speedometer question ????

http://files.automotiveforums.com/gallery/watermark.php?file=/500/100978133mph.jpg

if the limiter is supposed to be 130 [could it be different for the spyder?] and i have 18" wheels [bigger circumference = more distance per revolution = faster], how fast am i really going? 95 GSXracer said that the speedo becomes inacurate at higher speed. my limiter kicks in when my speedo reads 138 [or very close to it]. another question is if the speed limiter really kicks in at 130, how does it know better than the speedo where 130 is? how could the speed limiter have a more precise reading than the speedo?

this was sparked by another thread, so i *went to a closed circut* and hit the limiter a couple of times and got a pic.

I also have 18's, and I am in the same predicament as you. evidently there is a way to adjust for the larger rims and that might help with figuring out how fast you're actually going... as far as the limiter.. it boggles my mind as well

I know when you go to a larger diameter tire you are changing/tricking (for lack of better words), the gearing in your car. For example, a car with a gear of 3.42 with 26" tall tires would be/act like a car with 3.23 gears with 28" tall tires. Sometimes when racers run through the traps at 6k redline they put on larger diameter tires to lower the rpm's running though the traps. I think this is what is happening with the larger rims, it effecting the gears, hence the spedometer not reading right. Then again I could be completely out in left field with this one, but it makes sense to me. Wait for 95gsxracer's reply. Hopefully he will go easy on me if I am wrong.

Now that I read my reply, it doesn't really answer the questions... Sorry

also, more mass to spin = more force to turn ... but i'm just talking about accurate speedometer readings. could the speed limiter be raised inadvertantly with larger tires? if the limiter is set in an rpm/gear setting, it could still be the same; however a larger wheel spinning at the same rpm will be carry the car at a higher speed, reading more?!?!

i'm just wondering how fast i'm going...

EVeryone is pretty much on track. Let me see if we can clear some of this up.

The SpeedSensor is on the output of the tranny. All it really knows is the rpm of the tranny output. Using gear ratio and tire rolling circumference, it can calculate speed. The ECU and the speedo are both fed from that sensor. The ECU is all electronic though (math) and the guage is built by 10 year olds in third world countries. It is only as accurate as its quality will allow. The ECU is very accurate as far as translating the pulses sent from the speed sensor into a speed, but it asumes that the gear ratios and tire rolling circumference has not changed. So, if you go to different sized tires, they will both be wrong, but the ECU is still more likely to be more accurate. ;)

If you go to a tire with a larger rolling circumference (larger diameter/radius) the car will be going faster for the same tranny output speed, or the speedometer/ECU will read low, however you want to look at it. So if the speedo/ECU say 130, you are going faster than that, and if the car is really going 130, the speedo/ECU will read lower than that. The difference will be directly proportional to the difference in the rolling circumferences of the stock and new tires. So if the new combo has a circumference that is 3% more, the speedo/ECU will be off by 3% more than they were before in the slower direction. Since we can assume the ECU is accurate, its safe to say the ECU will read 3% slower. The Speedo could still be anywhere though, since you dont know how far off it was with the stock tires to start with.

One could easily datalog the speed sensor and compare it to the speedo, at say 30 mph, 60 mph, and 90 mph, and come up with a curve of how many % the speedo is off. ;)

The tach is a similar situation. When my ECU says 7000 rpm (pretty damn accurate) the tach says 7300-7400. This is quite common too. Guages are shyte. If it can be dataloged, always trust that number.

Hope that helps a little.

so gauges are shyte...
i need to add a datalogger to my list...
some turbo timers boast speedometers too...

thanks kevin...