Wheel size calculation

[kannabie]

Hi everybody!
I was wondering if there is any guide or "cheatsheet" on tire sizes comparison, or maybe some basic formula to adhere to when swapping rims with different diameter.
For example i have a Salem 1500 with stock tire/rim size "225/75R15" and I want to use R20 rims. Is it a "trial and error" only, or there is a way to calculate parameters of tires so that the wheel would fit nicely into fender arcs, or at least for its overall size to be the same as stock one?

[Capt. Carrot]

In real life, as you increase the rim diameter you need to reduce the sidewall height so that you maintain the same rolling circumference.

A rim diameter of 20 inches would probably require a sidewall height of 50

[axe.gaijin]

You can use this handy web page to calculate new wheel sizes: https://tiresize.com/calculator/ it'll show you various different options when you want to go with a wider tyre or biggest rim size, just keep the circumference within the tolerance.

[kannabie]

Originally posted by axe.gaijin:

You can use this handy web page to calculate new wheel sizes: https://tiresize.com/calculator/ it'll show you various different options when you want to go with a wider tyre or biggest rim size, just keep the circumference within the tolerance.

Thanks! I remember using some similar webpage for the same purpose in cms18.

Originally posted by Capt. Carrot:

In real life, as you increase the rim diameter you need to reduce the sidewall height so that you maintain the same rolling circumference.

A rim diameter of 20 inches would probably require a sidewall height of 50

"maintain the same rolling circumference" - Thanks, that's exactly what i wanted to say!
So, if we take stock Smith sizes 225/75R15, and take your suggested size 225/50/R20, this would mean that every inch of diameter is equal to 5 units of sidewall height. Right?

[axe.gaijin]

225/50R20 = 225 Tyre width in Milimeter (mm) 50 = 50% of 225mm (112.5mm)

[DutchGamer78]

225/75R15:
The first number is the width 225mm. The second number is the height, thats a percentage of the first number 75% of 225 = 168,75mm. Third is rim diameter in inch 15 x 2.54 = 38,1cm = 381 mm.
So wheel diameter is 168,75 + 381 + 168,75 = 718,5mm. Radius is half of that so 359,25mm Rolling circumference, 2 x pi x r = 2 x 3,14 x 359,25 = 2256,09mm = 2,256m

225/50R20
20 x 2,54 = 50,8cm = 508mm
112,5 + 508 + 112,5 = 733mm /2 = 366,5mm
2 x 3,14 x 366,5 = 2.301,62mm = 2,301m
Irl this will result in wrong speed gage info because it is calculated in revs in the gearbox.

225/45R20
20 x 2,54 = 50,8cm = 508mm
101,25 +508 + 101,25 = 710,5 / 2 = 355,25
2.230,97mm = 2,230m
This would be the best fit with 225 width.

[kannabie]

Originally posted by axe.gaijin:

225/50R20 = 225 Tyre width in Milimeter (mm) 50 = 50% of 225mm (112.5mm)

Originally posted by DutchGamer78:

225/75R15:
The first number is the width 225mm. The second number is the height, thats a percentage of the first number 75% of 225 = 168,75mm. Third is rim diameter in inch 15 x 2.54 = 38,1cm = 381 mm.
So wheel diameter is 168,75 + 381 + 168,75 = 718,5mm. Radius is half of that so 359,25mm Rolling circumference, 2 x pi x r = 2 x 3,14 x 359,25 = 2256,09mm = 2,256m

225/50R20
20 x 2,54 = 50,8cm = 508mm
112,5 + 508 + 112,5 = 733mm /2 = 366,5mm
2 x 3,14 x 366,5 = 2.301,62mm = 2,301m
Irl this will result in wrong speed gage info because it is calculated in revs in the gearbox.

225/45R20
20 x 2,54 = 50,8cm = 508mm
101,25 +508 + 101,25 = 710,5 / 2 = 355,25
2.230,97mm = 2,230m
This would be the best fit with 225 width.

Thank you for breaking this down for me, guys! A bit complex, but perfectly clear now ))

[TheCrispyBarrett]

Originally posted by DutchGamer78:

225/75R15:
225/50R20
20 x 2,54 = 50,8cm = 508mm
112,5 + 508 + 112,5 = 733mm /2 = 366,5mm
2 x 3,14 x 366,5 = 2.301,62mm = 2,301m
Irl this will result in wrong speed gage info because it is calculated in revs in the gearbox.

225/45R20
20 x 2,54 = 50,8cm = 508mm
101,25 +508 + 101,25 = 710,5 / 2 = 355,25
2.230,97mm = 2,230m
This would be the best fit with 225 width.

