If you are thinking of changing your car tyre size or are planning on buying a new wheel/tyre combination then you can use the flash tool below for a visual tyre size comparison of your stock wheel and tyre size versus your new wheel and tyre combination.
Got a comment or question regarding the current version of the tyre size calculator? Click here to leave a comment or ask a question.
Vesion 2 of the tyre size calculator is in development, if there’s anything you’d like to see in version 2 then please feel free to leave a comment here: Tyre Size Calculator V2: Wishlist.
Why is this important?
Matching the combined wheel and tyre diameter is important as it will ensure your ride height and speedometer readings don’t change drastically. An example of this would be fitting 17″ wheels to a car that has 14″ wheels as standard, there are three main consequences:
- the ride height will change, the car will be further from the ground.
- the top speed will increase slightly and the speedometer will under read.
- the car will acceleration slower.
Read on to see the reason for this in detail.
Tyre Size Guide
Ride Height
Take the default example in the flash tyre size calculator above, here we have a stock tyre size of 165/55R14 and a new tyre size of 215/45R17. There is a total difference of 8.82cm in the two wheel and tyre diameters, if we divide this by two we get a difference in radius of 4.41cm. If we ignore tyre pressures and the effects of adding larger, wider wheels to suspension geometry; then this is roughly how much higher the car will be from the ground.
Speedometer
Then there’s the speedometer, a cars speed reading is usually taken from the transmission and is set based on a number of predefined constants. When you change your wheel and tyre combination you are taking one of these constants and making it variable. The speedometer does not know the new value and so continues to read based on the original wheel & tyre size. If you look at the above default example again you will see that with a 215/45R17 wheel and tyre combination there is a difference of 16.4% in the speedometer reading over the stock setup. As you can see, at 60mph your speedometer should actually be reading 69.9mph!
Acceleration
If we fall back to the default example again, the 215/45R17 wheel and tyre combination has a larger overall diameter by 8.82cm. A larger diameter has a greater rotational inertia which requires more energy to accelerate. Or in other words: the bigger the wheel, the slower the acceleration.
It should be noted that putting a bigger wheel and tyre combination on your car does not reduce your cars power. Your cars power remains the same; however it takes more force to turn your car’s wheels resulting in slower acceleration.
Tyre Width Equivalency Table
You may also find the following table of recommended tyre widths for given rim widths useful when changing your tyre width.
|
Rim width
|
Min tyre width
|
Ideal tyre width
|
Max tyre width
|
|---|---|---|---|
|
5 .0 inches
|
155 mm
|
165 ou 175 mm
|
185 mm
|
|
5.5 inches
|
165 mm
|
175 ou 185 mm
|
195 mm
|
|
6.0 inches
|
175 mm
|
185 ou 195 mm
|
205 mm
|
|
6.5 inches
|
185 mm
|
195 ou 205 mm
|
215 mm
|
|
7.0 inches
|
195 mm
|
205 ou 215 mm
|
225 mm
|
|
7.5 inches
|
205 mm
|
215 ou 225 mm
|
235 mm
|
|
8.0 inches
|
215 mm
|
225 ou 235 mm
|
245 mm
|
|
8.5 inches
|
225 mm
|
235 ou 245 mm
|
255 mm
|
|
9,0 inches
|
235 mm
|
245 ou 255 mm
|
265 mm
|
|
9.5 inches
|
245 mm
|
255 ou 265 mm
|
275 mm
|
|
10.0 inches
|
255 mm
|
265 or 275 mm
|
285 mm
|
|
10.5 inches
|
265 mm
|
275 or 285 mm
|
295 mm
|
|
11.0 inches
|
275 mm
|
285 or 295 mm
|
305 mm
|
|
11.5 inches
|
285 mm
|
295 or 305 mm
|
315 mm
|
|
12.0 inches
|
295 mm
|
305 or 315 mm
|
325 mm
|
|
12.5 inches
|
305 mm
|
315 or 325 mm
|
335 mm
|
Hi Andy,
I’ve never had runflats but had heard they could be pretty harsh. I know the sidewalls on the runflat tyres are very stiff compared to regular tyres but I also found this which may be of interest:
I’d consult some other BMW owners before changing to non runflats first, this applies to changing the entire wheel and tyre package as well.
