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31st July 2008, 07:32 | #1 |
I really should get out more.......
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Wheel and Tyre combo puzzle
Looking in my Owners Handbook the Tyre Specification/Wheel sizes makes sense until you compare 17" and 18" where the tyre size quoted is the same (225/45), so surely the speedo/mileometer readings are going to be wrong for one of them?
Any thoughts anyone? Mick |
31st July 2008, 08:21 | #2 |
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All depentand on tyre width and profile. try this: http://www.alloywheels.com/tyrecalc.asp |
31st July 2008, 09:50 | #3 |
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How bizarre. Do you think a mistake was made when printing the handbook? Speedo will be way out with a 225/45/17 tyre.
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31st July 2008, 09:52 | #4 |
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31st July 2008, 11:35 | #5 |
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31st July 2008, 13:15 | #6 | |
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Quote:
I got the correct 225/45/18" fittement on my ZT. And my speedo is only a couple of mph out at even 100 mph compared with my sat-nav.
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31st July 2008, 14:30 | #7 |
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The differences between the 2 sizes you quote are massive in tyre terms. In particular if you consider the distance travelled per tyre revolution.
225-45-17 (1,930mm distance per rev) based on a new tyre 225-45-18 (2010mm distance per rev) based on a new tyre this equates to a difference of 80mm shorter distance travelled per rev. If you do the calculations again but compare a worn 17 against a new 18 the difference in the distance travelled is a massive 121mm per rev shorter Result increased acceleration decreased top speed & inaccurate speedo reading. Oh and less miles per gallon.
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31st July 2008, 15:16 | #8 |
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In the brochure's for the mk.1 Rover 75 the 17" Meteor and Serpent wheels were supplied with 225x45xR17 tyres..
In the Facelift brochure the 17" Star Spoke, Meteor and the Serpents were all supplied with 215x50xR17 tyres. The pre facelift MG ZT brochure does not show 17" wheels available. The facelift MG ZT has a 17" 5 Spoke with 215X50xR17 tyre. So maybe MG Rover thought there was a problem with the speedo readings and changed the profile of the 17" tyres tyres supplied when the facelift models were introduced. Although it took five years of production before the profile was changed.. Dave T |
31st July 2008, 16:19 | #9 |
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Mine was originally fitted with 15" Crowns and the speedo over-read by quite a bit even at 30 mph.
On fitting the 18" Straights due to the greater diameter of the 225/45/18 tyres the speedo still over-reads but only by 2mph at 50. Russ
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31st July 2008, 17:15 | #10 |
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We were wondering earlier if the tyre combo choice might influence the programming of the dash ecu after or on the build line - ie. the speedo correction factor? Or would that have been just too much work?
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