Tire diameter differences

Status
Not open for further replies.
Joined
Aug 7, 2011
Messages
394
Location
CT USA
Just purchased a 2015 Lexus GS350 which takes 235 x 45 x 18 tires. I have snow tires on wheels from my 2016 Lexus IS 300 which used 225 x 40 x 18 tires.
The alloy wheels are the same width, offset and bolt pattern. The diameter of the tire is 1.2" smaller. 26.3" v 25.1"

Can you think of any reason not to use these tires this winter? Ground clearance decreased 1/2" and speedometer error are the only downsides I can see.

Your thoughts?
 
What kind of TPMS does your car have? Some of them may need reprogramming when OD changes.

TPMS aside, personally I would not want to lose ground clearance for winter. I'd sell the current winter tires and buy new ones in correct size.
 
I made a spreadsheet some time ago to compare this kind of thing, although primarily for use when I was looking at a different tire, but wanted as close to the same rolling radius and circumference as possible to the original tire.

In your case, stock rolling radius is 334.35mm (13.1634in), and going to the 225/40 tire will result in a rolling radius of 318.6mm (12.5433in), and an overall rolling circumference only 95.29% of the spec'd tire. This means that when you're going 65, your speedo will say you're going 68.2. It also means your odometer will be off by 4.71%. For every 1000 miles you put on the car with those tires on, your odometer will say you went 1049.4 miles.
 
Originally Posted By: Quattro Pete
What kind of TPMS does your car have? Some of them may need reprogramming when OD changes.


The TPMS for Lexus has to be reprogrammed every time the tires are swapped. Only has room in the computer for one set.
If this is what you meant. Not sure I know enough about TPMS systems.
 
Originally Posted By: SirTanon
I made a spreadsheet some time ago to compare this kind of thing, although primarily for use when I was looking at a different tire, but wanted as close to the same rolling radius and circumference as possible to the original tire.

In your case, stock rolling radius is 334.35mm (13.1634in), and going to the 225/40 tire will result in a rolling radius of 318.6mm (12.5433in), and an overall rolling circumference only 95.29% of the spec'd tire. This means that when you're going 65, your speedo will say you're going 68.2. It also means your odometer will be off by 4.71%. For every 1000 miles you put on the car with those tires on, your odometer will say you went 1049.4 miles.


Great information. Thanks. Seems like although there will be a difference the errors will be to the conservative side.
 
I wouldn't worry about the ground clearance at all, especially since you already own the wheels and tires. Buying a new set of tires just because of the size would be foolish. A half inch isn't much and if the snow is so deep that the half inch does matter, you probably should be waiting for the plows to come through.

I have run different diameter winter and summer tires on my Sienna van for many years with no issues. In fact, the narrower winter tires do a better job of sinking through the snow and getting a better bite on the pavement below. The TPMS on this car didn't care about the size of the tires, only about the difference in circumference. That is how it sensed that one was low on pressure. It would rotate faster than the others. Granted, my Sienna probably has an older, less sophisticated TPMS technology than what you have.

Just use your GPS for speed readings since the speedometer will be off. And get those winter tires off as soon as the last chance for snow has passed. They will wear really quickly in warmer temps.
 
Originally Posted By: MajorCavalry
Just purchased a 2015 Lexus GS350 which takes 235 x 45 x 18 tires. I have snow tires on wheels from my 2016 Lexus IS 300 which used 225 x 40 x 18 tires.
The alloy wheels are the same width, offset and bolt pattern. The diameter of the tire is 1.2" smaller. 26.3" v 25.1"
Can you think of any reason not to use these tires this winter? Ground clearance decreased 1/2" and speedometer error are the only downsides I can see. Your thoughts?


Can't understand why one diameter is smaller when both tires are 18" ! I see that you have a small footprint and lower sidewall but the diameter ought to be the same.
 
Originally Posted By: Pelican
Originally Posted By: MajorCavalry
Just purchased a 2015 Lexus GS350 which takes 235 x 45 x 18 tires. I have snow tires on wheels from my 2016 Lexus IS 300 which used 225 x 40 x 18 tires.
The alloy wheels are the same width, offset and bolt pattern. The diameter of the tire is 1.2" smaller. 26.3" v 25.1"
Can you think of any reason not to use these tires this winter? Ground clearance decreased 1/2" and speedometer error are the only downsides I can see. Your thoughts?


Can't understand why one diameter is smaller when both tires are 18" ! I see that you have a small footprint and lower sidewall but the diameter ought to be the same.


I can't tell if you're joking...

How do you think tires are sized?

What do you think all those numbers, like 235/45, mean?
 
This is a very common swap in Germany when winter wheels are employed. Thinner tires with a slightly higher profile is also common.
The TPMS couldn't care less what they are in, you could throw them in a piece of sealed plastic pipe, pressurize it and toss them in the trunk if you wanted to run common valve stems.
All 4 TPMS in the pipe will read the same and keep the ecm happy.
 
