The new tires go on in the morning. On the first on-ramp the speedometer settles on 70, while a phone GPS beside it reads 74.3 mph, and the tachometer sits lower than it used to. Nothing broke. The truck now covers more road per turn of the wheel, and every number tied to the gearing moved with it. That is the whole question behind "what gear ratio for 35 inch tires": which axle ratio puts those numbers back where they were, and whether you want them back at all.
This walk-through follows one setup through the day of the swap: a 10-speed automatic with the 10R80 ratios (4.696 in 1st down to 0.636 in 10th), a 3.55 axle and factory 275/60R20 tires. Every figure below comes from the Transmission Gear Ratio Calculator and the RPM to MPH Calculator, so you can repeat it with your own truck's numbers.
Morning, before the swap: 33 inches, 3.55 and 1,610 rpm
A 275/60R20 tire does not say "33 inches" anywhere, but it is: the sidewall is 60% of 275 mm, so 165 mm top and bottom, plus the 20-inch rim, gives 32.99 in. That tire turns 611 times per mile.
On that baseline the engine turns 1,610 rpm at 70 mph in 10th and 1,744 rpm in 9th. First gear runs out at 35.3 mph at a 6,000 rpm redline. Write these three numbers down before the shop touches anything. They are what "it drives like stock" means in practice.
Install day: what 35s change before anyone opens the axle
A 35-inch tire is 6.1% taller than 32.99 in. That one percentage moves everything else by the same amount, and in the same direction for the driver: taller.
The tire now turns 576 times per mile instead of 611. To the engine and the transmission it is as if the 3.55 axle had been swapped for a 3.35: 3.55 x 32.99 / 35. First gear, which ended at 35.3 mph, now stretches to 37.5 mph. And at the same engine torque the force at the tire contact patch drops to 94% of what it was, because the same twist is now applied through a longer lever. The loss matters most when the engine needs all the force it can give: a start on a hill, a trailer, or both.
The speedometer is the part people notice first. It still counts 611 revolutions as a mile, so it now reads low. The gap grows with speed:
| Speedometer shows | Truck really does on 35s | Difference |
|---|---|---|
| 30 mph | 31.8 mph | +1.8 mph |
| 45 mph | 47.7 mph | +2.7 mph |
| 55 mph | 58.3 mph | +3.3 mph |
| 65 mph | 69.0 mph | +4.0 mph |
| 70 mph | 74.3 mph | +4.3 mph |
| 80 mph | 84.9 mph | +4.9 mph |
At an indicated 70 the truck is doing 74.3 mph. That gap stays until the speedometer is recalibrated for the new revolutions per mile, which is a separate job from any change to the gears. Also worth checking: the size on the sidewall is nominal, and a mounted, loaded tire can measure less, so measure the real height before you lean on any of these figures.
First highway run: the rpm drop, speed by speed
On the highway the change runs the other way. At a true 70 mph in 10th the engine now turns 1,518 rpm instead of 1,610, and the gap widens as speed rises. The RPM to MPH Calculator gives this for any single gear: choose "Road speed", type the speed, the gear ratio, the axle and the tire.
| True speed in 10th gear | 33-inch, 3.55 | 35-inch, 3.55 | Change |
|---|---|---|---|
| 55 mph | 1,265 rpm | 1,193 rpm | -72 |
| 65 mph | 1,495 rpm | 1,409 rpm | -86 |
| 70 mph | 1,610 rpm | 1,518 rpm | -92 |
| 75 mph | 1,725 rpm | 1,626 rpm | -99 |
So about 92 rpm less at 70. Whether that is good news depends on what the engine likes. In 10th each mph now costs 22 rpm instead of 23. To get back to the old 1,610 rpm at 70, the axle has to make up the full 6.1%.
What gear ratio for 35 inch tires? Scale the old axle by the tire
The arithmetic fits on one line: new axle = old axle x new diameter / old diameter. Here that is 3.55 x 35 / 32.99 = 3.77. Axle ratios come from a fixed list rather than any number you like, so in practice you pick a neighbor on that list: 3.73, almost exactly back to stock, or 4.10, deliberately past it.
The Transmission Gear Ratio Calculator runs all ten gears at once for each option. Here are the rows that matter for the decision:
| Setup | Rpm at 70 in 10th | Rpm at 70 in 9th | 1st gear at 6,000 rpm | Pull vs stock |
|---|---|---|---|---|
| Stock: 33.0 in, 3.55 | 1,610 | 1,744 | 35.3 mph | 100% |
| 35 in, 3.55 (no regear) | 1,518 | 1,644 | 37.5 mph | 94% |
| 35 in, 3.73 | 1,595 | 1,728 | 35.7 mph | 99% |
| 35 in, 4.10 | 1,753 | 1,899 | 32.4 mph | 109% |
"Pull vs stock" is the force at the tire for the same engine torque, compared with the factory setup. It is the gear ratio times the axle divided by the tire radius, nothing more, so it applies in every gear alike.
