As some of you might know, I sold my GTV6 but I recall Chip's post about my changing wheel and tire size:
<< 215/40/17 is about 1% smaller than stock in overall diameter, and 215/45/17 is around 2.5% larger than stock.
So with the 40's your speedometer will be 1% fast, and with the 45's it will be 2.5% slow. Neither one is enough to cause enough speedometer error to worry me. Even with the larger tires it will only be off aprox 1.5mph when travelling 60mph.
Chip >>
All I can say is – in theory! Although based on this
http://www.csgnetwork.com/speedocalibcalc.html
or this
http://www.miata.net/garage/tirecalc.html
my speedo should not have change much from stock
195/60 ZR15
to what I put in,
215/40/ZR17 in front, 215/45/ZR17 in back
Plugging in only the rear figure, that miata tirecalc site would indicate a change of 1.7%. The csgnetwork site indicates that 60 mph should be actually 61 mph.
NOT – the speedo was off like at least 10 mph when I changed up. I’d be going what read as 60 mph and the actual speed was 70 mph+.
The only way I knew I was in the ballpark was by RPM – around 3000 RMP in 5th was 70 MPH. Is this what RPM is 70 mph on your cars?
The reason this came up is that I was thinking of changing the tire size on my new Toyota Prius but will not do so. That car’s brain is so dependent on the correct speed reading – for all sorts of calculations and fancy functions – that to throw it off whack would be a killer.
Just leaving this as a legacy from personal experience - mine was that changing the tire size completely threw off the speedo, unlike what the expert charts indicated!
Matt
