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Old 06-01-2012, 11:42 PM   #43
Mitch
form follows function
 
Join Date: Nov 2011
Drives: RIP '13 BRZ
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Quote:
Originally Posted by serialk11r View Post
Only problem is, you still have the crappy ginormous gap between the ratios, so you will only get marginally better 1st gear acceleration :/

Apparently on some Japanese site the 3.727 gets about 5% better fuel economy on their test cycle, which is probably around what you can expect cruising on the highway. If you drive a lot the savings could add up quickly.
Base mpg: 30.0 mile / gal
New diff: 5% improvement
New mpg: 31.5 mile / gal

Premium fuel cost: $4.00 / gal

Base gas cost: $0.133 / mile
New gas cost: $0.127 / mile
Savings: $0.00635 / mile

Payback mileage (miles to break even, every mile after is actual savings): 157.5 miles per $1 spent modifying the gear ratio.
In other words, if you spend the $1106 mentioned earlier, it would take 174,195 miles before you broke even on the cost of the differential.

And for a little sensitivity study...
Payback mileage as premium fuel cost changes:

$3.00 / gal : 210.0 miles per $1 spent modifying the gear ratio
$3.50 / gal : 180.0 miles per $1 spent modifying the gear ratio
$4.00 / gal : 157.5 miles per $1 spent modifying the gear ratio
$4.50 / gal : 140.0 miles per $1 spent modifying the gear ratio
$5.00 / gal : 126.6 miles per $1 spent modifying the gear ratio
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