Tyre Question

There isn't a problem running different tyres front and rear as long as you aren't mixing them on the same axel. My camper will have 225/40/16 on the rear and

195/45/16 on the front when it's finished. There won't be a problem with the ECU.

MOT shouldn't matter as long as condition and loading is right and sizes are same on each side. Many cars have different sizes front/rear.

Shouldn't affect ECU at all, ABS might complain. As might an AWD system.

Why would you want your speedo recalibrating if it's now more accurate than it was before ?

Its not more accurate than it was before. I know speedo's over read as standard, but the wheels have made quite a big change e.g. around 30mp its not too bad as when the speedo says 25 i can tell its doing about 30mph from having driven the car loads. Its when it gets to say 70mph roughly and the speedo only says 60mph - its very noticable the speedo is a long way out. How is recaliberating done anyway? is it to do with the ecu or something else? Thanks Dazz

Go here -

Type in the size of your old tyres. Find a tyre in your new size ( was it

15"? ) thats about the same size, slightly smaller is ok, and get them on the car. Speedo's can't really be recalibrated.

-- Dan

ecu will be okay. will pass MOT, must have same size on each axle, but front and rear can be different. Advise you change them to all the same asap, for safety etc.

*yes* it is.

/me slaps forehead

Your car was overreading before, your previous reading is irrelevant. Your wheels are *very* unlikely to have changed more than 2 or 3 mph at the very most, unless you have made a seriously profile change.

Easiest way to find out, get a mate to match speed and tell you what he's reading (with your old wheels as well if you can).

What size have you actually changed from ?

So how is it done though? Isn't it something to do with gearing, increasing the overall radius will put your gearing off giving slower/faster acceleration and a higher/lower top speed? So you need to get the gearing adjusted (probably too expensive to be worth it)?

You're talking fractions of a second, if anything. It's completely inconsequential unless you're concerned primarily with figuring the car.

I've changed from 13" 165x55 all round to 15" 185x55 on the rear (at the mo) and 15" 195x50 on the front. I've looked at some of the online calculators for this and they reckon my speedo will be between 12 and 14% out. So if im doing 70, the speedo will say just about 60? Thanks Dazz

See my last reply to lordy for previous tyres. Thanks

Arse. Meant to change that to 63 ffs.

!!!!

A quick look at

will show you that yourcurrent tyres are *WAY* out.You're basically screwed - your old wheels were so ridiculously small, thateven the smallest 15" tyre (195/45-15) is still 8% too big. If you want bigger wheels, then a poverty-spec car isn't suitable. What make and model is your car ?

Return the alloys for a refund, until you own a suitable car. Your bigger wheels seriously affect your handling and performance, and they will cause increased wear-rates on your transmission - all of that is due to the way they raise the gearing of your car (by 14%). Leaving them on, is not an option !

Sorry my mistake. The original tyres were 165x65x13 and the new ones are 195x50x15 (the front ones are so thats where the speedo takes its reading). The rear ones are

185x55x15 (this is a bigger difference however as they are on the back dont matter - is this right? Thanks Dazz

Yes it has. You've now raised the gearing of your car. If aerodynamics weren't a factor, you'd now have a higher top speed, at the expense of acceleration. As it is, you've simply lost some acceleration.

Grip should certainly be - you've got an extra few cm of rubber on the road.

Ok thanks Nom. the 195x50 tyres are brand new so they'll be fine for a while yet, the 185s are a bit worn so wot il do is when the 185s wear down il get some 195x45 and put them on the front and stick the 195x50's on the back. As the difference is only about 6% at the mo, this shouldn't cause any gear damage etc? Thanks Dazz

Not worth it then :)

Good plan, but put the new 195/45 on the back, and the 195/50 on the front. Then the 195/50s will wear out quicker, allowing you to replace them with

195/45 to match the rear.

I'd also make sure the 185s are on the front right now - front tyres wear out quicker than rears, and you wanna get rid of the 185s ASAP.

But you may well be doing damage, that's the whole point. Wrong size tyres is a bad idea !

If I were you, I'd go to

and orderfour 195/45R15 tyres. Kumho Ecsta 712 are only £31 each, inc VAT. Yourproblem is then solved.