Free tool for 4x4s and off-roaders
Tyre size & gear ratio calculator
Fitting bigger off-road tyres to your 4x4? Enter the size you run now and the size you want. See your real speed, how much pull you lose, and the axle ratio that brings it back.
The size is printed on the tyre's sidewall. Or just type the height, like 33.
Popular: 31 to 3333 to 3535 to 37265/70R17 to 285/70R17
Going further Regearing? Find the right axle ratio For when bigger tyres leave the truck feeling lazy and you want the pull back.
What's an axle ratio? It's the gearing inside your diffs: how many times the driveshaft turns for one turn of the wheels. A bigger number, like 4.56, means more pull and more engine revs. Swapping it ("regearing") is how people get their performance back after fitting bigger tyres.
| Ratio | Against how it drives now |
|---|
"Now" means your current tyres on your current gears. Ratios shown are ones gear sets are commonly sold in. What you can buy depends on your axle.
Off-road Crawl ratio and crawl speed How slowly the truck can creep in low range, and what bigger tyres do to that.
What's a crawl ratio? It's first gear x low range x axle ratio: how many times the engine turns for one turn of the wheels in your lowest gear. A bigger number means slower, more controlled crawling and more engine braking downhill. Bigger tyres make the truck crawl faster, as if the number had dropped.
| Setup | Crawl ratio | Feels like | Speed |
|---|
"Feels like" is the crawl ratio you'd need on your current tyres to move at the same speed. Speeds assume no clutch or torque-converter slip.
How to read it
Speedo
Your vehicle works out speed by counting how fast the wheels turn. A taller tyre covers more ground per turn, so you travel further and faster than the speedo and odometer record. The error is the same percentage as the change in tyre height.
Gearing and pull
A taller tyre is a longer lever against the axle. In the same gear the engine turns slower at a given speed and the tyres push less at the ground, by about the same percentage. That's why a truck can feel lazy after a tyre upgrade.
Choosing a ratio (if you regear)
"Closest to now" returns the truck to how it drives today. It's the safe choice if you were happy with it. Going one step shorter gives more pull off the line and on climbs, and costs revs, noise and fuel at highway speed. People tend to go shorter when the truck is heavy, tows, or has a small engine or a manual gearbox, and stay closer to stock with a strong engine or a lot of highway driving.
Before you order gears
- On a 4WD, front and rear axles must be the same ratio. Budget for both.
- Check which ratios exist for your axles. Front and rear are often different designs with different options.
- After new tyres or new gears, the speedo needs recalibrating. On many modern vehicles the gearbox and stability control need the new values too.
- Lockers, carriers and some axle housings only suit a range of ratios. Ask your diff shop before buying parts.
Ride height
The axles rise by half the change in tyre height. This is the only upgrade that lifts the diffs themselves.
An off-road tyre size calculator, not a generic one
Most tyre calculators stop at diameter and speedo error. This one is built for 4x4s: it works with the 33, 35 and 37-inch sizes off-roaders actually talk about, shows the ground clearance you gain under the diffs, tells you how much pulling power bigger mud-terrain or all-terrain tyres cost, and covers regearing and low-range crawl ratio for the trail.
What gear ratio for 33s, 35s and 37s?
The nearest commonly sold ratio that restores stock gearing, starting from a 31.5-inch stock tyre. If your stock tyre is a different height, put your real sizes in the calculator above.
| Stock ratio | On 33s | On 35s | On 37s |
|---|---|---|---|
| 3.21 | 3.31 exact 3.36 | 3.55 exact 3.57 | 3.73 exact 3.77 |
| 3.45 | 3.55 exact 3.61 | 3.91 exact 3.83 | 4.10 exact 4.05 |
| 3.55 | 3.73 exact 3.72 | 3.91 exact 3.94 | 4.10 exact 4.17 |
| 3.73 | 3.91 exact 3.91 | 4.10 exact 4.14 | 4.30 exact 4.38 |
| 3.91 | 4.10 exact 4.10 | 4.30 exact 4.34 | 4.56 exact 4.59 |
| 4.10 | 4.30 exact 4.30 | 4.56 exact 4.56 | 4.88 exact 4.82 |
"Exact" is your ratio x new height / old height. Want more pull than stock? Go one step shorter than the table.
