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The speedometer reading is adjustable via the various drive gears fitted at the transmission. Depending on which final gear ratio (differential gears) are fitted, the speedometer gears are changed to keep the speedometer accurate.
Contents |
Overview
Stock 1200 Speedometer gear ratio:
pinion teeth/worm teeth on trans output shaft * 17/5 (3.90 diff) * 18/5 (4.11 diff) for Wagon or Ute
To adjust the speedometer for different diameter tires, change the transmission cable pinion cog.
* If speedo reads too fast, fit a bigger cog * If speedo reads too slow, fit a smaller cog
Given that stock is a 17 tooth pinion, moving to one more or less tooth is about 6% change. So by changing the pinion you can only get it to ±3 mph accurate. For example, at 100 mph, within 3 mph. Or at the standard 50 mph rating, a reading between 1½ mph (48½ to 51½) is the best you can hope for. For finer tuning the speedometer itself needs calibration. Or you could fit different diameter rear tires.
EXAMPLE: If you are reading say 60 when you are really going 50 then the speedo cable needs to slow down by about 20% to read more accurately. If the speedo drive has a 17 tooth cog in it try to find a cog that reduces by 20%. 17 X .20 = 3.4 teeth... 17 + 3.4 = 20.4 teeth, the closest is a 20 tooth cog.| Teeth | % change | 50 mph reading |
| 16 | 106.25% | 53.1 |
| 17 | 100.00% | 50.0 |
| 18 | 94.44% | 47.2 |
| 19 | 89.47% | 44.7 |
Correction Table, increase Teeth by:
| Actual mph | Desired mph | Tooth |
| 40 | 50 | 3+ |
| 43 | 50 | 2+ |
| 47 | 50 | 1+ |
| 50 | 50 | 0+ |
Estimated Tire Correction Table (for 3.9 diff)
| Size | Diameter | Difference | Pinion | ~Error |
| 165/55R12 | 19.1 | 88% | 19T | 0% |
| 165/60R12 | 19.8 | 91% | 18T | -2% |
| 165/70R12 | 21.1 | 97% | 17T | 3% |
| 155/80R12 | 21.8 | 100% | 17T | 0% |
| 175/70R13 | 22.6 | 104% | 16T | 2% |
| 195/50R15 | 22.7 | 104% | 16T | 2% |
| 165/70R14 | 23.1 | 106% | 16T | 0% |
| 205/45R17 | 23.5 | 108% | 16T | -2% |
| 185/60R15 | 23.7 | 109% | 16T/15T | 3% |
| 205/40R18 | 24.5 | 112% | 15T | 0% |
