Gear Ratio & RPM Calculator
Calculate cruising engine RPM at highway speeds for any axle gear ratio and tire diameter — and see what bigger tires really did to your effective gearing.
Cruising RPM at your speed2,714
Effective ratio with current tires3.93
RPM on stock-size tires3,006
| Axle ratio | RPM |
|---|---|
| 3.73 | 2,328 |
| 4.1 | 2,558 |
| 4.35 | 2,714 |
| 4.56 | 2,845 |
| 4.88 | 3,045 |
| 5.13 | 3,201 |
RPM = (gear ratio × speed × 336) ÷ tire diameter
Re-gearing for bigger tires: bigger tires raise your effective gearing — a 4.10 axle with 35 in tires behaves like a 3.70 with the stock 31.6 in. Multiply your old ratio by new diameter ÷ old diameter to find the ratio that restores stock feel: 4.10 × 35 ÷ 31.6 ≈ 4.54, so a 4.56 ring and pinion puts you back to stock.
Front/rear match matters on 4x4: if the axles differ by more than ~2%, part-time 4WD can bind, throw 4H faults, and over the long term chew up both diffs and the transfer case. Read the mismatched gear ratio guide — used 4x4 buyers check this before paying.
The cruising sweet spot: most engines sit happily between 1,800 and 2,400 rpm at cruise — enough load for efficiency, low enough for noise and wear. If your build puts you at 3,000+ rpm at 65 mph, an overdrive transmission or taller gears usually beats accepting the drone.
Assumptions & sources
RPM constant336RPM = ratio × mph × 336 ÷ tire_d — cross-checked against crawlpedia.com
Effective ratioold × new_d ÷ old_dRestores stock wheel-turns-per-mile
4x4 match limit≤ 2% front/rearCommunity-documented failure threshold (rules-inventory G2)
Frequently asked questions
How do I know my axle gear ratio?
Three ways: the build sheet or window sticker (often lists it), the RPO/door-tag code (GM G80 axle ratios are in the RPO list), or jack up one wheel and count driveshaft turns per wheel turn (about 2.2 turns per wheel turn ≈ 4.10 with an open diff — double it for one-wheel-up open-diff counts: 4.1 turns ≈ 4.10). The ring gear itself is stamped, e.g. 41:11 = 3.73.
Why 336 in the formula?
It folds together unit conversions: 60 minutes per hour, 63,360 inches per mile, and the 2π of wheel rotation. The result is the classic hot-rod constant: RPM = ratio × mph × 336 ÷ tire diameter in inches. It matches crawlpedia.com's calculator to the integer.
What RPM is too high on the highway?
Above roughly 3,000–3,200 rpm at 65 mph you are paying in noise, heat, and fuel for most stock engines. Comfort and economy usually peak between 1,800 and 2,400 rpm. Diesel trucks happily cruise lower; small engines run higher — compare against your stock number, not an absolute.
Do bigger tires change my gear ratio?
They change the effective ratio, not the metal. Each revolution now moves the truck further, so the engine turns slower for a given road speed — like shifting into a taller gear. Use the effective-ratio readout to see exactly how much taller, and which ring-and-pinion restores stock behaviour.