ok so i have hp tuners and i just bought new set of tires for my car. i went from 195/65/15 to 205/50/15.

i went to wild weasel's how to page and came up with this:

Stock Tire
Sidewall: 5.0in
Radius: 12.5in
Diameter: 25in
Circumf: 78.5in
Rev/m: 807

New Tire
Sidewall: 4.0in
Radius: 11.5in
Diameter: 23.1in
Circumf: 72.5in
Rev/m: 847


then it says:

"Speeedometer reading with non-stock tire is 7.6% too fast. when your speedo read 60mph, you are actully going 55.4

which leads me into this question......I have HP Tuners and if i go to speedo opion it brings up "VSS Pulses Per Mile" @ 24212 and also has "VSS Trans Revs Per Mile" @ 2747.7535..........so if wild weasels calculations are saying im 7.6% off and im doing 55.4mph when speedo says im doing 60mph...what if i decreased the numbers i just mentioned by 7.6% would that bring it back to corrected mph reading on the speedo?

24212 x .076=1840.112 (rounded to 1840)
24212 - 1840= 22372<-------corrected VSS Pulses Per Mile?

2747.7535 x .076= 208.829266 (rounded to 208.8293)
208.8293<-------Corrected VSS Trans Revs Per Mile??



MY 2003 SUNFIRE 15.17@90.82mph N/A....More times to come after turbo install

"A N/A ecotec is not gonna give Honda's and Mitsu's that much of a run for their
money unless their blown or bottle fed.GM is still smokin crack!"
~1QWKZ24
www.streetracing.org, 08/2001

I've never done these calculations before, but I think you need to add the number of pulses per mile off that your new tire size causes to the original value of 24212.

as in 24212 + 1840 = 26052

This is my reasoning: Since your new tires have a small diameter and also a smaller circumference than the stock size, it will take a greater number of rotations of your new tire to cover the same distance. For example, your old tire circumference was 78.5 inches, so in 10 rotations, it would cover a distance of 785 inches. For your new tires, which have a circumference of 72.5 inches, it would take:

785 inches / 72.5 inches/revolution = 10.82 revolutions to cover 785 inches.

So, you need to add VSS pulses to compensate for the additional number of revolutions for an equal distance.

ok i see what i did wrong. i was thinking changing the values would compensate my speedo to the tires..but what i really need to do is compensate the tires to the speedo. good catch



MY 2003 SUNFIRE 15.17@90.82mph N/A....More times to come after turbo install

"A N/A ecotec is not gonna give Honda's and Mitsu's that much of a run for their
money unless their blown or bottle fed.GM is still smokin crack!"
~1QWKZ24
www.streetracing.org, 08/2001

this would be nice for the guys that wanna run wider tires that would throw off the speedo a lot


300hp&tq coming soon
Team GREEN

heres actually the correct formula

PPM= Pulses Per Mile
Diameter= The Diameter of the Tire

PPM x (Old diameter/New diameter)= new PPM

Example:

24212 PPM x (25"/23")= 26392 PPM

Thanks to Chris from HP Tuners for supplying the formula



MY 2003 SUNFIRE 15.17@90.82mph N/A....More times to come after turbo install

"A N/A ecotec is not gonna give Honda's and Mitsu's that much of a run for their
money unless their blown or bottle fed.GM is still smokin crack!"
~1QWKZ24
www.streetracing.org, 08/2001

**12/09/06 9:45pm**
well i just flashed the pcm with the new tires on and PPM new numbers. well normally 3rd gear would top out around 87mph....now im hitting 80mph and thats top of 3rd gear....i didnt change the trans so thats probably why it did that. ill try that later tonight and see what happens

** 12/09/06 10:45pm**
ok i changed the trans and heres what i came up with. 3k rpms @70mph in 5th gear.....it was 3k rpms @ 80mph stock. still toping 3rd gear out at 80mph.

i just looked over my logs a little bit and the log shows, in 3rd gear, 6612rpms @80mph...normally i would be doing about 85-88mph somewhere along there bc i cant tell exact on the speedo. 2nd time i rev'd out 3rd gear it came to 6488@82mph. 4th gear seemed pretty close to normal 5934@ 102mph but still seems to be a couple hundred rpms off. 5th gear is off also 3034rpms@ 74 mph. normally at about 3000rpms the speedo would say roughly 80mph. im stumped right now and too tired to think.

thoughts anyone?



MY 2003 SUNFIRE 15.17@90.82mph N/A....More times to come after turbo install

"A N/A ecotec is not gonna give Honda's and Mitsu's that much of a run for their
money unless their blown or bottle fed.GM is still smokin crack!"
~1QWKZ24
www.streetracing.org, 08/2001