Visual Tyre Size Calculator

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:

  1. the ride height will change, the car body will be further from the ground.
  2. the top speed will increase slightly and the speedometer will under read.
  3. the car will accelerate 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

via pneus-online.co.uk

356 thoughts on “Visual Tyre Size Calculator”

  1. I have recently upgraded the tyre size on my Kia Sorrento from 245/70R17 to 265/65R17 and this has effected the handling of the car are they compensation setting’s that i can do to the wheel alignment to make the car more steady on the road.

  2. Hi Nadeem,

    In your example 145/80 and 155/80 the number 80 is the aspect ratio, the sidewall height as a percentage of the tyre width. If the aspect ratio remains the same (80), but the tyre width increases (145 to 155) then the sidewall height will increase as 80% of 155 is greater than 80% of 144. Hope that helps 🙂

    @Spike: I’ll add a section for special tyres in V2, do you happen to know of anywhere that I can get a comprehensive list of sizes, like a good mud tyre sales site?.

    @Michael: Without knowing the exact details of your cars setup… I would suggest camber adjustment. By adjusting the camber you could increase the contact patch of the wider tyres and thus increase the grip. But you couldn’t do this with your standard suspension. Is it overall poor handling or when your turning for example?

  3. hi i drive a 2008 saga blm which comes wif 175/70R13.. I wanna change to 205/40R16.. what would differ in terms of handling and stability and hw would that affect performance?would I have the risk of the tyres hittin on the walls of the tire well?

    1. Hi Ravind. I’d guess you’re moving from a 5.5/6″ wheel to a 6.5/7″ wide wheel, in that case you’ll have a greater tyre contact patch which will increase grip. The lower tyre profile will reduce sidewall flex which again improves handling but at the expense of comfort as the reduced flex means less shock absorption. Larger wheels also mean the engine needs to work harder to accelerate but you will travel further for each rotation of the wheel. Simply this means slower acceleration but increased top speed.

      Whether your tyres rub or foul on the tyre well depends on how tight things are to begin with and the offset you choose. It is pretty simple to work out what the ideal offset would be for your 205/40/16 wheels and tyres, so long as you know the stock offset, I will assume the offset of original wheel is +35:

      1) Get two bits of paper, or card which would be better.
      2) Cut one to 30mm by 175mm (strip A) and the other to the new width 30mm x 205mm (strip B)
      3) Centre of original tyre is 175/2 = 87.5mm, draw this line on strip A
      4) Centre of original tyre is 205/2 = 102.5mm, draw this line on strip B
      5) Offset is +35 so the offset line is 35mm closer to the face of the wheel from the centre line, mark this line on strip A
      6) Now, if you sit strip A on top of your original wheel (while it is still on the car) you can line up the center line of strip B and start sliding it back and forth to work out the ideal position of the new wheel. When your done, mark the position of the offset line on strip A onto strip B.
      7) measure the distance from the centre line on strip B to the offset line you just marked and this is your new ideal offset.

      Things to consider are the height of your new wheel, use my calculator to see how much taller the new radius will be (wheel & tyre) and bear this in mind as this may foul the arch liner or lip. Don’t use the rolling radius value on the calculator tho as it is showing the circumference.

  4. can u explain how effected on changed those things to the our lovely car if speedometer is more or lesser than original spec? your explaining
    will help me dude..tq

    1. NHC, You speedometer isn’t technically being affected, what is happening is that your cars speed is often read from the gearbox and the speedometer is calibrated to be fairly accurate across a small range of stock wheel sizes for that model. If the overall circumference of the wheel & tyre package increases (or decreases) then you risk moving out of this calibrated range and you can no longer be sure your speedometer reading is accurate… or at least as accurate as the manufacturer intended. My calculator gives you an idea of the changes you can expect.

      In my car for example, a 2008 Mazda 3 with nothing modified yet the speedometer over-reads by 5mph. I have confirmed this with three different makes of GPS. When my speedometer reads 50mph my road speed is actually 45, 60mph is 55, 70mph 65 etc.

      On my girlfriends old micra the stock tyres were an awkward size and quite expensive, by using the calculator I was able to find a more common size that was cheaper and the circumference change brought the road speed closer to the speedometer reading, although it still over-read by a couple of mph.

  5. can someone pl advise me on following:

    my honda civic currently has stock tyres of 205/55-R16. i want to change the tyres to 205/60-R16 to increase ground clearance. the overall dia increases by 18mm. pl advise if it can be done without comprising on cars performance and handling.

    1. Hi Maz. The sidewall height will increase by 9mm, you may find the ride is a touch more wobbly as there will be a little more flex in the sidewall but I doubt it would be noticable. If you’re on standard suspension then you shouldn’t have to worry about fouling on the arch liner either.

  6. very usefull but just cant get why cahnging the tyre width alters the diameter of the tyre/wheel combo,eg 265/70/17 is a bigger diameter than 235/70/17? any help on this would be greatly recieved.

    1. Hi Dave, the reason the overall diameter increases is because the tyre profile (the second number, 70 in this instance) is a percentage of the tyre width. So 70% of 235mm is 164.5mm and 70% of 265mm is 185.5mm.

      To get the overall diameter we convert 17 inches to millimeters: 431.8mm, and add 2x the sidewall height.
      235/70/17 is 431.8+164.5+164.5 = 760.8mm
      265/70/17 is 431.8+185.5+185.5 = 802.8mm

      I hope that helps 🙂

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