Can i put 235/65r18 instead of the 235/50r18 on a vanden plas 2004 whats the difference and can it effect the driving or car in general?

 

First number is the width = 235mm. Second number is the sidewall height expressed as a percentage of the width, i.e. 235/50 = 117mm, 235/65 = 152mm. Third is wheel radius (r) in inches. 235/50/r18 gives you a diameter of 693mm and a circumference of 2174mm while 235/65/r18 give you a diameter of 762mm and a circumference of 2395mm, for a difference of 9.9%. This means that if your speedo currently reads correctly with the 235/50, changing to the 235/65 will make it read 100km/h when you're actually doing 109.9km/h (increasing circumference makes the speedo read slow, decreasing circumference makes the speedo read fast). You can always just remember that your speedo is now reading 10% under however in my state in Australia "Speedometer accuracy must be maintained and adjusted where necessary" so changing circumference is technically illegal without a speedo recalibration.

You can feel a bit more tyre roll with taller sidewalls and it can make a car feel a little more "boaty," but whether you can feel that I don't know as it really depends on your driving, driving style, experience, etc. You might be supposed to recalibrate the ABS/traction control if changing tyre circumference but that's something someone else might be able to answer with more confidence.

If it physically fits without fouling the guards and your local laws allow it then it's possible, but unless there's a good reason to change your tyre size then I've always found it easier to stay within 5% of the placarded sidewall figure.

You can suss out tyre sizes and compare sizes yourself at https://tiresize.com/tyre-size-calculator/

 

I concur with dangoesfast. The laws over here also state that the accuracy of the speedometer must be maintained and adjusted where necessary.
Additionally, there is the clarification that it is permitted for a speedo to show more than your actual speed, but definitely not less!

 

Curious, as i was told that the O D of the tires were now the same.
That the sidewall, was made shorter to fit the larger diameter wheels.
Or so said the guy at the tire store, several years ago.
As i wanted to put bigger wheels, & tires, on my Honda Civic.
To fill in the wheel wells, like we used to do to the old American cars.
So i asked about the adjustment of the speedometer to read correctly

 

That aspect ratio is too much of a change. You'd have to either go to a much narrow tire, or a much smaller wheel, neither of which would be anything I'd want to try. Firstly, just going with 235/65-18 (if such a tire even exits) would put the speedometer about 10% or 11% low.

As for Wingrider's statement about the O.D. being the same, that's not possible by changing only the aspect ratio. You have to change the aspect ratio and then either go to a smaller wheel or a narrower tire if you want a higher aspect ratio. If you want a lower aspect ratio, you need a wider tire or a larger wheel to keep the same outside diameter.

 

Hi Pancho,
I think, as others have said, that profile 50% to 65% is too much of a jump, that's about an inch and a half extra sidewall, and it probably won't fit. Where I live, it would be illegal to change the size that much, as only 5% changes are allowed.
Having said that, I do think it would be ok to use a 235/55-18. That only changes the sidewall by half an inch and the speedometer by 3.3%. Most speedometers over-read by about 7% at 60 mph, so that makes the speedo more accurate, rather than less.


Comparison 50% to 55%


Pete M

 

The wheels & tires on our cars range from 17 to 20 inches.
Without any change in the cars speedometer
The 20" wheels are easy to damage or so i read.
Seems the sidewall is too short to absorb the shock.
What the salesman said, may have some merit.
I really don't know, just repeating what was said.

 

Because different widths and aspect ratios are mounted on those different wheel sizes. You can get a tire that's within a few percentage point of the same rolling diameter yet mounted on a 3-inch different wheel size, or even the same wheel size but a wider tire. If you want to change some tire dimension, whether it be width, sidewall height, or wheel size, without significantly affecting the speedometer, you have to do a little simple algebra and find the right combination of width and aspect ratio to get to the same tire diameter that you had before.

A 225/50-16 and a 225/50-18 will NOT be the same diameter tire!

 

So what the salesman said, had some merit.
The stock Honda wheel & tire sizes, were a similar setup to ours.
O D size is about the same size.

 

Being a geek, I use a home-built GPS speedometer (although my wife objects to it in our otherwise lovely Jag).
Speedometers, not just Jaguar, always read higher than actual (so does the odometer). This is the "safe" side for the manufacturer.
It varies but is in the order of 5kph. i.e. if your speedo says 85 you are only doing 80. I decline to give my address for speeding tickets.
(standard 19" tyres in my case)
P.S. the scaling of it makes me feel good, though not the Jag (100kph say what?)

 

I've always wanted to put Calisto's or Sepang's on my SV8, but didn't want to change the ride feel going from 19" to 20". The 19" chrome "custom's" have been just fine.

 

There are tire/wheel size calculators online and Discount Tire to compare different size tires and wheels and will show you the speed difference. 50 to 65 is too much. Speedometer error would be 6.6mph - 65/71.6. It might fit, but would probably rub the fender liners at the front.


Better ride, but more tire roll - less responsive handling.