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I am currently running on stock 16" alloy wheels with 205/55/R16 tyres. I've got my eyes on some used 18" aftermarket alloys that come with 225/45Z/R18 tyres already on them. My question is will it throw my speedo out a lot? I know my speedo is out by about 3-4mph anyway as when my speedo says im doing 30 my satnav says im doing 26-27mph. So will adding bigger wheels and tyres just level my speedo out, so when my speedo says 30 my satnav says 30 which would be good, or will I be travelling even faster than speedo says and be doing 33 or whatever when I think im doing 30 resulting in me getting flashed in speed traps. Will the 225/45Z/R18 tyres be ok or will I have to go for an even lower profile like 205/35/R18 and buy new tyres. I would be upgrading to some eibach springs as well to lower it when I get round to it.

Thanks people.
Hi Charlie,

your speedo is designed to be out by a few miles per hour. This is so that if you slip over the speed limit by a needle width, you wont panic and get done for speeding, because in essence, you will probably be doing 29mph or so. This is design and is best practice, because at least that way, you always know your speed is a Maximum you are doing.

By changing the wheels with no thought, you risk throwing that out one direction or the other. for example, 16 inches + 55mm of tyre means your wheel is overall 16" 5.5cm (overall about 18.2 inches), now, if you take an 18" wheel, with a 35mm profile tyre, you are then looking at roughly 19..3 inches of diameter, so you can see, there is a difference that will affect you and your speedo).

There are certain websites out there, and previous discussions on these forums about upgrading your wheel size and tyre profile with the minimum damage to the calibration of your speedo. You may be able to get it recalibrated if you really want to make the change, but as it stands, going out, buying wheels and hoping it works is not the best practice :)
I'd say that if you go up 1 inch on the rims drop 1 inch on the tyres, just be prepared for a harsher ride.
Thanks for the advice Jeebowhite, very wise indeed. But to be honest if I went for bigger wheels and the speedo evened out so when the speedo says 70 I'm actually doing 70, instead of the speedo saying 70 when I'm actually doing 66 I could live with it as I'm actually quite a careful driver and I'll usually do like 68 on the motorways according to my speedo which is a real speed of about 64 which is why I'm always being overtaken lol cause I'm in no rush and just wanna sit back and have an easy ride.

True say Stoney that's what I was thinking. Was going to go up an inch on the rims, down an inch on the tyres and lower it on some eibach springs but I won't be looking forward to going over the big bumps.
I did this on a previous vehicle. Upgraded the rims by 2inches and had the same profile tyres. It moved my speedo from about 3mph out to bang on accurate.

I could have "recalibrated", but didn't see the point, since a more accurate speedo was preferable for me anyway.

Try it, see what happens - use your GPS to guage your true road speed vs indicated road speed after the change, and if the error has gone too much in the other direction then have the speedo recalibrated or drop an inch or so on the tyre profile.
Nice one GSM exactly what I was thinking mate
Just remember, a SatNav speed is only as accurate as the completely perfectly flat surface your driving on. Any incline up or down, means that you are looking at traveling further or closer to the satelite and it can throw your speed readings out. Sat Nav is a good guide, but again its not 110% perfect / foolproof!

I have known people use the exuse "well my speedo said I was doing 30" traveling down an incline, yet the police actually snapped them at 33 using the gun, didnt do them, but advised strongly that they calibrate the VSS and ignore the satnav!

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