speedo vs gps tests

when i changed tires from stock to 225 65 17 my firend mechanic told me that renegades reads speed with the ABS sensor so the speed will always be right (2 miles under more or less) no matter what tire size you are running.

someone says that is not true, honestly i dont know but i tested my speedo with gps on straight road using cruise control on 50 and 100 kmh and it was 2 kmh under actual speed

i think it would be nice to do a little experiment:
for those who have bigger tires post some photos of the comparison speedo/gps, maybe at 30 and 60 mph in order to see if it is true or not

the perfect test would be an before and after but i've sold stok tires

 

I have stock tires, my Garmin and my speedo are the same.

 

Most abs systems are not counting the rpm....they are just looking for motion.

 

exactly the same?

 

Actually ABS looks at revolutions in the wheel as it relates to the other 3 wheels. If it notices a discrepancy it will actuate abs/traction control to bring a spinning/locked tire in line with the others.

 

Yes

 

Yup....not counting, looking for motion. The ring it uses causes pulses as it rotates, no pulse, no motion.

 

Now I'm not saying that the renny abs does not do as you say and comp for tire size, but this would be news to me. The sensors are just simple pulse generators, it may keep track of the amount of pulses per wheel to keep everything going as it should, but I do not believe it translates into mph. The ring never changes size so if the tire size is bigger, the only thing the sensor will know is that there are not as many pulses.

 

ok so maybe it is different between america end europe. i mean: here every car have the speedo setted up 3/4 kmh under the actual speed, some cars even 5/6 kmh...if you tell me that your speedo and garmin are the same maybe you dont have this difference between actual speed and speedo...i don't know, just wondering

 

Did you made a mistake, or your speedometer realy shows under actual speed ? We are "neighbours", but usualy in every car I've been driven speedometer showed over actual speed (as I noticed on the road warning signs for the current vehicle speed).
On stock tyres 215/60/17 at 100km/h speedo shows 3 kmh over actual speed(GPS). When I use second wheelset on TH (AT tyres ), 215/70/16, which are just under 3% bigger as stock, I get max 1 kmh difference betwen GPS and speedometer at all speeds (0-180 kmh)

 

i'me sure ...maybe it's just another italian weird thing, we are specialists