Fit a taller tire and three things change at once: your speedometer starts reading low, your effective gearing goes taller, and the engine turns slower at the same road speed. This works out all three, and tells you which axle ratio would put your original gearing back.
265/70R17 → 285/70R17 on a 3.73
- Diameter
- 31.61 → 32.71 in (+1.10)
- Revolutions per mile
- 638 → 617
- Speedometer
- Off by 3.5% — it reads low. At a true 70 mph it shows 67.6.
- Effective axle ratio
- 3.60 — the taller tire makes a 3.73 behave like this
- Engine speed at 70 mph
- 2,777 → 2,683 rpm, at a 1.000 transmission gear
To get your original gearing back
3.86
No GM ratio we can verify matches 3.86. We are not going to round you to the nearest one — a ratio that is close is still a different axle. The five we can stand behind are below.
What this does not tell you
Three things on purpose, because getting them wrong costs real money or hurts someone.
- Whether the tire will fit. That depends on your lift, wheel offset, backspacing, trim height and how the truck sits loaded. None of that is in a tire size, so this tool gives you geometry and stops. Test fit before you buy five.
- Load rating or inflation pressure. Those come off the tire’s own sidewall and the Tire and Loading Information label in your driver’s door jamb. We do not publish figures we cannot source, and a wrong number there is a safety problem.
- Exactly what your tire measures. These figures are geometry from the size on the sidewall. A real tire runs a little shorter than that under the weight of the truck, and two brands in the same size are rarely identical. A manufacturer’s published revolutions-per-mile is measured and will differ from ours by a percent or so. Use theirs when you have it.
- Whether your speedometer can be recalibrated. That is year and model specific. The error figure above is the size of the problem, not the fix.
The axle ratios we can verify
When the restoring ratio above lands on one of these, we name the option code. When it does not, we say so rather than rounding you to the nearest one — a ratio that is close is still a different axle.
| Code | Ratio | Where it is documented |
|---|---|---|
| GU4 | 3.08 | 1988–2020 |
| GU5 | 3.23 | 1988–2026 |
| GU6 | 3.42 | 1983–2026 |
| GT4 | 3.73 | 2000–2026 |
| GT5 | 4.10 | 2000–2019 |
GM has used well over two hundred codes carrying an axle ratio. These five are the ones with GM wording behind them in more than one document, across more than one decade. The rest are in the RPO code database with their evidence attached, including ten codes where GM’s own paperwork gives two different ratios for the same code.