Changing tyre size FORD FOCUSCC  

current size 205 50 17

propose      205 55 17

will it work ?

Not really your speedo will read wrong.

1 hour ago, Luke4efc said:

Not really your speedo will read wrong.

True, but by a very small margin. Speedos legally are allowed a +/- 10% anyway so not the most accurate instrument at best. If you want a truly accurate gauge of your speed, you'll need to use your SatNav (providing the one you have shows the speed!)

Quote

Speedos legally are allowed a +/- 10% 

I thought they were allowed + 10% BUT - 0% i.e they're not allowed to under read

For many years now since speedos became electronic and not mechanically driven they are incredibly accurate. The manufactures now set the speedos to read exactly 3% high so if you were to drive at 103 MPH according to your speedo, then your true speed would be 100 MPH. I should also mention you will also get a speeding ticket !!

The reasons behind this is many but mostly to ensure that you won't exceed a speed limit by accident. So with out all the maths you will be actually doing approx 29 MPH when your speedo shows 30 MPH.

Here is a very useful site that allows you to work out the diffrence that a tyre change will make to your speedo

https://www.tacomaworld.com/tirecalc?tires=205-50r17-205-55r17

As it happens in your case the difference is 3.1% so hardley anything to get worried about.

10 minutes ago, unofix said:

For many years now since speedos became electronic and not mechanically driven they are incredibly accurate.

 

I'm assuming then that my elderly Mk2 is mechanical? When checking the speedo against my SatNav, when doing 70MPH on SatNav, my speedo reads about 76MPH, which may explain alot of slow moving traffic on motorways from law abiding citizens thinking they are traveling much faster than they actually are LOL 🤣

All Focus MK2/MK2.5 have a electronically driven speedometer.

Very low spec Focus MK2/MK2.5 versions without ABS do have a vehicle speed sensor that is mounted on the gearbox. The VSS measures the revolutions of a shaft inside the gearbox. The vehicle speed is calculated based on the revolutions of the shaft.

All versions with ABS or IVD (ABS, ESP and traction control) use the ABS sensors. The vehicle speed is calculated based on the revolutions of the wheels.

On both systems the programmed tire size is crucial for a correct calculation. Therefore the tire size (circumference) that is programmed into the PCM must match the actual size of the tyres that are installed.

 

Hi JW1982, thanks for the info on the Focus speedo readings. As you state the tyre size is important to help ensure more accurate readings. Recently I had ForScan connected to my daughters 66 plate Ecosport and the Tyre size was 'Not Set' so I thought since I was hooked up I may as well set the tyre size. There is a drop down tyre menu with about 30 different tyre size combinations but not one with the correct size that the car has fitted (orginal Ford wheels with orginal size tyres). So I had to leave it 'Not Set'. Which left me thinking afterwards how does it know what speed it is doing ?

 

Ask yourself the question's:

Do i carry a lot of weight on a regular basis? If so, you may get tyre rub at extreme compression of suspension.

What am i trying to achieve with a 5% increase in side wall height?

If you can only get the tyre you want in the higher sidewall size then i feel 5% should be pretty safe.

On 4/29/2021 at 11:34 AM, unofix said:

The manufacturers now set the speedos to read exactly 3% high

As a matter of interest I have found that comparing my Mk3 speedo with my satnav it reads 3mph fast throughout the range rather than a percentage of the road speed. I found the same on our previous Mk1.5 Ghia estate except that was 4mph fast. Admittedly these results might be affected by the fact the satnav only displays integer mph.

10 hours ago, mjt said:

it reads 3mph fast throughout the range

mmmm........ so at zero speed your speedo shows 3MPH, and therefore it follows at 10MPH it must show 13MPH which is an error of plus 30% !!!!!!!!   🥴

You may need to consider getting a new speedo as any speed below 30 MPH you have an error that is greater than plus 10% which means it does not comply.

The manufactures of speedos Calsonic Kansei (Tyne & Wear UK) will confirm that the error on the units they presently manufacture is +3% -0%

 

Mine also shows 3 mph above the sat nav gps speed from 30mph to 70mph. At 0 it shows 0, I haven't tried it at 10 or 20mph. If I set the cruise control to 33mph the sat nav shows 30mph, cruise set to 73mph = sat nav showing 70mph. It's definitely not 3% on mine.

I suspect the issue is a phenomenon called the parallax error. Those who have cars with both speedos with pointers and a digtal speedo can try this for themselves.

Watch both the pointer and the digital speedo and you will notice as the pointer moves round the dial it appears to be some of the time the same as the digital read out and at other points on the dial it will seem faster or slower than the digtal read out.

https://www.google.co.uk/search?q=parallax+error&sxsrf=ALeKk00UCmHJ1qho3Jc99c-4qhkESyplFw%3A1619884313817&source=hp&ei=GXmNYKKZL7yijLsP6o6h2Ak&iflsig=AINFCbYAAAAAYI2HKRFJuBOujGRtjpwRED6U8cydJotj&oq=paralax&gs_lcp=Cgdnd3Mtd2l6EAEYATIHCAAQsQMQCjIHCAAQsQMQCjIECC4QCjIECAAQCjIECAAQCjIECAAQCjIECAAQCjIECAAQCjIKCC4QxwEQrwEQCjIECAAQCjoNCC4QxwEQowIQJxCTAjoECCMQJzoECC4QJzoFCAAQkQI6CAguELEDEIMBOgUIABCxAzoOCC4QsQMQgwEQxwEQowI6CAgAELEDEJECOgsILhCxAxDHARCjAjoCCAA6BQguELEDOggIABCxAxDJAzoFCAAQkgM6CAgAELEDEIMBOgcIABCHAhAUOggILhDHARCvAVCVD1iwMWCuVGgAcAB4AIABd4gBzQWSAQMyLjWYAQCgAQGqAQdnd3Mtd2l6&sclient=gws-wiz