Does anyone know how a HC calculates its speed?

[2ndAmend]

So I was able to hit the speed limiter on the '22 RE. Speedo displayed 202 but the official speed at 1.5mi was 204.980, thanks to a nice tailwind!

The car was still pulling, could have done maybe 1 or 2 mph more. Since the limiter kicked about 1sec before crossing the beams I figure my peak was over 205 before it throttled back.

I would like to see if I could improve on this a bit next year, maybe by buying tires that are 1-2% larger in dia thereby tricking the computer. I know I can buy 4 tires but what I am looking at are in the $500ea range, and I would prefer to buy just 2 if possible. Also, I can free up 2 wheels, but if I really need 4, I need to buy wheels as well.

As far as I can tell, the HC uses the ABS sensors to measure speed. But does it measure all 4 and average? Or does it measure just the rears?

I suspect it measures just the rears because when I ran with the stock 275 P0AS the speedo displayed lower than measured speed, but with the 305 PS4S on the rear only, the speedo was higher than measured speed.

OEM 275 P0AS: 28.8 dia
305 PS4S: 28.4 dia (1.4% smaller)

So, the question is: Does the HC use just the rears to measure speed? Or does it average in the fronts also?

Thanks!

 

[MJS73]

In MPH or Km/H

 

[2ndAmend]

In MPH or Km/H

Furlongs per minute.

 

[EricG]

The speedo has to use the rear tires as indicated by the speedometer surging when you peel out, but I believe for the built in timers over distance and such that are pretty accurate, I think they use the front wheels.

Would love to learn a little more about this. You can fool the 0-60 and 0-100 times with wheelspin, but the 1/8 mile and 1/4 mile don't get fooled by wheelspin, and the trap speeds at those distances are pretty close too.

 

[BULL]

I was under the impression that all four were used in some way, but with the trans being the main piece.

Could be very wrong

 

[MJS73]

Furlongs per minute.

You’re thinking of Mustangs.

 

[2ndAmend]

The speedo has to use the rear tires as indicated by the speedometer surging when you peel out, but I believe for the built in timers over distance and such that are pretty accurate, I think they use the front wheels.

Would love to learn a little more about this. You can fool the 0-60 and 0-100 times with wheelspin, but the 1/8 mile and 1/4 mile don't get fooled by wheelspin, and the trap speeds at those distances are pretty close too.

I love the Performance Display, it is a great source of amusement.

Mine says 0-60 in 0.1 sec!

 

[drag racer]

The FSM sez vehicle speed data comes into the ECM from the ABS module, which implies it comes from the wheel speed sensors. It does not go into great detail about the logic though.

 

[EricG]

I love the Performance Display, it is a great source of amusement.

Mine says 0-60 in 0.1 sec!

View attachment 170902

I think you have me beat by 0.1s

 

[16GoManGoHC2]

I was under the impression that all four were used in some way, but with the trans being the main piece.

Could be very wrong

M6 does not use input from the trans in any way. The PCM has no idea what’s going on in a M6 trans short of knowing when it’s in reverse.

 

[DGatzby]

So I was able to hit the speed limiter on the '22 RE. Speedo displayed 202 but the official speed at 1.5mi was 204.980, thanks to a nice tailwind!

The car was still pulling, could have done maybe 1 or 2 mph more. Since the limiter kicked about 1sec before crossing the beams I figure my peak was over 205 before it throttled back.

I would like to see if I could improve on this a bit next year, maybe by buying tires that are 1-2% larger in dia thereby tricking the computer. I know I can buy 4 tires but what I am looking at are in the $500ea range, and I would prefer to buy just 2 if possible. Also, I can free up 2 wheels, but if I really need 4, I need to buy wheels as well.

As far as I can tell, the HC uses the ABS sensors to measure speed. But does it measure all 4 and average? Or does it measure just the rears?

I suspect it measures just the rears because when I ran with the stock 275 P0AS the speedo displayed lower than measured speed, but with the 305 PS4S on the rear only, the speedo was higher than measured speed.

OEM 275 P0AS: 28.8 dia
305 PS4S: 28.4 dia (1.4% smaller)

So, the question is: Does the HC use just the rears to measure speed? Or does it average in the fronts also?

Thanks!

Ok. IMO you need a different way to get your speed, you really want to know EXACTLY your speed. One way, sir…a fast GPS device.

When we do lefts and rights with this thing, we may need to pay attention to the speeds entering each corner. At least if you think you are going to pilot one of these boats thru a turn near its max cornering ability. What we find is the speedo simply cannot read out the speed numbers with fast enough updates to our apps because of our rate of speed changes too fast. Both up (wicked acceleration from 30-150 ish before slowing its rate of change), down (braking certainly has a big rate of change at about any speed).

I know the acceleration above 170 (where I generally max out every lap) does mellow out so that the data MIGHT come in an bit more accurately but as you describe, you missed a couple MPH at 202.

SO! This is what I use in order to be absolutely confident of my speed anytime. This updates to my app, updates to my phone display and when I really need to quickly know my speed my eyes know where that readout is. The speedo in this car does not ever exist for me. That is how I do it on the track, if needed, and I could give a shit on the street except for setting my cruise control once in a while.

https://www.dualav.com/product/gps-solutions/gps/xgps160-skypro-gps-receiver/