Axle Ratio and Tow Rating

US & Canada

Two trucks with the same engine can carry different tow ratings, and the axle ratio is usually why. Here is what the ratio does, what the makers' tables show, and how to find yours.

Engine rpm from your gearing

1. Speed and gearing

mph
: 1

From the axle code on the door label (Ford), the RPO code (GM), the window sticker or your dealer. It is not in the VIN.

: 1

The ratio of the gear you are in. 1.00 is direct drive; overdrive gears are below 1. Your transmission's ratios are in the owner's manual or the maker's spec sheet.

: 1

Set 0 to skip the comparison.

2. Tire size

mm
%
in
in

For a flotation size such as 35x12.50R17, enter the 35 here. Leave 0 to use the size code above.

A size such as 275/65R18 reads width 275 mm, sidewall 65% of the width, 18 in wheel.

Result

0

How the number is built

This is the nominal tire size. A loaded tire rolls slightly smaller, so a tachometer usually reads a little higher than this.

Does axle ratio affect towing capacity?

Yes. The axle ratio is a torque multiplier: a 3.73 axle turns the driveshaft 3.73 times for every turn of the wheels, so the wheels get 3.73 times the driveshaft's torque. A higher number gives more pulling force at the wheels and a faster-spinning engine at any road speed. In Chevrolet's words:

"In general, a higher axle ratio offers higher trailer weight ratings, quicker acceleration and less fuel efficiency."

Chevrolet, 2026 Trailering Guide, Trailering basics: Axle Ratio, p. 7

The tow rating itself is not arithmetic from the ratio. The maker tests each engine, axle and body combination, for Ford and Ram pickups under SAE J2807, Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating, and prints a GCWR and a maximum trailer weight for it. That is why the only honest axle ratio towing chart is the maker's own table for your truck.

Axle ratio towing chart: what the makers' tables show

Each pair below is one configuration from a current towing guide, with only the axle ratio changing. Figures are as printed.

Same truck, different axle ratio, from the 2025 and 2026 towing guides
Truck Axle ratio GCWR Max trailer
2026 Ford F-150, 2.7L EcoBoost V6
Regular Cab 4x2, 141.5 in wheelbase
Ford, 2026 RV & Trailer Towing Guide, F-150 Pickup conventional towing, PDF p. 16 		3.15 or 3.55 12,300 lb 7,400 lb
3.73 13,300 lb 8,400 lb
2026 Ford F-250 SRW, 6.8L V8 gas
Regular Cab 4x2, 8 ft box
Ford, 2026 RV & Trailer Towing Guide, F-250 SRW Super Duty Pickup conventional towing, PDF p. 20 		3.73 21,000 lb 14,800 lb
4.30 23,500 lb 17,300 lb
2025 Ram 1500, 3.0L Hurricane (SO)
Quad Cab 4x2, 6 ft 4 in bed
Ram, 2025 Ram 1500 Payload and Towing Weight Capacities, Maximum Trailer Weight Capacities (SAE J2807) 		3.21 13,900 lb 8,470 lb
3.92 17,000 lb 11,550 lb
2025 Ram 1500, 3.6L Pentastar with eTorque
Quad Cab 4x4, 6 ft 4 in bed
Ram, 2025 Ram 1500 Payload and Towing Weight Capacities, Maximum Trailer Weight Capacities (SAE J2807) 		3.21 11,900 lb 6,480 lb
3.55 12,900 lb 7,480 lb
2026 Chevrolet Express Cutaway, 6.6L V8 gas
GCWR only, as printed
Chevrolet, 2026 Trailering Guide, Gross Combination Weight Ratings: Silverado Box Delete, 3500 HD Chassis Cab and Express Cutaway, p. 45 		3.73 16,000 lb Not printed
4.10 20,000 lb Not printed
  • 2026 Ford F-150, 2.7L EcoBoost V6: 3.73 instead of 3.15 or 3.55 adds 1,000 lb (454 kg) of GCWR and 1,000 lb (454 kg) of trailer rating.
  • 2026 Ford F-250 SRW, 6.8L V8 gas: 4.30 instead of 3.73 adds 2,500 lb (1,134 kg) of GCWR and 2,500 lb (1,134 kg) of trailer rating.
  • 2025 Ram 1500, 3.0L Hurricane (SO): 3.92 instead of 3.21 adds 3,100 lb (1,406 kg) of GCWR and 3,080 lb (1,397 kg) of trailer rating.
  • 2025 Ram 1500, 3.6L Pentastar with eTorque: 3.55 instead of 3.21 adds 1,000 lb (454 kg) of GCWR and 1,000 lb (454 kg) of trailer rating.
  • 2026 Chevrolet Express Cutaway, 6.6L V8 gas: 4.10 instead of 3.73 adds 4,000 lb (1,814 kg) of GCWR.

