Same type of gear, just a different ratio.
A "low" gear usually has a (numerically) high ratio - for instance, the "Granny low" in the SM420 runs down around 6.5-7.0:1 This means that the output shaft turns
once for every six and a half to seven turns of the input shaft. The reduction in speed is accompanied by an increase in torque.
Transmission ratios run from "low" up to a "direct drive" (1.0:1), and then anything past that is "overdrive". In the case of an overdrive, the ratio will be <1:1 (for instance, the 0.79:1 stated earlier. I'd have to look that up to confirm, but 21-22% sounds about right for the AX-15.)
Some six-speed transmissions have a "double overdrive", where fifth
and sixth gears both have a ratio lower than 1:1 (usually something like 0.75:1 and 0.6:1, from what I've seen.)
You saw me mention "% overdrive" - that's simple. If you have a "25% overdrive" on a gearbox, that simply means that the overdrive ratio is 0.75:1 (25% OD = 100%-25% = 75% gear ratio.)
Gear ratios are "input
utput", and are usually expressed as "input:1". This means that for every "input" times the input shaft turns, the output shaft turns once.
Gear ratios numerically larger than 1:1 are torque
multipliers, while gear ratios numericall smaller than 1:1 are torque
dividers (but they increase output speed relative to input speed.)
Let's take a hypothetical powertrain -
Engine Output: 100 pound-feet at 1000rpm
Transmission: Five-speed manual with ratios of 7.5:1, 3.5:1, 1.75:1, 1.00:1, 0.75:1 (trying to keep the maths simple...)
First gear would really only be used when towing or hauling - it fits in the "Granny low" class. It's good for getting a load moving, but not worth much for speed. In first gear, you're looking at an output speed of 133-1/3rpm (1000/7.5,) but an output
torque of 7,500 pound-feet. This would be further multiplied by the axle gear ratio (invariably numerically higher than 1:1,) so the output torque could be significant. Now you see how those large Diesels can get forty-tonne loads moving without stalling out...
Second gear would net you an output torque of 3,500 pound-feet (again, multiplied by the axle ratio,) with an output speed of 285.7rpm. Now you can turn your wheels a little faster...
Third gear? 1,750 pound-feet at 571.4rpm.
Fourth gear? Direct drive - 1,000 pound-feet at 1,000rpm.
Overdrive is only used on the highway for good reason. By that time, you don't need much torque to
keep the load moving (inertia handles the rest for you,) but you need to bring up output speed so you can turn your wheels faster. OD, in this case, would give you 750 pound-feet at 1333-1/3rpm.
Of course, this is a simplified example - I haven't taken tyre size into account, nor an actualised power output curve, rear axle ratio, ... However, it should give you the idea. I will summarise below
Input Torque Input RPM Ratio Output Torque Output RPM
1000 lb-ft 1000 7.50:1 7,500 lb-ft 133-1/3
1000 lb-ft 1000 3.50:1 3,500 lb-ft ~285-3/4
1000 lb-ft 1000 1.75:1 1,750 lb-ft ~521-2/5
1000 lb-ft 1000 1.00:1 1,000 lb-ft 1000
1000 lb-ft 1000 0.75:1 750 lb-ft 1333-1/3
Have you ever looked at a tractor? I learned to drive ('way back when, I was about seven) on an old Massey-Ferguson. Thing actually had
two gearboxes - a six-speed manual (six forward, one reverse, no OD) and a "three-way gear splitter" box behind it (one "underdrive" range, direct drive, and one "overdrive" range. I think the "underdrive" was about 2.5:1, and the overdrive was somewhere around 30%, or 0.70:1)
This actually gave an effective
eighteen forward gear ratios and
three reverse gear ratios. This can be very useful on a tractor - but not so much on a road car. However, you can get "overdrive" or "underdrive" boxes that fit between the transmission and transfer case to give you more ranges (the "low" range in a transfer case is technically an "underdrive" range - since it's lower - numerically higher - than "direct" drive.)
This is probably more than you were initially asking for, but read over it a few times and it will start to make more sense to you. This is the kind of thing we're talking about when we get heavily into planning to complement an engine mod selection with transmission gearing, axle gearing, overdrive/underdrive, tyre selection, ... for a dual-purpose rig.
