How to Determine Your Model A Ford Rear-End Ratio

Plenty of Model A Ford owners inherit a car whose rear-end ratio is unknown—or assume the standard 3.78:1 when something else is installed. That matters when you dial in A-Speed: the app's RPM calculator uses your rear-end ratio, tire diameter, and shift points to estimate engine speed on tour.

The good news is you can identify the ratio with a simple rolling test in your driveway or garage aisle. You never need to remove the differential cover or count ring-gear teeth.

The rear-end ratio describes how many times the driveshaft—and therefore the engine—turns for each revolution of the rear wheels. Change the ratio and you change how hard the engine works at any given road speed.

• Engine RPM: A taller ratio (higher number) spins the engine faster at the same MPH.
• Touring comfort: Matching your actual gearing helps you choose shift points and cruising speeds that feel right on long club runs.
• Speedometer accuracy: GPS speed in A-Speed is independent of gearing, but RPM estimates depend on it.
• Fuel economy: Honest MPG tracking is easier when RPM and speed readings reflect what the car is actually doing.
• A-Speed RPM calculations: Enter the correct ratio in Settings and the RPM calculator becomes noticeably more trustworthy at cruise—especially with overdrive configured.

Gather a few simple items before you start:

• A flat, level stretch with several feet of clear space ahead
• Chalk, tape, or a washable marker
• Wheel chocks for the front wheels (recommended)
• A helper to push while you watch the crank pulley

Prepare the car:

• Shift the transmission into high gear
• Remove all four spark plugs so the engine turns freely
• Block the front wheels if you are on even a slight grade

Place two reference marks before anyone pushes:

• Rear tire:Mark the sidewall at the bottom—the 6 o'clock position where the tire meets the ground.
• Crank pulley:Mark the rim at the top—the 12 o'clock position on the front crank pulley.

These marks are your start and finish lines. You will count how far the pulley turns while the tire makes exactly one full rotation.

Using a chalk mark, paint marker, or piece of masking tape makes it easy to identify when the tire has completed exactly one full revolution.

With your helper slowly pushing the car straight forward, watch the crank pulley mark. Count each full revolution of the pulley as the car moves.

Stop the moment the tire mark returns to its starting position at the bottom of the wheel—one complete tire revolution. Note both the number of full pulley turns and where the pulley mark ends up on its final partial rotation.

Move smoothly and steadily. A jerky push makes the final position harder to read.

For most Model A rear-end ratios, the crank pulley completes slightly more than three full revolutions during one tire turn. The stopping point of the pulley mark on that last partial turn is what identifies the ratio.

Think of the pulley face like a clock: compare where your mark lands after the tire finishes its rotation to the reference positions in the guide below. If you are between two marks, your ratio likely falls between those values.

Use this crank pulley chart to match your result. Each ratio shows roughly how many turns the pulley makes for one rear tire revolution, and where the mark typically stops.

Once you know your rear-end ratio, open A-Speed Settings and enter it under drivetrain configuration. A-Speed includes presets for common Model A ratios (3.54, 3.78, and 4.11), or you can type a custom value.

Pair the ratio with an accurate tire diameter and your shift points, then check the RPM Chart in Settings at a few cruise speeds. Your touring RPM calculator should now track much closer to what you feel on the road—especially helpful when judging overdrive engagement on highway legs.