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Tyres Increasing tyre sidewall to 40 - implications.

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oodLes

oodLes

Active Member
Nthusiast
Oct 14, 2019
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Australia
I'm currently looking at replacing my tyres (in the distant future) with the Michelin P4S' and when doing so increasing the sidewall to 40 from the stock 35.

Here in Aus the roads are pretty garbage and I've seen plenty of people with damaged wheels and tyres. I'm pretty worried about this, so I've been looking into ways to reduce the potential damage or what to replace the current tyres with if damage occurs.

So far the easiest solution seems to be going up to 40 from 35 sidewall. It might not be much but every bit will help with the roads I might be driving on.
What I'm trying to confirm now is what implications this will have, as far as I can see this will:
  • Add 23mm of ride height
  • Add 74mm to circumference to the tyre
  • Alter the Speedometer reading by -3.5%
The speedometer reading I'm not terribly concerned about, I've already tested the speedometer on my car and found it to read roughly 4.5km/h (2.8mph) higher than it should across the board, so with a 40 sidewall this difference will be almost completely cancelled out. (I did this testing with a GARMIN GLONASS High Accuracy GPS Speedometer, not just my phone lol)

Just wondering if I've missed something or this might be a nice easy solution to a slightly worrying problem. Biggest downside I can see is 23mm of ride height, but here in Aus that might just be a good thing as well.. :)
 
How did you get to +23 mm of ride height? +5% of tire cross section means +11,75 mm ride height afaik. Also speedo reading will be altered by +3,6%, which matches the additional circumference. Additional circumference and lower speedo readings dont go along.
Additionally you will have a less direct feeling for what the car and tire are doing because a higher sidewall works as kind of a softer spring compared to a lower sidewall. Also traction, both in longitudinal as in laterial direction, will change. However, if you can actually experience any of these changes will be up to your driving skill & style ;)
 
without said:
How did you get to +23 mm of ride height? +5% of tire cross section means +11,75 mm ride height afaik. Also speedo reading will be altered by +3,6%, which matches the additional circumference. Additional circumference and lower speedo readings dont go along.
Additionally you will have a less direct feeling for what the car and tire are doing because a higher sidewall works as kind of a softer spring compared to a lower sidewall. Also traction, both in longitudinal as in laterial direction, will change. However, if you can actually experience any of these changes will be up to your driving skill & style ;)
Click to expand...

I used websites like willtheyfit.com and tiresize.com/comparison to get these figures.

You're correct regarding the ride height, I was referencing the entire diameter rather than the increase in sidewall itself, this is why I 'measure 5 times cut once', not the best at maths haha!

As for the speedo correction, are you saying that what I have there is correct? My testing seems to show that at 114 on the digital speedo I'm doing 110/109.5kmh in my fastback, so if I increased to a 40 sidewall it would be 3.5% of 114kmh, or 3.99km/h.
 
I might have been confusing myself earlier. If you increase the circumference of your tire, you increase the speed you're going while the speedo still shows the same number. Assuming a perfectly precise speedometer, this means:

235/35R19: You're going 100 km/h, speedo shows 100 km/h
235/40R19: You're going 103,6 km/h, speedo still shows 100 km/h (that would be illegal at least in germany)

The reason for this is, that your speedo calculates the speed as a product of wheel revolutions per time, wheel radius (which is directly related to the circumference) and pi.

Edit: Every measurement device has some kind of error. Speedos are usually allowed to err on the positive side (i.e. show more than you do) but not on the negative side. Depending on your local regulations, assuming that the "error" from changing the tire dimensions compensates with the error of the speedo does not work. In germany that would still be illegal, i dont know about australia though.
 
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Although these calculations give a good approximation it isn't as simple as described above. You may have noticed a flat section at the bottom of your tyre when under load ;) The rolling radius is less than the theoretical value and depends on tyre pressure and sidewall stiffness for both 18" and 19" wheels. the only way to tell for real is to carry out a practical comparison experiment. Let us know :)
 
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No errors in speedo, no increase in ride height and I have been running 40 psi since I've purchased the Veloster N. The car will ride firmer and respond better.

