I have a 2000 Subaru Legacy GT with a 2005 Pontiac GTO/T56 combo in. I'm trying to get the factory Subaru speedometer to work correctly and continue to have issues with a ~50% offset in speed. The vehicle speed (as read by the ECM) is spot on and I used the gear/tire calculator to get that worked out.

I read that the factory cluster expects a 4104 pulse per mile input, so I've set the PCM output calibration "Mile" to 4104, which results in a speedometer that reads about 10x too high. Fine.

I do the math (25.03" tire, 3.56 rear end, 17 pulses per rev on the output shaft) and I get something like 49,116 pulses per mile. If I use that number I'm about 1.5x too high on the speedo (speedo 30mph, wheels 20mph).

Any number higher than 49119 is also 1.5x too high on the speedo. I've tried 56000, 98000, numbers up in the millions.... they're all 1.5x too high on the speedo.

If I try to walk the calibration in from 4104 it seems like it's not scaling linearly either.

4104 off by 10x
8208 off by 5x
16416 off by 3x
24624 off by 2x


I've pretty much run out of ideas at this point. It seems like either something is going wrong in the HP tuners calibration or something is odd with the speedo, but I'm lost.

Has anyone encountered anything similar? Any ideas?

Thanks