Axle Ratio Calculator for Tire Size

How to use the axle ratio calculator

1. Type your stock tire size. It's on the door sticker. Metric (265/70R17), flotation (33x12.50R15) and a plain height in inches (31) all work.
2. Type the tire you're going to. The diameter under each box updates as you type, so you can sanity-check it against the tire maker's spec.
3. Set your current axle ratio. Tap one of the common ratios or type your own. If you don't know it, the axle tag, the RPO code on a GM door sticker, or the original window sticker will tell you.
4. Set your cruise speed and top-gear ratio. Top gear is your transmission's last gear. Leave it at 1.00 for direct drive, or tap the overdrive figure closest to yours.
5. Read the result. The ladder shows cruise RPM for every common ratio on the new tires, with the one closest to your old RPM flagged. Copy it, share the link, or download the PDF to take to the gear shop.

Why this calculator is different

Most gear ratio calculators online were built by parts sellers to answer one question at a time. You type a tire height in inches, hit a button, and get a single number back. Here's what this one does differently:

• It reads the sidewall code. You don't have to work out that a 285/75R17 is 33.83 inches tall first. Type the size the way it's printed.
• Stock and new side by side. You see the RPM you have now, the RPM you'll have on the new tires, and the RPM after a regear, all at once.
• It snaps to gears you can buy. The exact math might say 3.80. Nobody sells 3.80s. The ladder shows the real ratios and which one lands closest.
• Transmission included. A lot of calculators assume 1:1 in top gear. On a modern truck with a 0.70 or lower overdrive, that overstates cruise RPM by more than 40%.
• Shareable and embeddable. The link carries your numbers, the PDF prints clean, and forums and clubs can embed it free.

How the math works

Everything here comes from one relationship between road speed, gearing and tire size, the same one gear shops have used for decades:

RPM = mph × axle ratio × top-gear ratio × 336.1 ÷ tire diameter (in)

The 336.1 isn't magic. It's 63,360 inches in a mile divided by 60 minutes and by π, which turns tire diameter and mph into wheel revolutions per minute. Shop math rounds it to 336; we use the full figure.

A taller tire covers more ground per turn, so it acts like a numerically lower gear. Two more lines follow from that:

• effective ratio = axle ratio × stock diameter ÷ new diameter
• ratio to restore stock RPM = axle ratio × new diameter ÷ stock diameter

Tire diameter for a metric size is the rim plus two sidewalls: rim + 2 × width × aspect ÷ 100 ÷ 25.4. A 265/70R17 works out to 31.61 inches.

The verdict bands are my shop rule of thumb, not physics. Under 3% is the same window plus-sizing stays inside to keep ABS and the speedometer close, so leave the gears alone. From 3% to 7% you'll feel it, and it's worth regearing if you tow. From 7% to 12% a regear is the right call for most trucks, and past 12% the truck will be lugging in top gear without one.

Limitations. The math uses unloaded tire diameter. A loaded tire rolls a little shorter, so real cruise RPM runs a bit higher than shown. It also assumes the torque converter is locked at cruise, which is true for most modern automatics. An unlocked converter adds a few percent.

Three worked examples

Half-ton going from 265/70R17 to 285/75R17

The stock 265s are 31.61 inches. The 285/75R17s are 33.83 inches, which is 7.0% taller. On 3.55 gears that drives like a 3.32. At 65 mph in a 0.67 top gear, cruise drops from 1,644 RPM to 1,536, and the transmission will downshift more often on grades. The exact fix is 3.80. The closest real ratio is 3.73, which puts you at 1,614 RPM. If you tow, this is the size jump where I'd regear. If you don't, you can live with it. Either way, buy good rubber; our tire brand rankings cover who makes LT sizes worth the money.

3.21 gears to 37-inch tires

Stock 255/75R17s are 32.06 inches. Going to 37x12.50R17 is 15.4% taller, and the 3.21s now drive like 2.78s. At 65 mph cruise falls from 1,466 RPM to 1,270, which is low enough that most gas engines will lug in top gear. The math wants 3.70; 3.73 is the closest you can buy, landing at 1,476 RPM. Plenty of builders go a step further to 4.10 for crawling and towing, and the ladder shows what that costs at highway speed. When the axle comes apart, a torque wrench is not optional. Pinion nut, cover bolts and lug nuts all have a spec.