Originally Posted By: MajorCavalry
Just purchased a 2015 Lexus GS350 which takes 235 x 45 x 18 tires. I have snow tires on wheels from my 2016 Lexus IS 300 which used 225 x 40 x 18 tires.
The alloy wheels are the same width, offset and bolt pattern. The diameter of the tire is 1.2" smaller. 26.3" v 25.1"

Can you think of any reason not to use these tires this winter? Ground clearance decreased 1/2" and speedometer error are the only downsides I can see.

Your thoughts?


You go from a 94 to a 92 load index...that would make me uncomfortable without knowing the weight of the car...

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

I would see what CapriRacer thinks. Perhaps both sets of tires are overkill for your car, or perhaps, you're overloading the smaller tires...
 
Last edited:
Originally Posted By: Trav
This is a very common swap in Germany when winter wheels are employed. Thinner tires with a slightly higher profile is also common.
The TPMS couldn't care less what they are in, you could throw them in a piece of sealed plastic pipe, pressurize it and toss them in the trunk if you wanted to run common valve stems.
All 4 TPMS in the pipe will read the same and keep the ecm happy.


True.

However, the tires in question are both thinner, and lower profile, with reduced load index, and the TUV would likely frown on the swap...
 
1/2" different diameter is only 1/4" from the axle to the ground and there is no such thing as reprogramming TPMS. The scaling for the speedometer is a value set in the ECU program and can be updated if you care to with a OBDC programmer or by the dealer. With slightly smaller diameter tires you will actually be going slightly slower than the speedometer shows. Unless your a nervous Nelly or obsessive compulsive those tires are fine for swap.
 
I see you have AWD on this vehicle? If so you should only run tires front to back difference of 1/4 inch circumference maximum. At least this is the recommendation from Subaru, AUDI and forgot the other AWD vehicle. Ed
 
Last edited:
Originally Posted By: Eddie
I see you have AWD on this vehicle? If so you should only run tires front to back difference of 1/4 inch circumference maximum. At least this is the recommendation from Subaru, AUDI and forgot the other AWD vehicle. Ed

Presumably he is replacing all 4, so this would not be an issue, but the lower load rating index might, as Astro noted.
 
Well of course the other reason is that with less sidewall, more likely to blow out a tire or get a bent/cracked rim. I have a set of 40's on one car and a set of 45's on the other. More blown tires/bent rims on the 40's than on the 45.
 
Originally Posted By: Astro14
Originally Posted By: MajorCavalry
Just purchased a 2015 Lexus GS350 which takes 235 x 45 x 18 tires. I have snow tires on wheels from my 2016 Lexus IS 300 which used 225 x 40 x 18 tires.
The alloy wheels are the same width, offset and bolt pattern. The diameter of the tire is 1.2" smaller. 26.3" v 25.1"

Can you think of any reason not to use these tires this winter? Ground clearance decreased 1/2" and speedometer error are the only downsides I can see.

Your thoughts?


You go from a 94 to a 92 load index...that would make me uncomfortable without knowing the weight of the car...

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

I would see what CapriRacer thinks. Perhaps both sets of tires are overkill for your car, or perhaps, you're overloading the smaller tires...


Load range index of 92 should support 5600 lbs (approx). Car’s curb weight is 3800 lbs (approx)
 
Originally Posted By: Astro14
Originally Posted By: Trav
This is a very common swap in Germany when winter wheels are employed. Thinner tires with a slightly higher profile is also common.
The TPMS couldn't care less what they are in, you could throw them in a piece of sealed plastic pipe, pressurize it and toss them in the trunk if you wanted to run common valve stems.
All 4 TPMS in the pipe will read the same and keep the ecm happy.


True.

However, the tires in question are both thinner, and lower profile, with reduced load index, and the TUV would likely frown on the swap...


They sure would, they have to be listed in the papers or carry an ABE for the application.
Today unlike years ago tire manufacturers are providing the ABE paperwork if the tire is compatible.
Back then It was a real fiasco and expensive to bump my bikes rear tire size by 10mm.
 
Quote:
Load range index of 92 should support 5600 lbs (approx). Car’s curb weight is 3800 lbs (approx)
But weight does not always spread evenly across all 4 tires. When you corner or brake, weight shifts quite a bit.
 
Your overall diameter and circumference are 5% less, probably not s big deal but most manufacturers allow 3%. The scrub radius will go positive by 5mm which may affect handling.

You need to consult a load/inflation table for your different load indexes. This will tell you how much to inflate your lower rated tire vs your stock size. Taking the 94 vs. 92 load index previously mentioned...If your car recommends 33 psi you will need to inflate your snows to 36 psi to carry the same load. Unless your winter tire are XL rated then you need to go to 42psi to carry the same load.

For the best answer post the size and load index located on the door placard and what your snows show on the sidewall. Or read this: Continental technical data book
 
This is a terrible idea. That's a drop of 2 tire sizes! That means the load carrying capacity is smaller and that increases the risk of failure. You don't want that to be happening in winter!

Sell the tires and get winter tires of the correct size. Your family will thank you (more likely they won't even know, but YOU"LL know!)
 
Status
Not open for further replies.
Back
Top