13 Free 4x4 Calculators and Build Tools That Do the Math Before You Spend

The moment it matters

You are at the parts counter with the phone out. The 35s are $1,640 mounted. The lift is $2,200. The guy behind the counter says you will "probably want gears at some point," the kind of sentence that costs $1,800 later. You know what the truck will look like. You have no idea what it will weigh, what the speedometer will do at 70, or whether the rear bumper you already ordered just used up the payload the rooftop tent needed.

None of those questions are hard. They are all arithmetic. The problem is that the arithmetic is scattered across forum threads from 2011, a Currie page that does one calculation, and a spreadsheet you meant to build and never did. So people skip it, buy the parts, and find out on the highway.

That is what the tools on 4x4playground.com/tools are for. This guide covers the 13 calculators and planners that answer "what happens if I do this?" with a number: what you type in, what you get back, and where each one can mislead you. (The four quiz-style matchmakers on the same page answer "what should I get?" and are a separate article.) A disclosure up front: these are our tools, built here because the alternatives were ugly, single-purpose, or both. That does not make them perfect, and the section on where calculators lie applies to ours as much as anyone's.

Where 4x4 calculators lie, including ours

Two things to know before trusting any output, from any site.

First, the odometer trap. Put 35s on a truck that came with 32s and the tires are 9.4 percent taller, so the odometer counts 9.4 percent fewer miles than you drove. Every fuel log after that is wrong by the same margin. A truck that actually gets 15.5 mpg on 35s shows 14.2 at a hand-calculated fill-up. A good share of the "I lost 4 mpg" posts on the forums are partly measuring odometer error, not fuel. Run the Speedometer Calibrator first, correct the mileage, then judge the tire.

Second, the door sticker is not your truck. Every payload calculator, including the one here, starts from a manufacturer curb weight: base trim, full tank, nobody inside. Your truck with the tow package, the tonneau, and the 40 pounds that lives behind the seat is 100 to 200 pounds heavier before you touch it. A calculator is only as honest as the number you give it. A CAT scale ticket costs $13 and settles the argument.

The six that do the tire and drivetrain math

Tire Size Calculator

Type in two sizes in either format (285/70R17 or 35x12.50R17) and the Tire Size Calculator gives diameter, section width, sidewall, circumference, and revolutions per mile side by side. It flags the usual rub points (fender, control arm, frame) and reminds you that two inches of tire diameter is one inch of ground clearance, since half the tire is above the axle.

The conversion most people get wrong: 285/70R17 is not "a 33." Sidewall is 285 times 0.70, or 199.5 mm, which is 7.85 inches. Double it, add the 17 inch wheel, and you get 32.7. Stepping to a true 35 is a 7 percent jump, not the 6 percent the "33 to 35" shortcut implies, and that decides whether you land in the regear band. Gears, speedo, MPG, and clearance all hang off this number, which is why it is the first tool to open.

Gear Ratio Calculator

Pick a vehicle preset or enter stock tire diameter, new tire diameter, and current axle ratio. The Gear Ratio Calculator returns the effective ratio you are running now, the ratio that restores stock performance, the closest ring-and-pinion sets actually sold, and a regear cost range of roughly $1,200 to $2,000 for a two-axle job.

Worked example: 3.73 gears, 32.7 inch stock tire, 35 inch new tire. Effective ratio is 3.73 times 32.7 over 35, which is 3.48. The truck now behaves like it came with 3.48s, which is why it hunts for gears on grades. Ideal is 3.99, so the calculator points to 4.10 as the closest common set. The bands: under 5 percent tire growth, leave the gears alone; 5 to 10 percent, regear if you tow, run a manual, or spend real time in low range; over 10 percent, regear or accept a slower, hotter transmission. The regearing 101 guide covers what the shop does and why it is a two-axle job.

Wheel Fitment Calculator

Choose the truck, lift height (stock to 12 inches), tire size, wheel diameter and width, and the stance you want (flush, mild poke, aggressive, extreme). The Wheel Fitment Calculator returns an offset range, the matching backspacing, and a bearing-wear table for how far outboard you are pushing the load.

The number that matters is the interaction between width and offset. A 9 inch wheel at zero offset puts its outer lip where an 8 inch wheel at negative 13 mm would. People chase poke with offset alone and forget the extra half inch of width on each side does the same thing to inner clearance and scrub radius. Use this before buying wheels, then read the offset and backspacing explainer so the terms stop sounding foreign.

Bolt Pattern Lookup

Enter year, make, and model from a 40-plus vehicle database, or search by pattern the other direction. The Bolt Pattern Lookup gives PCD (6x139.7, for example), hub bore, lug seat type, torque spec, and a list of other vehicles that share the pattern.

The part that saves money is hub bore. A Tacoma and a Silverado both run 6x139.7, so the wheels bolt on. The Toyota bore is 106.1 mm and the GM is 78.1 mm, so Toyota wheels sit off-center on a GM hub without a ring and the truck shakes at 60. Same with conical versus spherical lug seats: wrong seat, wheel works loose. It is a 20 second check before a Marketplace deal on "fits 6-lug."