Going down a size: 33x12.50R15 to 31x10.50R15

Smaller tires work the other way. Dropping 6.1% in height makes 4.10 gears drive like 4.36s. At 65 mph in a 0.70 overdrive, RPM climbs from 1,900 to 2,023, and your speedometer now reads high: an indicated 65 mph is really 61.1. The exact match is 3.85 and the nearest ratio is 3.92 at 1,934 RPM, but on a change this size most people leave the gears alone and recalibrate the speedometer. A smaller winter set is the usual reason for this one; see our winter tire picks.

Frequently asked questions

What axle ratio do I need for 35-inch tires?

It depends on what you're coming from. Multiply your current ratio by 35 and divide by your stock tire's diameter. Coming off 31.5-inch tires on 3.73s, that's 4.14, so 4.10s. Coming off 32-inch tires on 3.21s, it's 3.51, so 3.55s get you back to stock and 4.10 or 4.56 buys extra pull. Put your numbers in the calculator and read the ladder.

Do bigger tires change my gear ratio?

Not the physical gears, but the effect is the same as a numerically lower ratio. A tire 10% taller makes your truck feel like it has 10% less gear: slower off the line, more downshifts on hills, and lower cruise RPM.

How do I find my axle ratio?

Look for a metal tag on the differential cover bolts, a stamped number on the axle tube, or the axle code on the door sticker or window sticker. On GM trucks the three-character RPO code on the glovebox sticker maps to it (GU6 is 3.42, GT4 is 3.73). If all else fails, put the transmission in neutral (and the transfer case, on a 4x4), raise one rear wheel, and count driveshaft turns for one full turn of the tire. On an open differential, double that number. With a limited slip or locker, raise both wheels and don't double it.

Is it bad to run bigger tires without regearing?

Not dangerous on its own, but it costs you. The transmission works harder, shifts more, and runs hotter when towing. Fuel economy often gets worse, not better, because the engine leaves its efficient range. Brakes also have more rotating mass to stop.

Will my speedometer be wrong after a tire change?

Yes, by the same percentage the tire height changed. Taller tires make the speedometer read low. The calculator shows your true speed at your cruise setting. Most late-model trucks can be recalibrated with a handheld programmer or at the dealer.

Related tools

• Tire Height Calculator: compare any two sizes for diameter, speedometer error and lift needed.
• Tire Diameter Calculator: overall height and revs per mile from any sidewall code.
• Tire Pressure Calculator: the PSI your new tires need to carry the load, since LT sizes rarely match the door sticker.
• Towing Capacity Calculator: check what you can pull before and after the build.
• All free tools by Mike Reeves

Recommended for a tire-size change

• Best Tire Brands: who builds the sizes worth regearing for.
• Best All-Season Tires: for a size or two up, where the factory gears still work.
• Best Tire Pressure Gauges: a new size needs a new pressure, and you need a gauge you can trust to set it.

Sources and methodology

• Quadratec Knowledge Base: Tire & Gear Ratio Calculations, the RPM, ratio and diameter relationships.
• Rare Car Network: Calculating mph/rpm/Gear Ratio/Tire Size, a road test showing why the transmission ratio must be in the formula.
• AJ Designer: Gear Ratio Calculator, the same equation in tire-radius form (the 168 constant), used to cross-check every result.
• Metric tire diameter: rim + 2 × section width × aspect ratio, the same method behind our tire diameter chart.

Every result is checked on each build against six worked scenarios and the published 336 shop formula, including every figure quoted on this page. About Mike Reeves · Last reviewed October 29, 2026.

Embed this calculator on your site

Free for off-road clubs, Jeep and truck forums, gear shops, and auto shop classes. The attribution link is part of the snippet. No fee, no signup, nothing tracking your readers.

<!-- Axle Ratio Calculator by RevRated. Please keep the attribution link. -->
<iframe
  src="https://revrated.com/axle-ratio-calculator/embed/"
  width="100%"
  height="1180"
  loading="lazy"
  style="border:1px solid #d8dbe5; border-radius:16px; max-width:880px;"
  title="Axle Ratio Calculator (RevRated)">
</iframe>
<p style="font-size:12px;color:#666;margin-top:8px">
  Axle ratio calculator by
  <a href="https://revrated.com/axle-ratio-calculator/">RevRated</a>
  &middot; Reviewed by Mike Reeves, ASE Master Technician
</p>