Speedometers are an good approximation of measured speed. They are no means exact in anyway. The difference in ride height and speedometer readings are negligible. I will however say, your not getting a 23mm ride height increase by airing the tyres to 40 psi. Combining the weight of the car, sag of the suspension, you might get a 5mm difference. Once the car settles during driving, there will be no appreciable difference.

I've got even wear across all the tyres, so there is no appreciable wear difference either. With the proper rotation, I believe tread life is nominal.
 
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R Veloster N said:
No errors in speedo, no increase in ride height and I have been running 40 psi since I've purchased the Veloster N. The car will ride firmer and respond better.

Speedometers are an good approximation of measured speed. They are no means exact in anyway. The difference in ride height and speedometer readings are negligible. I will however say, your not getting a 23mm ride height increase by airing the tyres to 40 psi. Combining the weight of the car, sag of the suspension, you might get a 5mm difference. Once the car settles during driving, there will be no appreciable difference.

I've got even wear across all the tyres, so there is no appreciable wear difference either. With the proper rotation, I believe tread life is nominal.
Click to expand...

I'm talking about tyre sidewall not inflation pressure, were you as well?

Pastiche said:
I was also considering increasing the tyre sidewall but in Spain, since the law does not allow an external diameter difference of more than +/- 3%, the only possibilities are 225/40 R19 or 245/35 R19. That said, with the first one the wheels would be even more exposed, so it has to be 245/35 R19 or stay with the stock size.
Click to expand...

In Australia there isn't a restriction for such a small increase, or you would never ever be pulled up on it.
 
For these thoughts I‘d go for the 225/40 R19 idea. I think it‘s a good compromise between ride hight, comfort and costs. The sidewall is still higher as on 245/35.
 
Shadow-Flo said:
For these thoughts I‘d go for the 225/40 R19 idea. I think it‘s a good compromise between ride hight, comfort and costs. The sidewall is still higher as on 245/35.
Click to expand...

Forgive my ignorance but why would I reduce the tyre width but increase rim width, to keep a stiffer sidewall?
 
I haven‘t read in your posts anything about increasing rim width. And 225/40 are fine on 8“ wide rims. The sidewall of 225/40 is the highest of the mentioned ones accept the 235/40s.
225*0,4= 90,00
235*0,35= 82,25
245*0,35= 85,75
245 235*0,4= 94,00
 
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Shadow-Flo said:
I haven‘t read in your posts anything about increasing rim width. And 225/40 are fine on 8“ wide rims. The sidewall of 225/40 is the highest of the mentioned ones accept the 235/40s.
225*0,4= 90,00
235*0,35= 82,25
245*0,35= 85,75
245*0,4= 94,00
Click to expand...

You didn't mention anything but I swear someone in her was talking about 8.5 rims. I must have misread, my apologies.

The 225 on 8" wheel with a higher profile is simply to keep a stiff sidewall is that your thinking?
 
My thinking was only to increase the hight of the sidewall to raise the rim to get a better protection. To be honest, all the rim protectors on the tires won't help when smashing sidewards onto a kerb or crossing a kerb too fast. The most effective way is to keep the rim in distance to the kerbs and edges of potholes. Of course, when you can get a strong protection lip in addition, it can help in some situations. Goodyears are a good choice for that. The 235/40 R19 would be even higher, as calculated, for me a bit to big in diameter, what reduces the wheel torque. The best size would be 235/40 R18 :cool:
 
Hello folks just to clarify is it ok to fit 235/40 R18 tyres to my recommended 235/35/R19 8.OJ R19 rims if this is correct then that would open up more tyres to select from, I take it would give the same rolling distance and would not deviate too much from the recommended tyre size. Can anyone confirm Or give recommendations any advice would be very much welcome.
 
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Who! Me. said:
Hello folks just to clarify is it ok to fit 235/40 R18 tyres to my recommended 235/35/R19 8.OJ R19 rims if this is correct then that would open up more tyres to select from, I take it would give the same rolling distance and would not deviate too much from the recommended tyre size. Can anyone confirm Or give recommendations any advice would be very much welcome.
Click to expand...
No mate, 18 inch tyres will only go on 18 inch rims. Rollout is roughly the same between the tyres based on profile differences but it's the wheel (and therefore tyre) diameter that is the problem here.
 
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