MPG Impact Calculator

Enter current and new tire size, current MPG (default 18), driving style, tire weight class, lift and rack toggles, fuel price, and annual miles. The MPG Impact Calculator returns the new MPG range and a yearly dollar figure. Its all-terrain penalties are half to one mpg going 31 to 33, 1.5 to 2.5 going 31 to 35, and 2.5 to 3.5 going 31 to 37, with a 1.5x multiplier on tire weight because rotating mass costs more than static, another half mpg for a lift, and 2 to 4 mpg for a rack with a rooftop tent.

The dollars are the useful part. At 18 mpg, 15,000 miles a year, and $3.50 fuel, you burn 833 gallons, or $2,917. Lose 2 mpg to 35s and it is 938 gallons, or $3,281: $364 a year, about $30 a month, the true monthly payment on the tires nobody quotes. Add the tent and rack at another 3 mpg and you are at 13 mpg, $4,038, or $1,121 a year over stock. Over three years the tent costs more in fuel than it cost to buy. The 33s vs 35s vs 37s MPG breakdown has owner-reported ranges by platform.

Speedometer Calibrator

Two sliders, stock and new diameter, 29 to 40 inches. The Speedometer Calibrator shows the error percentage, true speed at an indicated 60, 70, and 80, annual odometer drift, and three fixes: a performance tuner ($300 to $600) that corrects every system reading wheel speed, an inline calibrator ($100 to $200) that only fixes the dash, or a dealer reflash ($50 to $200) where supported.

32 to 35 is a 9.4 percent error. Indicated 70 is a real 76.6, a ticket in a 65. The bigger cost is invisible: ABS, traction control, the shift map, and cruise control all read the same wrong signal, and only the tuner fixes all of them, which is why the cheap inline box is a half-fix. More in the speedometer calibration guide.

The five that price and plan the build

Build Budget Planner

Pick the vehicle class (midsize truck, full-size, body-on-frame SUV, or crossover), set a budget ceiling, and choose budget, mid, or premium in each of nine categories: tires, suspension, wheels, recovery, bumpers and armor, lighting, racks and storage, camping, and electronics. The Build Budget Planner returns a parts-only total, the gap to your ceiling, and a three-phase order ranked by trail capability per dollar.

Its headline claim is the one to remember: a $5,000 tire and recovery kit gets you down about 80 percent of the trails a $25,000 build does. Phase 1 is tires, recovery, maybe suspension. Phase 2 is durability and comfort. Phase 3 is what photographs well. Labor is excluded and the tool says so, so add 20 to 40 percent if you are not turning the wrenches. The wider cost picture is in what it costs to build a trail-ready 4x4.

Lift Cost Estimator

Choose one of 12 platforms and a lift type: leveling, spacer or body, coil spring, long-arm, coilover, or full custom. The Lift Cost Estimator tiers the total and itemizes what people forget: alignment (mandatory), brake line extensions, driveshaft work, and a 20 percent contingency.

The ranges are wide because the category is wide. Leveling kits are $75 to $400 in parts. A real 2.5 inch coil spring lift lands at $1,500 to $3,500 installed with alignment and lines. Long-arm kits are $2,500 to $6,000 in parts alone, coilovers $2,000 to $7,000. What the tool does well is stack dependencies: if the lift exists to clear 35s, the project total includes $800 to $2,000 in tires and $1,200 to $2,500 in gears, and a "$2,000 lift" becomes a $5,000 to $8,000 decision. Read driveline vibration after a lift before picking a height; that is the line item nobody budgets.

Gear Loadout Builder

Select one of eight vehicle classes, then check items across recovery, camping and shelter, navigation and comms, and safety and first aid, 25-plus items with budget, mid, and premium pricing. The Gear Loadout Builder totals cost and weight, adds vehicle-specific notes, and produces a shareable link, with a one-click essentials button for the baseline.

The weight total is the feature people overlook. A basic kit of boards, kinetic rope, shackles, compressor, shovel, 50 quart fridge, stove, water, and tent lands between 250 and 400 pounds before food and clothes. Carry that number straight into the next tool.

Payload Calculator

Pick the vehicle and the Payload Calculator pulls GVWR and curb weight. Add passengers (default 165 pounds each, optimistic for most of us), loose gear and food, fuel at about 6.3 pounds a gallon, trailer tongue weight, and toggle the build parts: steel bumpers, winch, rooftop tent, drawers, lift, bigger tires, rack, awning, auxiliary tank, electrical. It returns payload used and remaining, an under, at, or over verdict, and a rough axle split.

Run the example truck it shows: 5,050 pound curb weight, 6,250 GVWR, 1,200 pounds of payload. Steel front bumper and winch (about 240 pounds), rear bumper with tire carrier (150), rack and rooftop tent (180 to 250), drawers (120), fridge (80), 20 gallons of water (167), and recovery gear (60) comes to 1,000 to 1,070 pounds. Add two adults at a realistic 190 each and you are at 1,400 against a 1,200 rating, over by 200 before the dog, the cooler, or the trailer. That is the standard midsize overland build, and it is over. The payload and GVWR guide explains what "over" does to brakes and bearings.

