In the auto XR8 the standard diff ratio is 3.23 but you can option the 3.45,In the XR8 manual the only diff you get is the 3.23. I think all the GT`s have a 3.45 but im not sure.
They can be changed .
For example if you change from the 3.23 to a 3.45 in an Xr8 manual you will have a much quicker acceleration but a lower top speed.
Im not sure how it will affect the speedo ,but im sure who ever does it should know.I know in the older cars you just change the small cog at the end of the speedo cable that fits into the gear box.
Im going to change mine early next year hopefully to a 3.7 if possible.It will take a good second off the 1/4 mile time.
The auto XR8 has a 3.45 diff ratio standard while the manual XR8 has a 3.23. All GT's are 3.45. Changing the diff ratio to a 3.7 will not take anywhere near a second off your quarter times.
After putting 3.73's into my EF I used an 'electronic ratio box' under the dash to fix up the speedo signal. They're pretty good as you can alter it quickly if you change ratios again or get different sized tyres, etc. The guy who installed it set the speedo up exactly by driving around with a GPS locater.
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