Two cautions from the same guides. First, a higher ratio does not always change the rating: on the 2026 F-150's 3.5L EcoBoost, the 3.31 and 3.55 rows list the same GCWRs, and the higher figures appear only on the 3.55 row with the Tow/Haul Package and the optional Max Tow Axle. Second, the same ratio is rated differently on different engines, so "3.73 axle ratio towing capacity" has no single answer; Ram's 3.55 carries 12,900 lb (5,851 kg) of GCWR behind the 3.6L and 13,900 lb (6,305 kg) behind the Hurricane.

3.55 vs 3.73, and 3.21 vs 3.92: what the difference really is

The change in torque at the wheels, and in engine rpm at a given speed, is the ratio of the two ratios. Going from 3.55 to 3.73 is +5.1%; Ram's jump from 3.21 to 3.92 is +22.1%, which is why that pair moves the rating by 3,100 lb (1,406 kg) of GCWR while the 3.55 to 3.73 step on the F-150 2.7L moves it by 1,000 lb (454 kg).

Engine rpm by axle ratio at highway speed

Arithmetic, not a test: 65 mph (105 km/h) on a 275/65R18 tire (32.1 in, 815 mm nominal) in a 1.00 gear. Multiply by your gear's ratio for any other gear: a 0.75 overdrive is three quarters of these figures.

Engine rpm at 65 mph in a 1.00 gear on a 32.1 in tire
Axle ratio Engine rpm Against 3.55
3.15 2,146 -11.3%
3.21 2,187 -9.6%
3.23 2,200 -9%
3.31 2,255 -6.8%
3.42 2,330 -3.7%
3.55 2,418 Same
3.73 2,541 +5.1%
3.92 2,670 +10.4%
4.10 2,793 +15.5%
4.30 2,929 +21.1%

The formula: engine rpm = speed in mph × 63,360 ÷ (π × tire diameter in inches) ÷ 60 × axle ratio × gear ratio. The calculator above runs exactly this, and a loaded tire rolls a little smaller than its nominal size, so the real tachometer reads slightly higher.

How to find your axle ratio

It is not in the VIN: 49 CFR 565.15(b), Table I does not require a truck VIN to encode it. Each maker labels it differently.

Ford: the AXLE code on the door label

Ford's guide says to check the Safety Compliance Certification Label on the left front door lock facing or door latch post pillar: "Below the bar code, you will see the word AXLE and a two-digit code." Look the code up below.

Ford F-150 rear axle ratio codes, 2026
Ratio Open (non-limited slip) Electronic locking
3.15 15 Not available
3.31 27 L3
3.55 19 L9
3.73 Not available L6
4.10 Not available L4
Ford Super Duty pickup rear axle ratio codes, 2026
Ratio Open Limited slip Electronic locking
3.31 31 Not available 3H
3.55 35 3K 3J
3.73 37 3L 3E
4.10 Not available 4N Not available
4.30 Not available 4L 4M

GM: the RPO codes

GM identifies the axle with a three-character Regular Production Option code. GM's own documentation says "The vehicle's RPO list is printed on the Service Parts Identification Label." These are the axle codes in its 2026 Silverado 1500 list:

  • GU5: Rear axle, 3.23 ratio
  • GU6: Rear axle, 3.42 ratio
  • GT4: Rear axle, 3.73 ratio
  • G80: Limited slip (Positraction) rear axle, not a ratio

Other GM axle codes exist for other ratios and older years; this page lists only the ones read in that manual.

Ram and everyone else

The original window sticker or build sheet often shows the axle ratio, and a dealer can usually look it up from the build record for your VIN. Ram's towing chart is laid out by engine and axle ratio, so you need the ratio before the chart is any use.

Best axle ratio for towing

The best ratio is the one your engine's row rates highest, if you tow near the limit, and a lower one if you mostly drive empty. For the trucks asked about most:

  • F-150: on the 2.7L EcoBoost, 3.73 is rated 1,000 lb (454 kg) more than 3.15 or 3.55. On the 3.5L EcoBoost, the top ratings sit on the 3.55 row with the Max Tow Axle. Ford, 2026 RV & Trailer Towing Guide, F-150 Pickup conventional towing, PDF p. 16.
  • F-250: on the 6.8L gas V8, 4.30 is rated 2,500 lb (1,134 kg) more than 3.73, and the 7.3L gains 2,500 lb (1,134 kg) of GCWR on 4.30. Ford, 2026 RV & Trailer Towing Guide, F-250 SRW Super Duty Pickup conventional towing, PDF p. 20. The F-350 is in the same guide, on its own page.
  • Ram 1500: the Hurricane gains the most from 3.92. Ram, 2025 Ram 1500 Payload and Towing Weight Capacities, Maximum Trailer Weight Capacities (SAE J2807).
  • Ram 2500 and other heavy duty trucks: the maker's heavy duty guide for your model year, read by engine and ratio. This page does not reprint figures it has not checked.
  • Fifth wheels: the same logic, but the fifth-wheel columns of the guide are separate from the conventional ones, and pin weight counts against payload, so check that as well.

