mscowley Posted October 12, 2008 Share Posted October 12, 2008 Hi all, on my disco I'm running 235/85/16 tyres and what I'd like to know is how far out doe's it put the speedo. cheers Matt Quote Link to comment Share on other sites More sharing options...
BogMonster Posted October 12, 2008 Share Posted October 12, 2008 Using http://www.tyresave.co.uk/tyresize.html 235/70R16 are 28.95in diameter 235/85R16 are 31.73in diameter Difference 9.66% - so it will under-read by 9.66% in theory. However most Land Rover speedos over-read by about 5% due to the law - which says something like all speedos must over-read by somewhere between 0 and about 10% - so most manufacturers aim somewhere in the middle. So the most likely answer is that you will have a speedo which under-reads by about 5%. Only way to be 100% sure is calibrate it either against a measured mile, or with a GPS (these are quite accurate). Or drive through a 30mph speed trap at a noted speed (say 60mph) and see what it says on the ticket when you get it Quote Link to comment Share on other sites More sharing options...
Rolly Posted October 12, 2008 Share Posted October 12, 2008 I was talking a chap had the same tyre size on his Disco & said it was 6km/h difference, he had it radar tested. That would be 3.75 miles for you dark ages people Quote Link to comment Share on other sites More sharing options...
BogMonster Posted October 12, 2008 Share Posted October 12, 2008 I was talking a chap had the same tyre size on his Disco & said it was 6km/h difference, he had it radar tested.That would be 3.75 miles for you dark ages people At what speed? 6kph difference at 20kph is a lot different to 6kph difference at 100kph Mind you as kilofroggies are meaningless anyway it could be 1000kph difference for all the difference it would make to my understanding of what speed I was actually doing Quote Link to comment Share on other sites More sharing options...
Rolly Posted October 13, 2008 Share Posted October 13, 2008 Ah yes, very remiss of me, 6km at 60km/h (generic around town speed-limit here) which would male it 3.75miles at 37.5 miles an hour. Quote Link to comment Share on other sites More sharing options...
firemannotsam Posted October 13, 2008 Share Posted October 13, 2008 Hi all, on my disco I'm running 235/85/16 tyres and what I'd like to know is how far out doe's it put the speedo. cheers Matt Hi, I am running 235/85/16 on my disco - Discovery's come out of the factory with a 6% over read (with 235/75/16) on the speedo (as do a lot of cars). 235/85/16 is about 11% larger circumfrence which means they would cause the speedo to read 5% under. i.e at 100MPH they will read at about 95MPH. I have checked this with the use of my satnav and measured distance timing. I tend to drive with the needle at just below the speed I intend to drive at. Hope this helps. Quote Link to comment Share on other sites More sharing options...
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