Common 4x4 tyre sizes in inches
| Size | Height | Height |
|---|---|---|
| 265/65R17 | 30.6 in | 776 mm |
| 265/70R17 | 31.6 in | 803 mm |
| 275/70R17 | 32.2 in | 817 mm |
| 285/70R17 | 32.7 in | 831 mm |
| 295/70R17 | 33.3 in | 845 mm |
| 285/75R17 | 33.8 in | 859 mm |
| 305/70R17 | 33.8 in | 859 mm |
| 315/70R17 | 34.4 in | 873 mm |
| 265/75R16 | 31.6 in | 804 mm |
| 285/75R16 | 32.8 in | 834 mm |
| 305/70R16 | 32.8 in | 833 mm |
| 315/75R16 | 34.6 in | 879 mm |
| 275/65R18 | 32.1 in | 815 mm |
| 275/70R18 | 33.2 in | 842 mm |
| 285/65R18 | 32.6 in | 828 mm |
| 295/70R18 | 34.3 in | 870 mm |
Nominal height: rim diameter plus two sidewalls. Type any size into the calculator to convert it.
Questions people ask
What gear ratio do I need for 35-inch tyres?
Multiply your current axle ratio by the new tyre height and divide by the old one. From a 31.5-inch stock tyre, 3.73 gears need 4.14 to feel the same on 35s, so 4.10 is the nearest common set, and 4.10 gears need 4.56, so 4.56. Owners who tow, carry weight or run a manual gearbox often go one step shorter than that.
Is a 3.73 axle ratio OK with 35-inch tyres?
It will drive, but from a 31.5-inch stock tyre you lose about 10% of your pull and engine revs in every gear. A strong engine on flat highway copes. A loaded truck, hills and soft sand are where it feels lazy and the gearbox hunts.
Do bigger tyres affect the speedo?
Yes. The speedo counts wheel turns, and a taller tyre covers more ground per turn, so you travel faster and further than it shows. The error is the same percentage as the change in tyre height: about 6% going from 33s to 35s.
How do I find my axle ratio?
Look for a tag on the diff housing or an axle code on the door-jamb sticker, check the build sheet, or ask a dealer to look it up from the VIN. On a 4WD the front and rear ratios match.
How tall is a 285/70R17 in inches?
32.7 inches. Tyre height is the rim diameter plus two sidewalls: 17 inches plus 2 x (285 mm x 0.70), which is 831 mm. It is commonly called a 33.
Do I have to regear after fitting bigger tyres?
No. Regearing is a choice. It brings back the pull and revs you lost, at the cost of new gear sets for both axles on a 4WD. Small jumps in tyre size are often left alone. Bigger jumps, heavy trucks and towing are where it pays off.
Crawl ratio calculator
Open "Crawl ratio and crawl speed" in the calculator above and enter first gear, low range and axle ratio. It multiplies them into your crawl ratio, then shows how fast the truck creeps at a given engine speed on your current tyres, on the new ones, and after a regear. Bigger tyres raise crawl speed by the same percentage they raise tyre height, which is the other reason people regear: to get slow-speed control back, not only highway pull.
Sizes it understands
- Metric:
265/70R17,LT285/75R17,P245/75R16 - Inch (flotation):
33x12.5R17,35x12.50R17,37x13.5R17 - Just the height:
31,33,35 inch
Is "a 33" always 33 inches?
No. "33" is a nickname. A 285/70R17 works out at 32.7 inches and a 285/75R16 at 32.8, and both get called 33s. Tyres sold in inch sizes also tend to measure a little under their label once they're on the truck. Typing just the height is fine for a quick answer. For the exact one, enter the full size from the sidewall.
Want the pictures? Watch bigger tyres and your speedo and bigger tyres and your gearing.