Question about 3.45 diff tuning

[TheGr8WOP]

Hello all,
I just put a camaro 3.45 diff in my g8 and my tuner put in the tie info and the 3.45 gearing and my speedometer was way off. We went back and forth with tire size gearing and could not figure it out. So we started to dramatically change the gearing in the tune and started to get somewhere by first putting 3.73 and then got it perfect with a final gearing of 3.90. So my question is why does the gearing have to be set at 3.90? Did I get a 3.90 diff because I didn't think those fit? We were both confused by this and any info about this would be great. Thank you all in advance.

[Greg@PacePerformance]

The 1LE diffs are 3.90 ratio and will fit just fine in the G8, or someone upgraded the 3.45 diff with 3.90 gears before parting it out.

-Greg

[-Ray-]

Interesting. What rpm are you running at 70? My 3.27 puts me at 2150 rpm at 70.

[Murky]

The only way to really know is to get under the car and count how many driveshaft revolutions make one wheel revolution.

[TheGr8WOP]

50~1600
60~2000
80~2600

[Featherburner]

50~1600
60~2000
80~2600

This works out to be 3.91 gears. (3.91x80x336)/26.7-33%=2637rpm