the F40 outputs roughly 60,000 PPM, the '7730's KSPDDIV setting in the PROM can be configured as such:

A B C
0 0 0 Divide by 1 (ie for a 4000 ppm VSS) (=0) (or any # less than 32 as the lower bits are ignored)
0 0 1 Divide by 9 (36000 ppm VSS) (=32)
0 1 0 Divide by 7 (28000 ppm VSS) (=64)
0 1 1 Divide by 11 (44000 ppm VSS) (=96)
1 0 0 Divide by 6 (24000 ppm VSS) (=128)
1 0 1 Divide by 10 (40000 ppm VSS) (=160)
1 1 0 Divide by 8 (32000 ppm VSS) (=192)
1 1 1 Divisor disabled, no output (>=224 again as the other bits are ignored)

(A B & C are the 3 most significant bits of that byte value, a byte can represent a # from 0 to 255, eight bits add up as such: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255, the lower 5 bits are ignore by the chip in the '7730 that controls the speedo output)

So in theory, you can't control the Fiero speedo with the '7730 and F40 6speed ... HOWEVER! I have confirmed that no matter what code mask you run that the BC1 pin is always outputting the 2000PPM signal. So you would need a divisor of 15 if the ECM supported that but it doesn't. However, switching over output from the 4000PPM out to the 2000PPM out on BC1 gives you a free division of 2. So using a divisor value of 64 (div of 7 and div of 2) you get a divisor of 14. Using 192 you get a divisor of 16. Basically a 6.67% margin of error either way. Given I have oversized tires that puts me 2% faster going from a diameter or 24.5" to 25" then my margin of error is down to 4.67% ... and I can live with that rather than having to wire up a Dakota Digial SG-1 unit.

Note: you still need to convert the square wave(digital) signal output back to a sine wave(analog) signal for the Fiero speedo as before.

[This message has been edited by lou_dias (edited 05-20-2012).]

A bump for people who don't login on weekends...

How accurate is it relative to the speed the ECM shows in the log? I like to have as close to accurate as I can get get on my data particularly when wrestling with tuning. I posted the proceedure I used to setup the SGI with the F40 and digital cruise. The only hard part about it was the price of the module at just under $100 about 4 yrs ago.

To get the internal speed accurate, you have to put in a ROAD SPEED CONSTANT (KSPDSEN) of 59942

quote Originally posted by lou_dias: To get the internal speed accurate, you have to put in a ROAD SPEED CONSTANT (KSPDSEN) of 59942

So you are able to get an accurate speed signal to register in the ECM? I'm asking because I know the odometer reading can differ from what registers in the ECM. I calibrated mine by GPS.

The internal speed is actually computed in the code. The chip that controls the speedo sends it a pulse count every .8 seconds. From there it does math taking into account the road speed sensor constant. You can change that # to be higher or lower than 60,000 to reflect your tire size better. I'm running the $A1 mask but the $88 does it exactly the same way.