Solve for engine RPM, road speed, gear ratio, or tire diameter โ then check every gear, correct for tire growth, and A/B test two setups.
Solve for RPM, Speed, Ratio, or Tire Diameter
Pick what you want to find; fill in the other three. Tire growth is applied to the effective tire diameter.
Per-Gear RPM / Speed Table
Transmission preset ร final drive ร tire. Enter a speed to see RPM per gear, or enter RPM to see speed per gear. Tire growth applies to the effective diameter.
Before / After Setup Comparison
Two setups side by side โ ratio, tire, and growth each โ with the full delta table.
Step-by-Step Formula Trace
Every calculation shows its work here: formula, numbers plugged in, result. Updates live.
Share & Saved Setups
Your inputs are encoded in the page URL โ copy it to share or bookmark this exact calculation.
Worked Examples
Frequently Asked Questions
What is the 336.13 constant in these formulas?
It folds unit conversions into one number. A mile is 63,360 inches and an hour is 60 minutes; 63,360 รท (ฯ ร 60) = 336.13. It converts tire circumference (inches per revolution) and RPM into miles per hour.
What is the difference between gear ratio and final drive ratio?
A transmission gear ratio is one gear inside the gearbox (e.g. 1.52:1 in 2nd). The final drive ratio is the differential (e.g. 3.73:1). Multiply them for the overall ratio the engine sees: 1.52 ร 3.73 = 5.67:1 in 2nd gear.
How much do tires actually grow at speed?
Street radials: effectively zero โ use 0%. Drag slicks grow as centrifugal force stretches them: 2โ4% at the top end is typical, which is why trap RPM often reads lower than the static-diameter math predicts. Bias-ply street tires: ~1%.
Why does my tachometer read higher than the calculator?
Common causes: torque-converter slip in an automatic (adds 100โ300 RPM until lockup), tire slip, an inaccurate speedometer, or a tire that measures smaller than its sidewall says. Locked-converter cruise on the highway should match within ~2โ3%.
Should I measure tire diameter or trust the sidewall math?
Measure if you can: the loaded rolling radius (axle center to ground ร 2) is what the physics actually uses, and it runs slightly smaller than the unloaded diameter. The metric-code formula gives the unloaded nominal size โ close, but a measurement beats it.
Limitations
An rpm calculator answers a simple question: at a given speed, what will your engine’s RPM be โ or, flipped around, what speed does a given RPM give you? Enter your overall gear ratio, tire diameter, and either speed or RPM, and you get the missing value instantly. It’s a quick way to check whether a gear swap will raise your highway cruise RPM, or whether taller tires will drop it.
How to Use the RPM Calculator
- Choose what to solve for. Most people solve for RPM: enter your speed, overall gear ratio, and tire diameter, and the calculator returns engine RPM. But you can also solve for speed (enter RPM and get mph), for gear ratio, or for tire diameter.
- Enter your overall gear ratio. This is the total ratio the engine sees โ your transmission gear multiplied by the rear-axle ratio (e.g. 1.52 ร 3.73 = 5.67 in 2nd gear). On the Per-Gear Table tab, pick a transmission preset and the calculator combines it with your axle ratio for you.
- Enter your tire diameter in inches. Measure the actual tire if you can โ the loaded rolling radius is slightly smaller than the nominal size printed on the sidewall.
- Read the per-gear table. The results include a full table of RPM at your speed for every gear, so you can see exactly where each shift point lands and what your cruising RPM will be in top gear.
- Try the presets. Common transmission gear sets (TH350, 700R4, 4L60E, T5, Powerglide, C4) are preloaded on the Per-Gear Table tab โ pick one close to your vehicle and adjust from there.
The Formulas
The core relationship between speed, RPM, gearing, and tire size is:
RPM = mph ร gear ratio ร 336.13 รท tire diameter
The 336.13 constant folds together the unit conversions: inches per mile (63,360), inches of tire circumference per revolution (ฯ ร tire diameter), and minutes per hour (60). 63,360 รท (ฯ ร 60) = 336.13. The “gear ratio” here is the total ratio โ transmission gear multiplied by the axle ratio.
Worked example: solving for RPM
Cruising at 65 mph with 3.73 gears and 28-inch tires, in a 1:1 transmission gear:
- 65 ร 3.73 ร 336.13 = 81,494.7
- 81,494.7 รท 28 = 2910.5 โ ~2911 RPM
Rearranged to solve for speed:
mph = RPM ร tire diameter รท (gear ratio ร 336.13)
Worked example: solving for speed
At 3000 RPM with the same 3.73 gears and 28-inch tires:
- 3.73 ร 336.13 = 1253.76
- 3000 ร 28 = 84,000
- 84,000 รท 1253.76 = 67.0 โ ~67.0 mph
Common Mistakes
- Using tire width instead of overall diameter. A “285” is the section width in millimeters, not the height. Overall diameter for a 285/70R17 is about 32.7 inches โ using 285 anywhere in the formula gives a meaningless number.
- Ignoring torque converter slip in automatics. The formula assumes a locked converter. An unlocked converter at cruise slips 100โ300 RPM, so your tach will read slightly higher than the calculator says.
- Confusing engine RPM with wheel RPM. The formula gives engine RPM. Wheel RPM is engine RPM divided by the total gear ratio โ a much smaller number, and not what your tach shows.
- Mixing metric and imperial. Tire diameter in millimeters with mph on the other side breaks the 336.13 constant. Keep everything in inches and miles, or convert first.
- Forgetting overdrive. A 0.70:1 overdrive gear multiplies the axle ratio down โ 3.73 gears become an effective 2.61 in overdrive. Run the numbers in your actual top gear.
Frequently Asked Questions
What RPM is too high for highway cruising?
Most modern gasoline engines are comfortable cruising at 1,800โ2,500 RPM. Sustained cruising above ~3,000 RPM gets loud, thirsty, and wears the engine faster โ if your calculator shows that at your normal highway speed, consider taller gears or an overdrive transmission. Diesels typically cruise lower, around 1,500โ2,000 RPM.
Why is my actual RPM different from what the calculator says?
The usual culprits: torque converter slip (100โ300 RPM in an unlocked automatic), tires that are worn or underinflated (smaller effective diameter raises RPM), and using the nominal tire size instead of the measured diameter. A small error in tire diameter compounds across the whole calculation.
Do diesels and gas engines cruise at different RPM?
Yes. Diesels make peak torque low (often 1,600โ2,000 RPM) and are geared to cruise there โ many diesel trucks sit near 1,700 RPM at 70 mph. Gas engines make power higher up, so 2,000โ2,500 RPM at cruise is normal. The calculator doesn’t care which you have; it just needs your ratios and tire size.
Does tire wear change my RPM?
Slightly. Worn tires are smaller in diameter โ losing half an inch of diameter raises cruise RPM by roughly 1.5โ2%. You won’t notice it on the tach, but it’s one reason measured diameter beats the number printed on the sidewall.
How does overdrive change the numbers?
Overdrive is a transmission gear below 1:1 (commonly 0.65โ0.80), and it multiplies your axle ratio down. A 4.10 axle in a 0.70 overdrive behaves like a 2.87 final ratio at cruise โ which is exactly why modern trucks can run deep gears for towing and still cruise quietly on the highway.