RPM Speed Calculator: Free Vehicle Speed Tool
Engine RPM means nothing on its own: the same 3,000 RPM is 82.5 MPH on one truck and 65 on another, because the gearing and the tires decide what each crankshaft revolution is worth. This calculator runs that chain for you in three directions. Enter RPM, overall gear ratio and tire diameter to get road speed, work backwards from a target speed to find the RPM you need, or type the three numbers off your sidewall and let the tire-size mode derive the diameter itself.
Overall gear ratio = transmission gear × axle ratio. Cruising in 4th (1.00) with a 3.42 axle, enter 3.42; in an overdrive 6th of 0.70, enter 2.39.
Theoretical speed assuming no slip. Real speed varies with tire wear, load, inflation and torque-converter slip in automatics.
Quick Answer
At 3,000 RPM through an overall gear ratio of 3.42 on 31.6-inch tires, road speed is 82.5 MPH (132.7 km/h). Working backwards, holding 65 MPH with a 2.611 overall ratio on the same tires needs 1,805 RPM. Type a sidewall code like 265/70R17 into the tire-size mode and it derives the 31.61-inch diameter itself.
Key takeaways
- Speed = wheel RPM × tire circumference × 60 ÷ 63,360. Wheel RPM is engine RPM divided by the overall gear ratio, which is transmission gear times axle ratio.
- Entering only the axle ratio is the classic mistake: in an overdrive 6th of 0.70 with a 3.73 axle, the number that matters is 2.61, not 3.73.
- Bigger tires lower your cruise RPM but make the axle behave like a numerically lower ratio; re-gearing restores the original feel.
- These are theoretical speeds. Tire wear, load, inflation, and torque-converter slip all make the real number slightly lower.
- Quick Answer
- How to use the RPM speed calculator: three modes
- The core formula, and where 336 comes from
- Worked example: 3,000 RPM, checked three ways
- Why “overall gear ratio” is doing quiet work
- Bigger tires change your gearing, and the math tells you by how much
- Why your speedometer disagrees with the math
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the RPM speed calculator: three modes
Pick the tab that matches your question. The results stay hidden until you click the calculate button.
- RPM to speed: enter engine RPM, overall gear ratio and tire diameter. Pre-filled with 3,000 RPM, 3.42 and 31.6 inches, which gives 82.5 MPH.
- Speed to RPM: enter your target speed, overall gear ratio and tire diameter. Pre-filled with 65 MPH, 2.611 and 31.6 inches, which needs 1,805 RPM.
- Tire size to speed: enter engine RPM and overall gear ratio, then the three sidewall numbers: width in mm, aspect ratio, rim diameter in inches. Pre-filled with 265/70R17, which derives a 31.61-inch tire and gives 82.5 MPH.
Use the units selector for metric (km/h and mm). A blank gear ratio is treated as 1.00, direct drive. Tabs support arrow-key navigation. Copy my results and Share results sit side by side under the results. If you are planning a trip rather than a gearing change, the driving time calculator turns that speed into arrival times.
The core formula, and where 336 comes from
The whole calculation is a chain: engine RPM ÷ overall gear ratio gives wheel RPM, and wheel RPM × tire circumference gives distance per minute. In miles per hour with the tire diameter in inches:
Wheel RPM = engine RPM ÷ overall gear ratio
Tire circumference = π × diameter
The 1,056 is a pure unit constant: 63,360 inches per mile divided by 60 minutes per hour. Many gearing references, from AJDesigner’s gear equation page to the PowerNation gear ratio calculator, write the same relationship as RPM = MPH × gear ratio × 336 ÷ tire diameter, where 336 = 63,360 ÷ (60 × π) ≈ 336.13. This tool uses the wheel-RPM form of the same equation, which is algebraically identical, and prints the formula with the numbers filled in under every result so you can check it by hand.
