New tires...speedo calibration

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)
2747.7535 - 208.8293= 2538.9242 <-------Corrected VSS Trans Revs Per Mile??



or am i doing the inverse operation and should add them instead of subtracting?

All you need to do is enter the new tire size at the top and put in the same gear ratio that is in the current box and it will do all the adjustments to the vss for you.

i dont have that option. unless im missing something completely. i included screen shot of what i have.

If anyone else has this problem or in the future has this problem the answer to it is the following

PPM x Old diameter/New diameter= new PPM

Example:

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

Thanks goes out to Chris for supplying the formula

yeah you don't have the same options that I have in mine, thanks for that formula, it may come in handy in the future.

How did this work out for you? I am interested in doing the same thing. I just went from 15 to 18 on my ecotec and it shows the same options in the speedo section. Did you change anything besides General VSS pulese per mile? Did you mess with trans rev/mile also?

Thanks

well i just flashed the pcm with the new tires 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

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 dunno maybe someone has some answers or ideas.


Edit* i just looked over my logs a little bit and the log shows 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?

yeah its because the overall tire size is smaller. For any given RPM you overall speed will be slower.

yea i keep reading into it last ngiht kinda figured that. but wasnt sure thanks for clearin it up. just dont like doing 70mph @ 3000rpm other then that i can live with it.

If I might ask why did you decide to go with a smaller tire? Better off the line performance?

well i wanted to keep my stock steelies on there and the option was either 55 or 50 and there wasnt much of a difference so i took the lower profile for less tire flex and be more repsonsive on the road.

sorry to bump up and old thread but is there a similiar formula for doing gear changes? I have the same options that are shown on the screen shot above and have changed gear ratios in the rear and would like to re-calc the speedo.

Thanks.