Bigger tires and axle ratio

A taller tire covers more ground per turn, which acts like a lower axle ratio. Moving from the stock 32.1 in tire to a 35 in tire, a 3.73 axle behaves like a 3.42 and the engine turns 2,328 rpm at 65 mph instead of 2,541. To get the stock rpm back takes 3.73 × 35 ÷ 32.07 = 4.07, so 4.10 is the closest of the ratios in the tables above. That restores the engine speed; it does not change the tow rating printed for the truck as built.

The maker's rating is for the truck as it was built. If you have changed tire size, treat the printed figure as belonging to the original setup and the calculator's rpm as the part you can check.

Sources

Ratings checked against each guide on 2026-10-24. Guides are revised during the model year; for a purchase or a heavy tow, read the current edition for your exact configuration.

Common questions

What does axle ratio mean on a truck?
How many times the driveshaft turns for each turn of the wheels. A 3.73 ratio turns the driveshaft 3.73 times per wheel turn, so the engine spins faster and the wheels get more torque than on a 3.55. The ratio is written as a number with ": 1" after it, and a "higher" ratio means a bigger number.
Does axle ratio affect towing capacity?
Yes, and the makers say so. Chevrolet, 2026 Trailering Guide, Trailering basics: Axle Ratio, p. 7: "In general, a higher axle ratio offers higher trailer weight ratings, quicker acceleration and less fuel efficiency." On the 2026 F-150 with the 2.7L EcoBoost, the guide lists 7,400 lb (3,357 kg) on a 3.15 or 3.55 axle and 8,400 lb (3,810 kg) on a 3.73, for the same regular cab 4x2.
Is 3.55 or 3.73 better for towing?
3.73, when the maker's table lists both for your engine, because it multiplies torque 5.1% more. It also turns the engine about 5% faster at every speed, which costs fuel. On some engines the two carry the same rating: Ford's 3.5L EcoBoost F-150 lists identical GCWRs on 3.31 and 3.55 until the Max Tow Axle is added. Read your engine's row, not the ratio alone.
What is a good axle ratio for towing?
The one your engine's row in the maker's towing guide rates highest. There is no single number: the 2026 F-250 6.8L gas engine gains 2,500 lb (1,134 kg) of trailer rating going from 3.73 to 4.30, while a diesel Super Duty is offered on 3.31 and 3.55. Higher ratios tow more and burn more fuel.
Where do I find my axle ratio?
On a Ford, the door-jamb Safety Compliance Certification Label: "Below the bar code, you will see the word AXLE and a two-digit code." On a GM truck, look for the axle RPO code (GU5, GU6, GT4 and so on); GM says "The vehicle's RPO list is printed on the Service Parts Identification Label." Otherwise the original window sticker or a dealer working from your VIN can tell you. The VIN itself does not encode it.
Is the axle ratio in the VIN?
No. 49 CFR 565.15(b), Table I lists what a truck VIN must make decipherable, and axle ratio is not on that list, any more than towing capacity or GCWR. A dealer can look the build up from the VIN, but the characters themselves do not carry the ratio.
What does G80 mean on a GM truck?
A limited-slip (Positraction) rear axle. It is not a ratio. GM's 2026 Silverado 1500 RPO list gives G80 as "AXLE POSITRACTION - LIMITED SLIP", and the ratio is a separate code such as GU6 (3.42) or GT4 (3.73).
What gear ratio do I need with 35 inch tires?
Taller tires lower the effective ratio. A 3.73 axle on a stock 32.1 in tire behaves like a 3.42 on a 35 in tire, and getting the engine back to stock rpm needs about 4.07, so 4.10 is the nearest ratio in the tables above. The calculator does this for any size. Regearing restores rpm; it does not raise the ratings on your labels.
Can I change my axle ratio to tow more?
You can change the gears, but not the rating. The tow rating and GCWR belong to the configuration the maker tested and printed on your labels, under SAE J2807, Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating for current Ford and Ram pickups. A regear can make an existing rating easier to live with, especially after bigger tires, but it does not let you exceed it.