Er, the ratio is the number, 3.90 is higher than 3.08, no?
The effect of a higher ratio in the rear diff is to reduce speed for given revs, or to increase revs for a given speed.
This has come up before so I can just paste in the actual figures for my car:
revs per min A 6000 6000 A eng rev / hour B 360000 360000 B = A * 60 pick gear ratio C 0.833 0.833 pick gear prop rev / hour D 432,173 432,173 D = B / C pick diff ratio E 3.08 3.90 pick diff axle rev / hour F 140,316 110,814 F = D / E tyre circ (cm) M 180 180 pick tyres trav cms / hour N 25,257k 19,947k N = M * F speed kms / hour P 253 199 P = N / 100,000 actual spd mph Q 158 125 Q = N / 160,000
This is a confusion which comes about from the use of the same word for gears and gear ratios. The gear *ratios* of a gearbox go from about 4 (in first) through to about 1 for fourth gear, and maybe 0.833 for fifth gear as above (which means every four turns of the engine returns five turns of the gearbox output shaft, instead of four turns in fourth, or a mere one turn in bottom gear.)
The problem is, the highest *ratio* gears in the gearbox, are the lowest
*speed* gears, and vice versa. Hence you can see why "highest gear" is the "lowest gear ratio". With the rear diff there's no confusion, the higher the ratio, the higher it will rev for a given vehicle speed.Hence why I am putting a lower ratio rear diff in, as you can see I am redlining at 125 mph in fifth, which is bad for fuel economy, and would rather be able to do 160 mph in fifth before redlining, not that the car can reach 160 but it will give me something like 30% better fuel economy, and generally cruise better.
Ah yes, brakes are one reason to do this, bigger wheels mean bigger spaces for the larger discs. I was forgetting that.