Racing Teams use different gear ratios to suit the characteristics of each track and obtain maximum acceleration. Successful autocrossers know the advantages of a quicker ratio, but as a street/highway performance option it is often overlooked. This may be because it seems more difficult than changing computer chips or adding a suspension package. Also, many local shops and even dealers lack the special tools and expertise to rebuild and change gear sets in BMW differentials. With our racing experience we are very proficient with BMW differentials and we have the special tools needed to make modifications correctly. UPS and other carriers have increased their weight limits and can now pickup and deliver your differential at your front door. The usual diff removal or installation time for most BMW's is 1.5 hours.
Changing to a quicker ratio provides faster acceleration in all gears at all rpm's. It does not affect engine emissions or idle speed and can make up for changes in tire diameter. Some BMW models were originally geared very long for increased fuel mileage. These cars benefit the most from quicker ratios.
Generally, longer ratios yield better fuel mileage and shorter ratios offer more acceleration and higher fuel consumption. In practice, the fuel difference is often slight. If you have such a long ratio you must frequently downshift to pass or accelerate quickly, then in reality you do not get the theoretically higher fuel mileage that the longer ratio would have you expect. And, if you switch to a shorter ratio for more acceleration, you may not burn much more gas than before, as you are now able to upshift more quickly into the higher gears.
In a similar way, you would expect that changing to a shorter ratio for more acceleration will reduce your top end speed a you would run out of revs in high gear. However, few cars have enough top end torque to pull full revs in high gear. Changing to a shorter ratio will usually enable the engine to pull more revs against the high speed drag resistance. Though you will turn more revs per mile, your actual top end speed may be as good or better than before, especially if you have added some additional performance improvements.
The following formula will enable you to calculate your possibilities, but keep in mind actual results will vary. Drag, rolling resistance, tire characteristics, spoiler packages and engine modifications all affect acceleration and achievable top end speed. And don't forget road conditions, speed limits, and radar! It is usually more practical to gear for greater acceleration than higher top end speed. You can enjoy it more often with fewer penalties.
MPH=
(RPM)(TIRE DIAMETER) in inches
(GEAR RATIO)(336)
*See Tire Diameter Formula to calculate above diameter
A +/- 7% gear ratio change is recommended for most street driven cars. To obtain a percentage change, divide your original gear set ratio by your proposed new gear set. The resulting percentage also equals your RPM change at a given speed.
Thanks to Korman Autoworks
