Solve for the ratio, not the result
Rather than checking what an existing ratio produces, this works backwards from a target: the engine speed you want at a given road speed, in a specific gear. Useful for planning a build, matching a highway cruise RPM, or choosing a ratio for a large tire size before buying gears.
| Ratio | RPM at 70 mph | Difference from target |
|---|---|---|
| 2.73:1 | 1626 rpm | -374 rpm |
| 3.08:1 | 1834 rpm | -166 rpm |
| 3.15:1 | 1876 rpm | -124 rpm |
| 3.21:1 | 1912 rpm | -88 rpm |
| 3.31:1 | 1971 rpm | -29 rpm |
| 3.42:1 | 2037 rpm | 37 rpm |
| 3.55:1 | 2114 rpm | 114 rpm |
| 3.73:1 | 2221 rpm | 221 rpm |
| 3.9:1 | 2323 rpm | 323 rpm |
| 4.1:1 | 2442 rpm | 442 rpm |
| 4.3:1 | 2561 rpm | 561 rpm |
| 4.56:1 | 2716 rpm | 716 rpm |
| 4.88:1 | 2906 rpm | 906 rpm |
| 5.13:1 | 3055 rpm | 1055 rpm |
| 5.38:1 | 3204 rpm | 1204 rpm |
Want to check what your current ratio produces instead of starting from a target? The gear ratio calculator works in that direction, or see the ring and pinion teeth a given ratio typically uses.
Getting a number you can act on
- 01Decide on a target engine speed
This is usually a comfortable cruising RPM, or an RPM that keeps a torque converter or a camshaft in its efficient range. There is no universal right answer — it depends on the engine and the goal.
- 02Set the road speed that target applies at
70 mph is a common highway reference in the US, but use whatever speed actually matters for the build — a tow rig's cruising speed differs from a daily driver's.
- 03Enter the top gear ratio
Whichever gear the vehicle spends the most time in at that speed — usually the highest gear, often an overdrive below 1.00.
- 04Enter the tire size that will actually be fitted
This has to be the final tire size, not the current one — the whole point of a regear calculation is usually to compensate for a size that has changed or is about to.
- 05Compare the exact answer against what is actually available
Ring and pinion sets come in a fixed set of ratios. The table shows how each standard option performs against your target, so the choice is made with numbers rather than a guess.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
axle ratio = (target RPM × π × tire diameter) ÷ (mph × gear ratio × 1056)The standard RPM formula solved for axle ratio instead of RPM — the same relationship, rearranged for the question being asked.
Why start from a target instead of checking a result
The gear ratio calculator answers a forward question: given this axle ratio and this tire, what RPM results? That is the right tool when a ratio is already fixed and you want to know its consequence.
This tool exists for the opposite situation — planning before anything is bought. Someone building a tow vehicle, fitting a significantly larger tire, or simply wanting the engine to sit at a specific RPM on the highway does not yet have a ratio to check. They have a target, and the practical question is which ratio gets them there.
Both tools solve the identical underlying relationship between RPM, road speed, gear ratio, axle ratio and tire diameter — they simply solve it for a different unknown, which is why either can be used to check the other's answer.
Why the exact answer usually isn't for sale
Ring and pinion sets are manufactured in a fixed set of tooth-count combinations, which means the ratios available in the real world cluster around familiar numbers — 3.55, 3.73, 4.10, 4.56 and similar — rather than forming a continuous range.
Solve for a target RPM and the exact figure that comes out will almost never be one of those numbers. The practical decision is which side of it to land on: a numerically higher ratio than calculated slightly over-corrects, giving marginally more RPM and torque than the target asked for; a numerically lower ratio under-corrects, leaving some of the shortfall in place.
For a vehicle that tows or works for a living, over-correcting is usually the safer error — extra torque is rarely unwelcome. For a vehicle built mainly to cruise efficiently, under-correcting keeps the engine a little quieter and more relaxed. The table above shows exactly how far off each standard option leaves you, so that choice is made deliberately.
Regear Calculator FAQ
How do I calculate the axle ratio I need?+
Multiply your target RPM by π and the tire diameter, then divide by road speed, the transmission gear ratio and 1056. This tool does that calculation directly from the numbers you enter.
What RPM should I target when regearing?+
There's no universal figure — it depends on the engine and the goal. A common approach is choosing a comfortable, efficient cruising RPM at a typical highway speed, often somewhere between 1,800 and 2,500 rpm on a modern engine.
Why doesn't my calculated ratio match anything I can buy?+
Ring and pinion sets are manufactured in a fixed set of tooth-count combinations, so the calculated figure almost never lands exactly on a real option. Choose the nearest available ratio and check how far off it leaves you.
Should I round up or down to the nearest available ratio?+
For towing or heavy use, rounding up (numerically higher) usually suits better, trading slightly more RPM for more torque. For a vehicle mainly cruising, rounding down keeps the engine quieter.
Do I need to know my current gear ratio to use this?+
No — this tool works from a target RPM forward to the ratio needed, not from your current setup. If you want to compare against what you have now, the gear ratio calculator checks a known ratio's result instead.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗02The Tire and Rim Association — standards filing (NHTSA docket)TRA has been the US standardising body for tire and rim interchangeability since 1903; this filing sets out dimensional practice.↗Calculated, not measured. These figures assume nominal tire sizes and the ratios you entered. Real rolling radius changes with load, pressure and wear, and a vehicle's actual axle ratio is not always the one on the door sticker. Verify the ratio on the differential tag before spending anything on gearing.