Wrong tyres

Car Mechanics (UK) - - Help! -

Re­cently I fit­ted new tyres to my ve­hi­cle. How­ever, I 'm re­li­ably in­formed the new 245x65/17 tyres shouldn't have been fit­ted, be­cause the man­u­fac­turer's OE tyres are 235x65/17. What prob­lems have I cre­ated and how detri­men­tal are they as a whole in ve­hi­cle per­for­mance? The new tyres fit­ted are Da­vanti Ter­ra­toura 245x65/17.

Dave Oates

Un­for­tu­nately, the orig­i­nal reg­is­tra­tion num­ber you supplied only in­formed me that the ve­hi­cle is a John Deere and un­taxed. Be­cause of this I am pre­sum­ing it is a farm ve­hi­cle and so my ad­vice will be based on that in­for­ma­tion as I am un­able to find the OE spec for the ve­hi­cle.

What is not al­ways re­alised is that be­cause the rim size and as­pect ra­tio of the tyre are the same, this will not mean that the over­all di­am­e­ter is the same.

The 245x65/17 tyres will have a side­wall height 6mm above the rec­om­mended fit­ment and a di­am­e­ter of 13mm above the rec­om­mended fit­ment – this will give a cir­cum­fer­ence of 41mm above the rec­om­mended fit­ment.

This means the cir­cum­fer­ence is 1.8% greater than the orig­i­nal fit­ment and the speedome­ter will be wrong. But as I am pre­sum­ing this is an off-road ve­hi­cle, that is not a ma­jor ob­sta­cle.

Me­chan­i­cally speak­ing you may no­tice a slight de­crease in power, and pro­vided the tyres have plenty of clear­ance there should be no other prob­lems. From the rec­om­mended tyre width of 235 I would ex­pect the wheel rims to be 8.5 and this size of rim will eas­ily ac­com­mo­date the 245-width tyre. In fact, it could safely use any­thing from a 225-255-width tyre.

I then re­ceived a re­ply that in­formed me the orig­i­nal reg­is­tra­tion let­ters were mis­tak­enly trans­posed: the ve­hi­cle was in fact a 2009 Land Rover Free­lander that was taxed and MOT’D.

As your ve­hi­cle is road­wor­thy, I would first men­tion that if the garage ad­vised you to have the tyres fit­ted and sold them to you, they should re­place them, but if you bought the tyres sep­a­rately and then had them fit­ted, the garage have sim­ply car­ried out your re­quest.

The ad­vice would be the same on any ve­hi­cle with re­gard to fit­ment of th­ese tyres to the rims. But as your ve­hi­cle is used on the road you should be aware that the speedome­ter will read in­cor­rectly. As you have fit­ted all four tyres I do not be­lieve this will have an ad­verse ef­fect on the four-wheel-drive sys­tem.

Land Rover do not spec­ify the 245x65/17 as an op­tion. Pro­vided you take into ac­count that your ac­tual speed will be 1.8% higher than that in­di­cated on the speedome­ter there should be no other con­cerns. As most speedome­ters do have a lee­way of around 5% this should not be a ma­jor ob­sta­cle. The ac­tual speed of the ve­hi­cle could be eas­ily checked us­ing a sat­nav or dash­cam, which have GPS built-in. An app that will achieve the same re­sult could also be down­loaded on to a smart­phone.

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