The 3.73 row is the "drives like stock" answer: 1,595 rpm at 70, 15 below the factory 1,610, first gear to 35.7 mph, and 99% of the original pull. The 4.10 row trades highway rpm for pull: 1,753 rpm at 70, 143 more than stock, but 109% of the original force at the tire, which is the number that matters for a trailer or a steep trailhead.
End of the day: three ways the truck can leave the shop
By evening the choice comes down to three rows, and none of them is wrong. Each one simply moves the numbers in a known direction.
Keep the 3.55. Cheapest by far, since nothing inside the axle changes. The truck cruises 92 rpm lower at 70, starts with 94% of its old pull and needs the speedometer recalibrated either way. A reasonable call for a truck that rarely tows and mostly sees highway miles.
Regear to 3.73. The engine, the shift points and the pull come back to within about 1% of stock. This is the answer to "do I need to regear for 35s" for anyone who liked how the truck drove before and wants it to feel the same, only taller.
Regear to 4.10. More pull than the truck ever had (109%), a shorter first gear (32.4 mph to redline) and 1,753 rpm at 70. The option for a truck that tows, carries weight or crawls, where the extra force earns its keep.
Running the same check on your own truck
The example above uses one transmission and one factory tire, but the method does not depend on either. It takes four inputs and about five minutes, and the order matters less than getting each number from the truck itself rather than from memory.
- Measure the height of a mounted tire on the truck, ground to the top of the tread, instead of trusting the number on the sidewall. That height is the diameter the calculators need.
- Find the forward gear ratios of your transmission in the owner's manual or the manufacturer's spec sheet, all of them, down to the top overdrive.
- Confirm the axle ratio. If the paperwork is gone, the FAQ of the Transmission Gear Ratio Calculator explains how to count it with the truck safely raised.
- Enter the stock tire and axle first and note rpm at 70 mph, first gear at redline and the 9th or 5th gear figure you actually cruise in. Then change only the tire, then only the axle, and compare each run with the first one.
Changing one input at a time is the whole trick. It shows how much of any difference comes from the tire and how much from the gears, which is exactly the split a regear decision rests on.
The same day with 37-inch tires
Go two inches further and every effect grows. A 37-inch tire is 12.1% taller than the stock 32.99 in, turns 545 times per mile, and makes the 3.55 axle behave like a 3.17. At an indicated 70 the truck is really doing 78.5 mph. The scaling rule gives 3.55 x 37 / 32.99 = 3.98, so here 4.10 is the "back to stock" ratio and 4.56 is the one that goes past it.
| Setup | Rpm at 70 in 10th | 1st gear at 6,000 rpm | 10th at 2,000 rpm | Pull vs stock |
|---|---|---|---|---|
| 37 in, 3.55 (no regear) | 1,436 | 39.6 mph | 97.5 mph | 89% |
| 37 in, 4.10 | 1,658 | 34.3 mph | 84.4 mph | 103% |
| 37 in, 4.56 | 1,844 | 30.8 mph | 75.9 mph | 115% |
On 37s the 4.10 gives 103% of the factory pull at 1,658 rpm at 70, close to the stock feel. Keeping the 3.55 leaves the truck at 89% of its original pull, an 11% shortfall in force on every climb. The 4.56 is the trailer and trail option at 1,844 rpm on the highway.
Where the gearing tables stop
These are all gearing numbers: they say how fast the engine turns at a given speed and how much force reaches the ground. They do not say how fast the truck can actually go, because that depends on power against air drag, and a lifted truck on big tires has more frontal area than it had stock. The Top Speed Calculator works that out from power, weight, drag coefficient and frontal area; a 200 hp automatic sedan with Cd 0.28 and 23 sq ft of frontal area tops out at 155.1 mph, for comparison.
Every figure here uses the nominal tire diameter and assumes the torque converter is locked. A loaded tire rolls slightly smaller than its size, and an unlocked converter lets the engine spin faster than the tables show. For your own truck, measure the mounted tire and use your exact gear ratios.
Where each number in this article came from
- Transmission Gear Ratio Calculator - all your gears at once: speed at redline, rpm after each upshift and cruising rpm at 55 to 80 mph, with metric or flotation tire sizes.
- RPM to MPH Calculator - one gear, both directions: road speed from rpm or the rpm at any speed.
- Top Speed Calculator - the speed where power meets air drag and rolling resistance.
Other tire and drivetrain tools
- Tire Size Comparison Calculator - diameter, circumference and speedometer change between two sizes.
- Wheel Circumference Calculator - circumference, diameter and revolutions per kilometer from a tire size.
- Fuel Consumption Calculator - real mpg and L/100 km from your own fill-ups, before and after the change.
- Acceleration Calculator - an estimated 0-100 km/h time from power, weight and drivetrain.
- Speed Converter - miles per hour, kilometers per hour and meters per second.
- Tread Depth Calculator - how much tread is left, which also shrinks the rolling diameter.