Sorry for the necropost, but is there any way you could elaborate on what you mean by the whole wrong speed gage and gearbox part, as well as what makes the other option the appropriate fit? If you don't really feel like it I definitely get it. Sent you a steam award as an apology for an ancient necropost, because the original info was super useful.

[ZombieHunter]

You can alter width and not affect the speedometer. If you increase tire height it will cause the speedometer to read slower than actual b/c the tire has more circumference and travels farther than stock. If you reduce the tire height from stock, the speedometer will read faster than normal because the tire travels less distance than stock.

Anytime you adjust tire height or diameter, you must re-calibrate the speedometer.

[astrofelix59]

Originally posted by TheCrispyBarrett:

Sorry for the necropost, but is there any way you could elaborate on what you mean by the whole wrong speed gage and gearbox part, as well as what makes the other option the appropriate fit? If you don't really feel like it I definitely get it. Sent you a steam award as an apology for an ancient necropost, because the original info was super useful.

Don't let anyone hassle you for "necro posting" it shows you know how to use the search bar, a lost art in most threads, much preferred over posting the same thread every week, search function is encouraged in the official threads.
Only problem with really old threads is the poster may not still be playing, but as above someone will jump in to help.

[DutchGamer78]

np, if you are looking to switch rims Irl and want your car to (almost) behave the same and don't want or can't re-calibrate the speedo it's best to match up rolling distance of one wheel revolution as much as possible. So your new wheels almost travel the same distance each rev as stock wheels.
Also avoids problems with wheels hitting the body or frame. That could be fixed with stiffer suspension but would alter car behavior.

[Mardoin69]

To add with @DutchGamer78 post...
Changing Rim Sizes (IRL) can lead to problems... bearings wearing out faster, scraping against swing-arms, etc. etc. You may end up needing to change the 'offset' value for a larger rim in some cases. Usually a professional Tire shop can look up what will work and what won't for whatever vehicle you have. I say 'usually' cuz sometimes the guy / gal working there might not have a good enough understanding of all this and then either doesn't know what WILL work or sells you something that won't work 'properly.' Had to grind the edge on my swing-arms (IRL) on a vehicle once in order to keep the larger rims from scraping on a vehicle I had that someone put 17" rims on--default were 15".

[dogwalker1]

Google 'Tire Size Calculator' - there are tools that will tell you which sizes are compatible. ie: enter the tire dimensions and you will see a list of alternate tire dimensions that will give the same overall wheel diameter, within acceptable tolerances. Sorry I don't have the link atm but it's easy to find.

Edit: this is the one I use: (you can change rim sizes as well and find the corresponding tire sizes to match your previous wheel diameter)

https://tiresize.com/calculator/

[Rumpelcrutchskin]

There is easy way to roughly calculate how much you need to reduce tire size if you scale up the rims. In your example 225/75R15 if you scale this to R20 you need to take off 10 for each inch of added rim size. So 225/25R20. Can probably get away with 225/30R20 too.

[Mardoin69]

Originally posted by Rumpelcrutchskin:

There is easy way to roughly calculate how much you need to reduce tire size if you scale up the rims. In your example 225/75R15 if you scale this to R20 you need to take off 10 for each inch of added rim size. So 225/25R20. Can probably get away with 225/30R20 too.

Yeah, that's a pretty rough way to do it since the second number is a percentage. But, for quick easy (off the top of my mind) reference, I'd just drop 5 points off the second number for each jump up in rim diameter... skipping some diameters that just aren't normally used. So, like... 225/75 R15 to R16 would drop to 70, R17 would drop to 65, R18 would drop to 60, R19 would drop to 55, and the size in question... R20 would drop to 50.... 225/50 R20. And, that is *fairly* close to original manufactures intended wheel circumference. It will be slightly off though and depending what the intended circumference is... may need to round down and go to 225/45 R20. I wouldn't go down to a 225/30 R20 cuz I hate super low profile tires... makes road surface noise horrendous on vehicles not well insulated against outside noise.

[bjthedeejay11683]

Originally posted by dogwalker1:

Google 'Tire Size Calculator' - there are tools that will tell you which sizes are compatible. ie: enter the tire dimensions and you will see a list of alternate tire dimensions that will give the same overall wheel diameter, within acceptable tolerances. Sorry I don't have the link atm but it's easy to find.

Edit: this is the one I use: (you can change rim sizes as well and find the corresponding tire sizes to match your previous wheel diameter)

https://tiresize.com/calculator/

This was mentioned in an older post and I have been using this for maybe a year or more for nearly every build where I change the rims. It will give you a list of options that match the TOTAL diameter (rim and tire) of the stock setup. It gives you options within 3% smaller or bigger than equal.