Quote:
Originally Posted by Driven97
Yeah changing gearing isn't magic, it just shifts (pun!) stuff around.
Pulling hard through 3rd at 80 on the small tires the car is about to run out of gear and needs an upshift to 4th. On the larger tires there's still ~1,000rpm left to play with in 3rd. But note by 90mph the smaller tire crosses over to pulling harder in 4th as the larger tire car reaches redline in 3rd, then holds an advantage to about 105. They swap again at 115 and then a final time at 130.
An important thing to note is that gear changes take time (especially for us 915 guys) so while lowering the effective ratio looks appealing, in the real world it can end up slower, situation dependent. For example, if you geared a 911 to reach 60mph at the top of 5th the thrust chart would look insane. However, you'd need 4 shifts to get there, and gears 1-3 would be pretty useless.
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Changing the gear, changing the gear ratio or changing the drive tire OD all affect the thrust curve
A good example is seen in this comparison of 2 993 transmissions, the /20 is a stock street trans and the /30 is the stock racing trans in a Cup or RSR
All the gears in the /30 except first and second are shorter and all are closer together to minimize engine rpm drop w/ shifts, 1st is easy to see the taller 1st in the /30(blue) reduces thrust at all points and moves the peak torque to the right or higher speed, in 1st the /20 car w/ the same engine would beat the /30 car to 31mph, but maybe not to 37 because a shift would have to occur which would slow the average rate of acceleration. You'd need to time the cars to actually see. The same occurs in 2nd but to a lesser degree because the difference in gearing is less. For the rest of the gears the opposite occurs, the shorter /30 gears move thrust peak to the left but again to a lessor extent because the difference in gear ratios is less. But in every case the trans w/ the shorter gearing will out accelerate the taller for most of the time spent in one of the shorter gears, Additionally if the curves for a given transmission cross, that is an indication that a short shift is called for, because the higher gear now provides more thrust than the lower gear.
