Speedometer Off
Short answer: A speedo that reads off after a rear-end swap or a tire-size change usually just needs the correct speedo driven gear (the colored plastic gear on the cable at the transmission) to match the new ratio/tire size. It’s an easy, ~$8 part at a GM dealer (Classic Industries sells them too — give your trans type and rear ratio), or a shop can recalibrate. If originality matters, fitting reproduction original-size tires is the other route.
Why it’s off: a rear-axle swap changes the gear ratio, or a taller/shorter tire changes the rolling distance, but the trans speedo drive gear wasn’t changed to match. Quick tire-diameter estimate: error = 1 − (new dia ÷ old dia). Going 225/70-15 (27.4″) to 245/50-16 (25.6″): error ≈ 6.6%, and the speedo reads faster than you’re going (shows ~70 at ~65.5). Tire-size decode: 225/70-15 = 225 mm tread width, 70 = sidewall as % of width, 15 = rim diameter.
| Size | Diameter |
|---|---|
| E70-14 | 25.8″ |
| F70-14 | 26.2″ |
| 195/75R14 | 25.5″ (5.1″ tread) |
| 205/75R14 | 26.1″ (5.3″ tread) |
| 215/70R14 | 25.9″ (6.0″ tread) |
| 225/70R14 | 26.4″ (6.3″ tread) |
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Content last modified: June 13, 2026 at 10:06 am
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