Worked example: 3,000 RPM, checked three ways
The calculator is pre-filled with a truck turning 3,000 RPM through an overall ratio of 3.42 on 31.6-inch tires. Clicking Calculate reproduces this:
Every result also carries a reference table for 1,000 through 5,000 RPM at your gearing, which is the quick way to answer “what if I cruise 500 RPM lower?” without re-entering anything. Switch to the Speed to RPM tab with the same tires and a 2.611 overall ratio (0.70 overdrive × 3.73 axle): 65 MPH needs 1,805 RPM, wheel RPM 691.4. And the tire-size tab with 265/70R17 derives 17 + 2 × 265 × 0.70 ÷ 25.4 = 31.61 inches, then lands on the same 82.5 MPH, which cross-checks the whole chain.
Why “overall gear ratio” is doing quiet work
The single most common mistake is entering only the axle ratio. Overall gear ratio means transmission gear times axle ratio: the number that matters is the ratio between the crankshaft and the wheels in the gear you are actually in. Cruising in 4th at 1.00:1 with a 3.42 axle, enter 3.42. In an overdrive 6th of 0.70 with that same axle, enter 2.39, because 0.70 × 3.42 = 2.394. Enter 3.42 while cruising in overdrive and the calculator will understate your speed by a third.
If you are unsure of the numbers, the transmission ratios are in the owner’s manual and the axle ratio is usually on a tag on the differential or the door-jamb sticker. For the inverse problem, sizing the axle itself for a target cruise RPM, the RPM to radians calculator in this site’s RPM family converts the rotational speed into the angular units driveline equations use.
Bigger tires change your gearing, and the math tells you by how much
A larger tire covers more ground per revolution, so the engine turns slower at a given speed, but the axle also behaves like a numerically lower ratio: acceleration softens and the transmission hunts more. The effective ratio is the old axle ratio times the old diameter divided by the new one. Going from 31.6-inch to 35-inch tires on 3.73 gears gives an effective ratio of 3.73 × 31.6 ÷ 35 = 3.37, so the truck drives like it has 3.37 gears on the original tires.
| Setup at 70 MPH, 33-inch tires, 0.70 overdrive | Overall ratio | Engine RPM |
|---|---|---|
| 3.73 axle (stock) | 2.611 | 1,862 |
| 4.10 axle (re-geared) | 2.870 | 2,046 |
To keep the original cruise RPM after moving from 31.6 to 35 inches, scale the axle ratio by the diameter change: 3.73 × 35 ÷ 31.6 = 4.13. Axles come in fixed steps, so 4.10 is the closest real option, which is exactly the recommendation the re-gearing guides converge on. Both rows above are real runs of this calculator’s speed-to-RPM mode, so you can verify them yourself. The RPM to distance calculator extends the same math to total distance covered over time.
Bigger tires: the upsides
- Lower cruise RPM and quieter highway running
- More ground clearance off road
- Slightly better steady-state fuel economy if the engine stays in its efficient band
Bigger tires: the costs
- Effective gearing drops (3.73 behaves like 3.37), so acceleration softens
- The speedometer reads low unless recalibrated, typically a few percent
- Heavier rotating mass and more rolling resistance around town
Why your speedometer disagrees with the math
The calculator gives the theoretical speed for the diameter you enter. Three things make reality differ, all in the same direction. First, a worn tire is smaller: losing 6 mm of tread all round shrinks the diameter by 12 mm, about 1.5% on a 31.6-inch tire, so the car travels less per revolution than the sidewall size implies. Second, load and inflation change the effective rolling radius; an under-inflated tire squats and its rolling circumference shrinks. Third, automatics lose a few percent to torque-converter slip, which is why the results note says theoretical speed assuming no slip.
The practical rule: if your GPS says 68 when the speedometer says 70, check the actual measured tire diameter and the inflation pressure before suspecting the gearing. Speedometer error from a tire swap follows the diameter ratio directly: 35-inch tires where 31.6-inch were stock read about 10% low.
What competing calculators miss
RPM-to-speed tools agree on the formula and diverge on everything around it:
- Guide-style pages (Auto Engineering Tools): the clearest explanation of the transmission-times-final-drive chain, with a worked 62.2 MPH example. But it is a page, not a tool: no reverse calculation, no tire-code input, no reference table.
