19" wheels

[bikesteve64]

hi just bought a ds 5 hybrid it is fitted with genuine citroen wheels but they are 19" is this standard fitment. as most of the other ds,s i have looked at were 18" they are in quite bad condition what effect would it have on the car if i were to put 18" wheels on (i.e would it affect speedo reading etc) any help apreciated

[GiveMeABreak]

Let me have your VIN (please post your VIN in full without spaces - it will be automatically masked from public view after submitting your post & will only be visible to staff) and I'll check what was factory fitted, but 19" wheels were certainly an option.

[mickthemaverick]

hi just bought a ds 5 hybrid it is fitted with genuine citroen wheels but they are 19" is this standard fitment. as most of the other ds,s i have looked at were 18" they are in quite bad condition what effect would it have on the car if i were to put 18" wheels on (i.e would it affect speedo reading etc) any help apreciated

I had a similar situation when I bought my Subaru Legacy. It came with 19" wheels bearing 205/45/19 tyres which I found gave a very firm ride. I swapped them out for 18" wheels fitted with 205/55/18 tyres and improved the ride no end. The increase in tyre wall depth making up for the reduction in wheel diameter kept the rest of the sensors within acceptable tolerances as the overall radius was very similar!!

[Sloppysod]

Yes, I investigated the same to replace the 18" for lower spec 17" wheels on my C4. I was told that my car computer will need to be adjusted accordingly.
Look & play here to see the differences in speed of each tyre size.
https://tiresize.com/speedometer-calibration/

[Rp0thejester]

I'm very shocked at that. As far as I was aware speed was speed. The only difference in rim size would be volicity not speed. Ride comfort would be affected by tyre wall 'height' but wouldn't affect the speed just velocity. 10 inch mag wheels doing 30mph would be the same as 22 inch wheels doing 30mph. Or am I missing something? Like have newer vehicles got a speed sensor aimed at the tyre or.....??

[Paul-R]

Velocity and speed are surely the same thing, Ryan? What do you think is the difference?

[Rp0thejester]

Well speed is a measurement. If you measured the speed of the wheel near the hub it would be the same as the tyre speed but velocity wise, the centre would be going at a higher velocity than the outside. Or have I got that wrong? Always willing to learn so if I'm wrong, let me know please

[mickthemaverick]

Without getting too involved in a debate the difference between speed and velocity is that velocity is speed in a given direction whereas speed does not include direction. So both are measured in terms of distance travelled in a given time, eg miles per hour or feet per second, but velocity specifies in a straight line whereas speed is distance travelled in any direction. The speed of rotation is angular velocity.

That said the speedometer reading is usually taken from the gearbox or final drive where it measures the number of revolutions of the output shaft per minute. The ECU knows the circumference of the tyre which dictates the distance travelled per revolution of the wheel and the gear ratio between the shaft being measured and the wheel itself. Thus if you change the circumference of the wheel then it will result in travelling a longer or shorter distance per revolution and thus alter the distance actually travelled for one revolution of the measured shaft, resulting in an inaccurate reading on the speedo.

However if the change is very small as mine was then the difference in the actual reading is so small as to make no material difference. n my case the new arrangement would show 30.43mph when I was actually travelling at 30mph. Nothing to worry about!!

[Rp0thejester]

https://www.jagranjosh.com/general-know ... 14530821-1 trying to explain it to the wife, I found this which helped

[bikesteve64]

anybody know where i can get a descent set of 19" alloys for ds 5 at a reasonable price

[MattBLancs]

anybody know where i can get a descent set of 19" alloys for ds 5 at a reasonable price

Confused me there, thought you already had 19" and we're seeking 18" ??

As to the other discussion above, yes at 30mph a mini on 10 " wheels is going the same speed as a range rover sport on 21" bling rims who is also doing 30mph. As discussed, the rotational speed of the little wheels is much higher.
As also said, put the 21" wheels onto the mini and (it'll look awful, ride terrible and) still do 30mph, but it's speedometer will under read terribly as the distance covered by each revolution of the massive wheel is much greater than distance the little wheel will cover (so it'd only say indicate 10mph on its speedometer when actually moving down the road at 30mph. Or would actually move along at 90mph when the Speedo was just showing 30!)

The other thing said was important - changing the tyre width and aspect ratio both impact the overall diameter of the wheel so a swap from 19" to 18" can have no negative impact on Speedo accuracy so long as the other tyre attributes are altered to keep the same overall diameter.

[Rp0thejester]

Dropping down a size can add a cushioning effect on the ride as the tyre wall is bigger so more air to cushion the pot hole impacts. When it comes to speed, is it measured against the tyre or is it rotational speed of the driveshafts? If it's done by shaft speed 30mph is 30mph regardless of tyre/wheel size. There are discrepancys, is that why the police give us 10% leeway on speed?

[MattBLancs]

Dropping down a size can add a cushioning effect on the ride as the tyre wall is bigger so more air to cushion the pot hole impacts. When it comes to speed, is it measured against the tyre or is it rotational speed of the driveshafts? If it's done by shaft speed 30mph is 30mph regardless of tyre/wheel size. There are discrepancys, is that why the police give us 10% leeway on speed?

Agree on tyres, more sidewall = more comfy.

Unfortunately you've got in a muddle on the speedometer front - if you think about it, a speedometer really is just like an rpm gauge in its function - it measures the driveshaft/wheel rotational speed, only the scale you read has a conversion where this is effectively a bit of multiplication:

Wheel rotational speed (I e. Revolutions per minute) X wheel circumference (I.e. distance per rotation) = distance per minute. Obviously can then convert from that to miles per hour by multiplying by 60.

This is only true (or rather the Speedo is only correct) when the wheel circumference is as designed. On the hypothetical mini swapped onto 21" rims, the Speedo will still be set up as though each revolution moved the car forward the circumference of a 10" wheel. But it would move a much greater distance per revolution= faster for any given rotational speed

Hope that makes sense!

The other thing not talked about with more tyre sidewall Vs more inches of alloy wheel (i.e. keeping the same diameter of wheel overall) is that the car will accelerate slower with low profile tyres on big rims! This is because the sidewall weighs very little - certainly easily less than a cast aluminium wheel rim an inch bigger:

This bit of thin rubber gets bigger, doesn't weigh much! Picture from Google

And rotating masses have a greater impact than just dead weight (so the 10" wheel mini with the 21" wheels in the boot would be faster than wearing the 21" inch wheels and carrying the 19" ones in the boot instead! Obviously this daft example also ignores the terrible effective gear ratios the huge wheels would create, so the mini more likely to explode it's clutch than get any meaningful 0-60 times! )

[Rp0thejester]

Thank you so much for that info, I'm not joking, Thank you. I'm always willing to learn. Everyone else can do 1

[MattBLancs]

Thank you so much for that info, I'm not joking, Thank you. I'm always willing to learn. Everyone else can do 1

No problem at all! I like to understand stuff just as much as I like help others to do the same! All to the benefit of everyone really.

There's so much stuff on here, much of it from say a decade ago in some cases = a cracking resource of folks sharing their problems and fixes, is so much available to learn from. And a good craic too!