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A change in tyre size changes your overall gear ratio. With a 4x4 that
has "high" gears (numerically low such as 3.07, 3.31, 3.54), the effect
on acceleration from a radical swap can be like starting in second gear.
The effect of going from a 28-inch tall tyre to a 30-inch tyre is like
trading 3.54 gears for 3.32s. More radical tyre changes produce more
radical gearing changes.
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From the other perspective, going from a 30-inch to a 33-inch tyre is
like swapping 4.56 gears for 4.10s, and this could be advantageous in
the highway cruising department. The moral here is that if you figure
out what diameter you want, you can pick a gear ratio that gives you the
best off-roading gearing, combined with a livable highway cruising rpm.
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| Effective Gear Ratio |
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| Effective Ratio of New Combination |
= |
Old Tyre Diameter
New Tyre Diameter |
x Original Ratio |
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| Engine RPM at a Given Speed |
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| Engine RPM |
= |
mph x Total Gear Ratio x 336
Tyre Diameter (Inches) |
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| Equivalent Ratios for New Tyres |
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| New Gear Ratio |
= |
New Tyre Diameter
Old Tyre Diameter |
x Original Ratio |
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| Speedometer Correction (Tyre Swap Only) |
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| Actual Speed (mph) |
= |
New Tyre Diameter
Old Tyre Diameter |
x Indicated mph |
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| Speedometer Correction (Tyre and Gear Swap) |
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| Actual Speed (mph or kmph) |
= |
New Tyre Diameter
Old Tyre Diameter |
x |
Old Gear Ratio
New Gear Ratio |
x Indicated mph or kmph |
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