- Multi-solve tools (AJDesigner): solves for any variable, adds effective gear ratio and crawl ratio. The trade-off is a tire radius input, which nobody measures, and a dated interface.
- Re-gearing tools (PowerNation TV): the best axle-step tables and the effective-ratio concept, aimed at trucks going to 33s and 35s. No tire-code mode, no metric, no reverse direction.
- Truck calculators (Drivewyze): millimetre tire diameter and overdrive inputs for semis. Narrow by design: one direction, one vehicle class.
- Formula-reference sites (calculator.city): documents both constants, 336.135 for MPH/inches and 530.516 for KPH/cm. Thin surrounding content and no worked checks.
- The gap this page fills: all three directions in one tool with the gearing intact between them, a sidewall-code mode so nobody looks up a diameter table, metric and imperial, a 1,000-to-5,000 RPM reference table printed with every result, the formula shown with your numbers in it, and the re-gear and speedometer guidance that the single-direction tools leave to forum threads.
Frequently asked questions
The total reduction between the crankshaft and the wheels: transmission gear ratio times axle (final drive) ratio. In 4th gear at 1.00:1 with a 3.42 axle the overall ratio is 3.42; in an overdrive 6th of 0.70 it is 2.39. Always enter the product, not just the axle number.
It is a unit constant: 63,360 inches per mile divided by 60 minutes per hour divided by pi, which is about 336.13. The formula RPM = MPH × gear ratio × 336 ÷ tire diameter is the same physics as this calculator’s wheel-RPM form, just rearranged.
In a code like 265/70R17, 265 is the width in millimetres, 70 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the rim diameter in inches. This calculator’s tire-size mode takes those three numbers and derives the overall diameter: rim + 2 × width × aspect ÷ 100 ÷ 25.4.
The calculator uses the diameter you enter, while the car drives on the actual rolling tire. Tread wear shrinks the diameter, low inflation shrinks the rolling circumference, and torque-converter slip in automatics loses a few percent. After a tire size change, the speedometer error follows the diameter ratio: 35-inch tires where 31.6-inch were stock read about 10% low.
No. A blank gear ratio is treated as 1.00, which is direct drive with no reduction. Zero or negative entries are rejected with a message, and non-numeric entries like “abc” are rejected too, while commas in numbers (3,420) are stripped and accepted.
Yes. Switch the units selector to metric and the calculator works in km/h with the tire diameter in millimetres, converting internally. The tire-size mode always takes width in mm and rim in inches regardless of the unit setting, because that is how sidewalls are marked worldwide. Note: switching units changes the labels but does not convert the diameter already typed, so re-enter it in the new unit (31.6 in = 803 mm).
Your road speed at 1,000 through 5,000 RPM in 500-RPM steps, using the same gear ratio and tire diameter you entered, in both MPH and km/h. It answers gearing what-ifs, like dropping the cruise by 500 RPM, without re-entering anything.
Yes, proportionally to the diameter increase, but the axle then behaves like a numerically lower ratio, so acceleration softens. To keep the original cruise RPM when moving from 31.6-inch to 35-inch tires on 3.73 gears, you would need about a 4.13 axle, and 4.10 is the closest real step.
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References
- Tire Rack. How to Read Your Tire Size: Decoding the Sidewall (width, aspect ratio, construction, rim diameter, load index, speed rating). Accessed October 23, 2026.
- AJDesigner. Gear Ratio Calculator: Speed, RPM and Tire Size (vehicle speed equation v = r × RPM ÷ (168 × G), effective gear ratio, tire diameter from metric size). Accessed October 23, 2026.
- Auto Engineering Tools. How to Calculate Vehicle Speed From RPM (engine RPM ÷ transmission ratio ÷ final drive = wheel RPM; worked 3,000 RPM example at 62.2 MPH). Accessed October 23, 2026.
- PowerNation TV. Gear Ratio Calculator (axle ratio selection for tire size, effective gear ratio, re-gearing tables). Accessed October 23, 2026.
- Bureau International des Poids et Mesures. The International System of Units (SI Brochure) (the inch defined as exactly 25.4 mm, the mile as 1,609.344 m). Accessed October 23, 2026.