I have a question for any of you mathmatically gifted beast owners. I just installed a set of Dunlop Wintersport M2s on basic 17" borbet type E rims (Thank you Tire Rack, $1,270 delivered). My question is how much speedometer error occurs going from the 18" to the 17" wheels? It seems as though the speedometer is reading faster than the car is going by about 10 mph when travelling around 70 mph. Is this just my imagination? Thanks for the help!
HotWheels
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HotWheels
'02 M5 Le Mans/Caramel/Walnut
'59 Buick Electra 225 Convertible
It's pretty easy to figure out yourself; rsbj is right, we just need the numbers off the new tires.
Or you can just go to http://www.miata.net/garage/tirecalc.html and it'll do all the math for you.
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'06 M Coupe #308 - Silver Gray/Sepang Brown Dark
E36 M3 gone
E24 645 gone
My snows are 235x45x17, and the nifty calculator site tells me that this creates a 1.5% speedometer error. Put another way, when the speedo says I'm doing 60, I'm really doing 59.1.
As a side note, I had my first chance to try out the new snows today driving 30 miles in heavy snow. The Dunlop Wintersports handled it great. Thanks to all those on this site who recommended them over the Blizzacks. Great in snow and decent (not squirrely) on dry pavement.
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HotWheels
'02 M5 Le Mans/Caramel/Walnut
'59 Buick Electra 225 Convertible
Originally posted by Fast6 It's pretty easy to figure out yourself; rsbj is right, we just need the numbers off the new tires.
Or you can just go to http://www.miata.net/garage/tirecalc.html and it'll do all the math for you.
Thats a great site, thank you for posting it
all the best
adam
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1995 Alpina B8 4.6 #10-(engine blew up. sold)
2001 BMW ///M5(sold)
2003 RUF R turbo
1990 RUF CTR
2006 VW Golf R32 DSG
2005 e60 M5 (sold)
2007 997 GT3 RS Carrara White
2008 Renault Clio 197
"You know it is kinda ironic, these old people are being kept alive by the organs of the young people they ran over" - Chief Clancy Wiggum
Nifty calculator, but you should know also that there can be a difference in how the sizes are implemented by different manufacturers. For example, a 275/35-18 tire could be a bit larger or smaller in width or circumference from manufacturer to manufacturer, or even within the same mfr but across tire models. Almost how you need to try on shoes or clothing nominally of the same size to make sure the particular pair or article you're shopping for actually fits you, you need to see each mfr's specs to verify how they may be compared to your current tires. The calculator will give you a rough idea, however, when doing plus (or in your case, minus) 1 type changes so you generally know what size should be comparable to stock in terms of diameter or rolling circumference, which is the factor affecting speedo accuracy. So, the 17 in. size with the same circumference as the stock 245/40-18 front tire is a 245/45-17. That would give you a tire as wide as stock, and the same rolling circumference in a 17 in. size., which should not affect your speedo accuracy. I don't know if that size would fit (but it seems it should). Bridgestone and Dunlop make winter tires in that size. I don't know if anyone else does. Perhaps not surprising is that the 275/35-18 rears are just about the same circumference as the 245/40-18 fronts. (The 17 in. size for the rear would then be 275/40-17 if you wanted to keep the same section width, but I don't think anyone makes this size.)
You should also know that BMW speedos are notoriously optimistic, e.g. can read 80mph when you're really doing only 75.
Last edited by Need4Spd; 6th December 2002 at 23:36.
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