Try the follwoing forums and clubs for owners views:
http://www.bmwcarclubforum.co.uk/forum/forum_topics.asp?FID=29
http://www.pistonheads.com/gassing/forumGroup.asp?c=77&catname=BMW
I have a BMW 2008 320d M Sport with 225/45R17 & 255/40R17 run flats and the ride is very jittery. I like the stiff suspension, but I do not like the high frequency vibrations coming through which I think is through the tyres being really stiff.
In peoples opinion, what effect will have replacing the tyres (and wheels?) to
A) the same size non run flats
B 205/55R16 wheels and tyres non runflat.
Thanks in advance Andy
Thnak
Peter, it might be worth hitting CTRL+F5 or clearing your browser cache. The address for the calculator has changed and you browser is likely still looking for it in the old location.
Not bored, I like learning new stuff
Hi Insomniac. No calculator yet I’m afraid just a big white space
Still banging on about this tyre size against speed thing, most modern sport-ish bikes have electronic speedos even if they are analogue rather than digital. They assume a standard rear tyre and use the engine revs and gear position to compute the speed. However…. on a lot of trail bikes like my old Suzuki DR600 the speedo is mechanical driven by a cable from a small gearbox on one end of the front hub. So it will be the front tyre size which will affect the reading. Getting boring now ? Peter.
@Peter:
No there’s nothing wrong with your flash player, I lost all my site files when the latest version of my blog deleted every file on my hosting account. I got distracted trying to setup my site again with a new theme and better backup routine, I’ve restored the tyre size calculator and you should see it again now.
Thanks for the additional info too, I notice the front wheels can go as low as 90 in width so I may tweak the options a little to include some values from that site.
To Insomniac. Hi, with regard to motorcycles the size range was perfect for me.The standard rear for my bike is only 150.70×17 I am running a 160.70×17 at present which puts me on the wrong side of the speedo error thing. I will be changing my tyres soon and if it will go in I will have a 170.60×17 rear. This will mean the speedo will be slightly under reading. To see typical bike sizes go to….. http://www.motorcycletyresuk.com/
Which is a local supplier.
Incidentaly the calculator has recently gone somewhat less visual. In fact it is not apearing at all. Everything else on the page is present. Do I have a problem with flash player?
just fitted 215/60/17`s to back of my previa to try to eeck a bit more mpg. found your website and tyre calculator most helpful, thanks, Danny
@Peter:
I’d not thought of the tyre size calculator in terms of motorbikes, although the basics should all remain the same. Are the range of widths, sidewall heights and tyre sizes sufficient for bikes or would it be useful if I added more?
Glad the tool was of use
@Danny:
I’m glad my little utility helped, always good to get feedback and to know that people appreciate it
I have just used the visual tyre size calculator to look at the effect of possible alternative sizes for motorcycles. It was exactly what I needed as I particularly wanted to check the indicated/actual speed situation. I will be posting a message on the Suzuki Owners Club site as I’m sure others will find it usefull. Many thanks,
Peter.
I have a 2005 BMW 7 Series and the Tyre Size is 275 34 R20. I also have a set of Tyres from my old 1997 BMS 7 Series which I have now sold. Would I be able to use my old wheels and tyres on my new car. They are 235 60 R16
@Malcolm: There actually isn’t a whole lot of difference between the two sets of wheels & tyres in terms of overall rolling radius; your old 16″ wheels being a little under 4cm shorter with those tyres. What you would need to watch is the PCD, Center bore and offsets of the two sets of wheels. I believe that BMW are pretty consistent with their PCD/Center bores which are usually 5×120 and around 72mm for the C/Bore I think, although you should check because I’ve been wrong before. The offset will almost certainly be different as there is a 40mm difference in width between the two sets of tyres.
I’ve a visual offset calculator that would help if I ever got round to finishing it, but off the top of my head (and without checking offsets) I expect your old wheels and tyres would fit but sit fairly deep inside the arches.
I hope thats of some help.
How much effect on riding quality does tyre size have (particularly wall thickness)? Say 245/45 R17 against 255/50 R127 tyres, which would give the softer ride on the same suspension?
@Barry: As a rule of thumb a bigger sidewall will often have a softer ride. In your example, if you were comparing a 245/45R17 Toyo Proxes to 255/50R17 Toyo Proxes the 50 profile tyre sidewall will be 17mm deeper, and assuming all tyres were made equal they would offer the more comfortable ride while sacrificing a tiny amount of handling due to the increased sidewall flex. However all tyres aren’t equal and there are variances in rubber compound and construction that could make a difference.
Hope that helps