, I thought it was the total diameter. When I just measured the standard wheels it came out to .378"  or something like that which translates to 33".
That means the John Bull wheels are 29".

[Terry Toenges]

It looks like Kadee and Intermountain have 28" wheels. I didn't find any 29" ones. I measured again and it is close to .326 or .327 which translates to 28.362 to 28.449. I guess the 28" wheels will be good.

[jward]

RIvarossi passenger cars used 31" wheels, and i believe IHC sold replacements for them. You might be able to find some on Ebay.

[Terry Toenges]

I'm ordering wheels for Bachmann's John Bull passenger cars. They are close to 28".

[Terry Toenges]

Thanks Calypso.
I received the new 28" Intermountain wheels today.
They are a little smaller than the stock wheels.

The IM wheels are longer, too I had to drill holes through the trucks in order to make them fit.

Now the heavier wheels are causing some wheel slippage on the Bull going up my 2%grade. I'm trying to figure out how to add weight to the shed with the motor. There's not much room for anything. I can maybe do a little in a couple of places.