One Mod Away

Save your rig in the Garage profile (year, make, model, tires, lift, lockers) and One Mod Away checks it against the trail database, shows which trails you can run today, then simulates one upgrade at a time (bigger tires, a lift, lockers) and counts how many new trails each opens.

It reframes "which mod is cool" as "which mod gets me somewhere." For a stock 4Runner the answer is usually tires and a rear locker before a lift, and the tool shows that as a trail count instead of an opinion. Two caveats: the database is a few hundred trails, so a region with thin coverage skews the result, and difficulty ratings are a template, not ground truth. Use it to rank options, not to decide a trail is safe.

The two for the trail

Air Down Calculator

Choose terrain (sand, gravel, mud, rock, snow), tire size, vehicle weight class, street pressure (default 35), and whether you run beadlocks. The Air Down Calculator returns a target PSI range, contact patch before and after in square inches, a maximum speed while aired down, and a re-inflate reminder. Its bands are 12 to 18 PSI for soft sand, 22 to 28 for hard pack and gravel, 18 to 22 for mud, 8 to 15 for rock, and 20 to 26 for snow.

The contact patch figure is the persuasive part: 35 to 15 PSI takes the example tire from about 30 to 70 square inches, 133 percent more rubber on the ground. The safety line matters as much. Without beadlocks, most wheels risk unseating a bead below 12 to 15 PSI, especially in a turn, so the bottom of the rock band is a beadlock number, not a stock wheel number. The 25 mph ceiling is conservative and correct. The airing down guide covers why the same PSI behaves differently on a light Tacoma and a loaded 2500.

Recovery Gear Checker

Pick a weight class (midsize around 5,500 pounds, half-ton around 7,000, HD or van around 9,500) or enter your loaded weight, then terrain, whether you usually have signal, and solo or group. The Recovery Gear Checker returns a three-tier list with the load ratings shown: baseline (rated recovery points, boards, shovel, plug kit, compressor), remote additions (kinetic rope, soft shackles, satellite messenger), and solo additions (winch, snatch block, ground anchor, damper).

The sizing rules are the ones from any good recovery course, applied to your number. Kinetic rope at 2 to 3 times gross vehicle weight, sweet spot 2.5x, so a 7,000 pound rig wants a 17,500 pound rope. Winch at 1.5x GVW minimum, so 10,500, which in practice means a 12,000 pound winch because line pull drops with every wrap on the drum. Shackles and recovery points rated at or above the rope. The weight you enter is the weight it sizes for, which is why the Payload Calculator comes first: a 5,500 pound midsize truck is a 6,500 pound truck loaded, and a rope sized for the sticker is under-rated for the trip. The recovery gear guide covers brands and technique.

Which 4x4 calculators to open, in what order

Chaining them: a 35s build in ten minutes

A midsize truck on 265/70R17 (31.6 inches) with 3.73 gears, going to 285/75R17.

Tire Size: 31.6 to 33.8 inches, a 7 percent increase, one inch of clearance gained. Gear Ratio: effective ratio drops to 3.48, 4.10s restore it, $1,200 to $2,000. Speedometer: 7 percent error, indicated 70 is 74.9 real. MPG: budget 1.5 to 2 mpg, about $300 a year at 15,000 miles. Wheel Fitment: 17x8.5 at zero offset with a 2 inch lift. Bolt Pattern: 6x139.7, 106.1 hub bore, before you buy anything used. Lift Cost: 2.5 inch coil lift, $1,500 to $3,500 installed. Build Budget: tires, lift, gears, and recovery kit at roughly $6,500 to $9,500 in parts. Payload: lift and tires added 80 to 120 pounds, and the rear bumper you were eyeing puts you over with two people and water aboard. Gear Loadout: kit at 300 pounds. Recovery Gear Checker: 17,500 pound rope for the loaded weight. Air Down: 22 to 28 PSI for the forest road on Saturday.

Ten minutes, and every number that was going to surprise you at the parts counter, on the highway, or on the scale is already on the screen.

Who should use these, and who should not

If you are planning a first build, or a second one after learning the expensive way, the tools are the fastest path to a budget that survives contact with reality. If you are a shop or a builder who knows the ratios and weights by heart, the value is narrower: Payload and Recovery Gear still earn their keep because those numbers change with every load and every trip, and Bolt Pattern is faster than the catalog. If you want a tool that says a specific trail is safe for your rig this weekend, none of these do that, and neither does anything else online. That is what the drive and the recovery kit are for.

Related guides

• Regearing 101: when bigger tires demand new gears
• How much MPG do 33s, 35s, and 37s really cost
• Payload, GVWR, and overland weight guide
• How to air down tires off-road: PSI guide
• Wheel offset and backspacing fitment guide
• Speedometer calibration after bigger tires
• Overlanding recovery gear guide
• 33s vs 35s vs 37s: everything that changes