rs6er Posted March 7, 2008 Share Posted March 7, 2008 My 17" winter wheels and tires have a 25" diameter (833 RPM). My stock 18s have a diameter of 25.7" (811 RPM). How is this affecting my speedo and acceleration (if at all)? Also, will it affect my alignment when I switch back to the summer wheels? Link to comment Share on other sites More sharing options...
SubLGTaru Posted March 7, 2008 Share Posted March 7, 2008 http://www.miata.net/garage/tirecalc.html You always want to get your car realigned after you do any changes to suspension/tires & wheels combo. Link to comment Share on other sites More sharing options...
mwiener2 Posted March 7, 2008 Share Posted March 7, 2008 At what speed are those RPM's? That's the difference you're gonna notice. My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons Link to comment Share on other sites More sharing options...
rs6er Posted March 7, 2008 Author Share Posted March 7, 2008 At what speed are those RPM's? That's the difference you're gonna notice. Those are revolutions per mile, not per minute. Link to comment Share on other sites More sharing options...
rs6er Posted March 7, 2008 Author Share Posted March 7, 2008 http://www.miata.net/garage/tirecalc.html You always want to get your car realigned after you do any changes to suspension/tires & wheels combo. Thanks for that calculator. It says that with the setup I have now, when my speedo reads 60mph, I'm really going 57.7 mph. I guess that's a nice margin of safety. Also, from your avatar, your car looks like mine from the front except mine's DGM (RBS Painted too) Link to comment Share on other sites More sharing options...
EL PAALO Posted March 7, 2008 Share Posted March 7, 2008 http://www.miata.net/garage/tirecalc.html You always want to get your car realigned after you do any changes to suspension/tires & wheels combo. you do not need an alignment just for a wheel/tire swap. Link to comment Share on other sites More sharing options...
mwiener2 Posted March 8, 2008 Share Posted March 8, 2008 + 1 My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons Link to comment Share on other sites More sharing options...
SubLGTaru Posted March 10, 2008 Share Posted March 10, 2008 you do not need an alignment just for a wheel/tire swap. I thought that if the diameter and size of the swap is different that it is a good idea for alignment. Link to comment Share on other sites More sharing options...
mwiener2 Posted March 11, 2008 Share Posted March 11, 2008 if you make a large change, yes but you didn't and most people don't You'd have to go from a 215 series to a 245 to need an alignment and have it worth it My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons Link to comment Share on other sites More sharing options...
boostsr20 Posted March 11, 2008 Share Posted March 11, 2008 if you make a large change, yes but you didn't and most people don't You'd have to go from a 215 series to a 245 to need an alignment and have it worth it Even then a stock spec'd alignment would be fine for 245's since a factory spec alignment isn't aggressive. If you had an aggressive alignment done and went shorter then you might want to get it checked so you don't have excessive camber wear. Even then it would still only be a front end alignment since the rears aren't camber adjustable. Link to comment Share on other sites More sharing options...
AZZMAN Posted March 11, 2008 Share Posted March 11, 2008 I have 17" this winter with a smaller diameter too. The local tire shop said I do not need an alignment balance when I swap wheels. He thought the Speedo would be off by about 3mph... showing 60mph when my actual speed was around 57mph. I should have asked him when the speedo is at 90 am I going 87 or is it closer to 80.. it feels closer to 80. Link to comment Share on other sites More sharing options...
boostsr20 Posted March 11, 2008 Share Posted March 11, 2008 Its off by a percentage. At 90mph your going more like 86. Also, shorter tires will rack up the miles faster on your odometer. 4% more per tank would be roughly 12 miles per fillup. It will add up eventually. Link to comment Share on other sites More sharing options...
Vimy101 Posted March 11, 2008 Share Posted March 11, 2008 My 17" winter wheels and tires have a 25" diameter (833 RPM). My stock 18s have a diameter of 25.7" (811 RPM). How is this affecting my speedo and acceleration (if at all)? Also, will it affect my alignment when I switch back to the summer wheels? You can check out your indicated speed vs actual by comparing your speedo to the info system GPS indicated speed . Link to comment Share on other sites More sharing options...
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