The MGA With An Attitude
GEAR RATIOS Commonly Used In The MGA - GT-102A
| Standard Gearbox |
| | Input | 4th Gear | 3rd Gear | 2nd Gear | 1st Gear | Reverse |
| Mainshaft Teeth | 21 | direct drive | 25 | 31 | 28 | 28 |
| Reverse Idler Teeth | | 17/13 |
| Layshaft Teeth | 30 | - | 26 | 20 | 11 | 11 |
| Standard Ratios | 1.000:1 | 1.374:1 | 2.214:1 | 3.636:1 | 4.755:1 |
| Close Ratio Gearbox |
| | Input | 4th Gear | 3rd Gear | 2nd Gear | 1st Gear | Reverse |
| Mainshaft Teeth | 26 | direct drive | 29 | 32 | 28 | 28 |
| Reverse Idler Teeth | | 17/13 |
| Layshaft Teeth | 25 | - | 22 | 19 | 11 | 11 |
| Close Ratios | 1.000:1 | 1.267:1 | 1.619:1 | 2.448:1 | 3.201:1 |
| Standard Gearbox with Various Final Drives |
| | 4th Gear | 3rd Gear | 2nd Gear | 1st Gear | Reverse |
| Standard Gearbox -- | 1.000:1 | 1.374:1 | 2.214:1 | 3.636 | 4.755:1 |
Overall - Short Track Gears
41/9 = 4.556:1 | 4.556:1 | 6.258:1 | 10.087:1 | 16.566:1 | 21.663:1 |
Overall - 1500/1600
43/10 = 4.300:1 | 4.300:1 | 5.907:1 | 9.521:1 | 15.636:1 | 20.448:1 |
Overall - 1600 MK II
41/10 = 4.100:1 | 4.100:1 | 5.632:1 | 9.078:1 | 14.909:1 | 19.497:1 |
Overall - MGB gears
43/11 = 3.909:1 | 3.909:1 | 5.370:1 | 8.656:1 | 14.215:1 | 18.589:1 |
| Close Ratio Gearbox with Various Final Drives |
| | 4th Gear | 3rd Gear | 2nd Gear | 1st Gear | Reverse |
| Close Ratio Gearbox -- | 1.000:1 | 1.267:1 | 1.619:1 | 2.443:1 | 3.201:1 |
Overall - Short Track Gears
41/9 = 4.556:1 | 4.556:1 | 5.774:1 | 7.377:1 | 11.150:1 | 14.581:1 |
Overall - 1500/1600
43/10 = 4.300:1 | 4.300:1 | 5.450:1 | 6.964:1 | 10.524:1 | 13.762:1 |
Overall - 1600 MK II
41/10 = 4.100:1 | 4.100:1 | 5.197:1 | 6.640:1 | 10.035:1 | 13.123:1 |
Overall - MGB gears
43/11 = 3.909:1 | 3.909:1 | 4.955:1 | 6.331:1 | 9.568:1 | 12.512:1 |
If the gear ratio calculation is still a mystery to you, it works like this. A smaller gear driving a larger gear results in a speed decrease. A large gear driving a smaller gear results in a speed increase. The change of speed is a ratio commonly represented by a fraction using the numbers of the teeth on each gear, putting the driven gear number on top and the driving gear number on bottom. A speed reduction has a ratio greater than 1, and a speed increase has a ratio less than 1. This is because we commonly speak of these speed changes as "reduction ratios". For instance, the input gear ratio for the standard gearbox is 30/21 = 1.429, which gives a speed reduction from input gear to laygear. The input gear ratio for the close-ratio gearbox is 25/26 = 0.962, which gives a speed increase from input gear to laygear.
Most selected gears will have two (or three) speed changes in series between the input shaft and the output shaft. Series connected gear ratios are multiplied to find the overall gear ratio, like this:
Standard gearbox 2nd gear: 30/21 x 31/20 = 2.214
Standard gearbox Reverse gear: 30/21 x 17/11 x 28/13 = 4.755
Fourth gear is direct drive from input shaft to output shaft, resulting in no speed change, or a reduction ratio of 1/1 = 1.000.
Notice also that each gear mesh will change direction of rotation from the driving gear to the driven gear. An even number of gear meshes in series will result in the output shaft turning in the same direction as the input shaft. An odd number of gear meshes in series will result in the output shaft turning in the opposite direction as the input shaft (reverse gear).
Addendum December 2010:
You can determine gear ratios of the gearbox without disassembly. This is where the starting handle comes in handy (with a little help from a friend). Jack at least one rear wheel off the ground, release hand brake, and engage the gear to be tested. Make a mark on propshaft. Have a friend turn the engine and count turns while you watch the propshaft go exactly one turn of the propshaft. Number of turns of the engine (or gearbox input shaft) is the gear reduction ratio.
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