Will running 20"s affect my Speedo?

[MrWebby]

Hi,

I'm putting 20" rims on my VE wago, (Z Series 2012 series 2), I was told the cars Speedo is automatically set for 18"s and 20"s, is this true? Or will I need a re-calibration. Thanks

 

[07GTS]

it will change the speed but maby only by a little, just check it with a gps speed app on your phone to see if its still ok

 

[_R_J_K_]

You can see speed differences by plugging your old and new setups into this - www.willtheyfit.com

Offsets won't matter for speedo so just put in whatever.

 

[Sean880]

Hi,

I'm putting 20" rims on my VE wago, (Z Series 2012 series 2), I was told the cars Speedo is automatically set for 18"s and 20"s, is this true? Or will I need a re-calibration. Thanks

In basic terms, the speedo of the car is accurate (within whatever tolerance is set by the manufacturer ex factory) with the diameter of the stock tyres fitted.

Therefore if you change the diameter of the tyres fitted compared to the stock size by a significant figure the speedo will be inaccurate. Fitting a significantly higher diameter tyre will result in the previously accurate speedo under stating the actual vehicle speed because you have increased the diameter of the tyre.

It does not matter if you use 18 or 19 or 20 inch wheels. What matters is the diameter of the tyre fitted to those wheels compared with the diameter of the stock tyre.

So you match the diameters as closely as possible to the stock tyre size fitted to retain your speedo accuracy .
For example:

Stock Wheel and tyre:........................... 18 x 8 inch,, tyre 245 45 18 ...... typical diameter 677 mm
20 inch Replacement wheel and tyre........ 20 x 8.5 inch,, tyre 245 35 20 ..... typical diameter 680 mm
19 inch Replacement wheel and tyre ........19 x 8.5 inch,, tyre 245 40 19 ...... typical diameter 679 mm

 

[MrWebby]

In basic terms, the speedo of the car is accurate (within whatever tolerance is set by the manufacturer ex factory) with the diameter of the stock tyres fitted.

Therefore if you change the diameter of the tyres fitted compared to the stock size by a significant figure the speedo will be inaccurate. Fitting a significantly higher diameter tyre will result in the previously accurate speedo under stating the actual vehicle speed because you have increased the diameter of the tyre.

It does not matter if you use 18 or 19 or 20 inch wheels. What matters is the diameter of the tyre fitted to those wheels compared with the diameter of the stock tyre.

So you match the diameters as closely as possible to the stock tyre size fitted to retain your speedo accuracy .
For example:

Stock Wheel and tyre:........................... 18 x 8 inch,, tyre 245 45 18 ...... typical diameter 677 mm
20 inch Replacement wheel and tyre........ 20 x 8.5 inch,, tyre 245 35 20 ..... typical diameter 680 mm
19 inch Replacement wheel and tyre ........19 x 8.5 inch,, tyre 245 40 19 ...... typical diameter 679 mm

Now that's better. Will it have the diameter printed on the tyre?

 

[Cam-ralph]

Now that's better. Will it have the diameter printed on the tyre?

No will not have the diameter on the tyre, will have the info above eg.245(tyre width),35(side wall ratio), R20(wheel diameter) from that info you can work out the tyre diameter on a site like this https://tiresize.com/tyre-size-calculator/
You can even compare your original size to your new size and will give you an idea of any changes in speed you would expect to see

 

[VS 5.0]

I think the key term missing here is "rolling diameter".

 

[07GTS]

the percentage difference in rolling diameter is the same percentage your speedo will change

 

[losh1971]

If you're buying new wheels and tyres the tyre shop should be able to sell you the tyre and rim combination that will be close to the original set up.

 

[MrWebby]

So a quick question to add on, will 275's bolt onto the rear without needing adjusting? Haven't lowered the car. They'll be 275/30/20 20x9