MJP911

I have an AIM Solo DL and SmartyCam connected to the car for data acquisition. I have been using the GPS sensed speed for my data overlays but I was wondering if the speed signal supplied by the car is more accurate. Thoughts?

Spyerx

No, gps is more accurate. My car speedo reads quite optimistic once over 130mph.

38D

Gps speed is normally better for actual ground speed than wheel speed as you don't have to worry about tire slippage. Wheel speeds are used or over/under steer calc, seeing when the lsd engages, etc.

MJP911

Thanks. And nice avatar, 38D!

Matt Romanowski

Tires also change circumference with load and speed, so the actual wheel speeds are not usually completely accurate. Like the guys above said, use GPS speed for car to car comparison, track metrics, and other performance measures. Then use actual wheel speed to look for tire lockup, diff lock/unlock, tire slip, etc.

gobuffs

agree on using GPS.

However, just because your speedo reads optimistically doesn't mean the speed the car is using is wrong. I've seen the speed the ABS reports be very close to GPS but be very different from the speedo reports.

2BWise

ABS speed is calculated differently from the speedo. Often times the speedo is referencing trans or diff speeds for output. ABS uses wheels speeds and then processes them to get actual vehicle speed, but there's a whole lot of fuzzy logic that goes into it.

amso3

Accuracy is going to be different on a straight vs a hard braking zone. GPS refreshes at 10 Hz and in a hard braking zone speed can change a lot in 1/10 sec. So GPS is an approximation where wheel speed (approxomately 40 pulses per rev of the wheel) is more accurate relative to changing speed. As Matt said, if you are comparing car to car, GPS would be the way to go. If you are looking for changes in data with your car, wheel speed might be better. If you are looking for bragging numbers, use the higest! Here are three views of data of a Cayman at Road America

ProCoach

^^This^^

Spyerx

You guys are completely right. But the question asked is which is more accurate for actual measured speed. That will gps for all the reasons listed above.

But yes wheels speeds tell more to the story then just raw speed. I use them for other car dynamics. But for general comparisons of "speed" it's gps.

So the real answer is "it depends"!