The tire size on my Cav is a 215/40r17 which throws the speedo off approx 2.5%
Is there a table that can be changed to accomodate the smaller tire (going for the most cost efficient measure since the tires are fairly new)? I have to have someone that has HPT change it for me, so the more info that I have, the better.
Edited 1 time(s). Last edited Friday, February 16, 2007 12:24 PM
id just get the correct size tires and save your wheels , lol
[quote=97trd(???™)]id just get the correct size tires and save your wheels , lol
well truly any tire size you get is not exact to the factory set-up, they are close, but not exact. You can adjust it to read correctly which I think is a nice feature.
FU Tuning
[quote=97trd(???™)]id just get the correct size tires and save your wheels , lol
Beauty of it is I don't have potholes out where I live, so no worries about bent rims
yeah darkstars and i were talking about this the other day. sounds like we need some math people to chime in here.
Not hard. You divide the diameter of the new tires by the old. This will give the ratio of linear translation per 360° rotation of the wheel (ratio of circumference or distance traveled to the layman). You multiply this value by whatever the current value the VSS is.
It's actually:
[(pi*D1)/(pi*D2)]*VSS value.......but the pi's cancel out.
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"Youth in Asia"...I don't see anything wrong with that.
Ok... so if I read that right, the stock 195/70 r14 diameter being 24.7 and the 215/40r17 being 23.8 gives a value of 1.0378 (not sure how far to take it out decimal wise).
http://www.miata.net/garage/tirecalc.html
Taking that # and multiplying it by the VSS value should calibrate the speedo or would that be the other way around using the following:
0.9636 (17in wheel/14in wheel)
It should be the smaller one, the new diameter is smaller, therefore the circumference will be shorter and the pulses will come more frequently.
Diameter of the new wheel divided by the original. So the second one...
Kardain wrote:Ok... so if I read that right, the stock 195/70 r14 diameter being 24.7 and the 215/40r17 being 23.8 gives a value of 1.0378 (not sure how far to take it out decimal wise).
http://www.miata.net/garage/tirecalc.html
Taking that # and multiplying it by the VSS value should calibrate the speedo or would that be the other way around using the following:
0.9636 (17in wheel/14in wheel)
Or if you wanted to make it look like you are going 200 mph on the highway (traveling at the 65mph with your new tires), you could multiply by 2.9649
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"Youth in Asia"...I don't see anything wrong with that.
back from the dead, I tried that it didn't work too well (read faster). So, after doing some thinking, I came up with this: Starting size / end size = ratio. VSS PPM gets multiplied by that ratio and it should read dead on, in this case multiplied by 1.0378. this was after reading some of the posts above and looking at the values on the VSS output and somehow blatently missing this post:
http://www.hptuners.com/forum/showpost.php?s=55c83dd017ac6a567c87181e3a4e1666&p=64433&postcount=4
Quote:
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
with multiplying it by the .9636 value, it actually makes the car think that there's a bigger tire (less tire rotations per that 1 mile).
Yay
I accurately know how fast I'm going now
lol...sorry Kardain. Looking at it now, it makes perfect sense. Don't know what I was thinking..lol
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"Youth in Asia"...I don't see anything wrong with that.
Its all good. Now I just have to remember that its been fixed so I don't get popped with a ticket
yea a -4mph speedo screwed me once. damn ticket.
rodimus prime is gonan correct my speedo when i get new wheels..hopefully he can do that math
My car may run 18s, but I can do your taxes in 10 seconds flat.
JBO lube - they would never have enough in stock and we'd never see RodimusPrime again
Ok, now that I've had a couple days to drive around, one unforeseen side-effect: Trac Off light come on intermittently....
Kardain -
Remember that the 5spd cars didnt come with trac control. That was something that stayed on the 4spd auto's only. Since you're running 5spd with this setup, I'd look through and see if you can't just disable the TC forever. Should fix the problem.
Its for my Cav, which is an auto
