Do You Need a Tune After 37s?

A tire-size recalibration tells the PCM (powertrain control module) and TCM (transmission control module) the real rolling diameter of your new tire, so the speedometer, odometer, shift points and cruise control reference reality again. A performance tune changes the fuel, spark, boost and shift-firmness tables to make more power. Almost every truck that goes to 37s needs the recalibration. Most do not need a power tune to go with it, and the two get sold together more often than they should.

Put 37s on a Gen 2 or Gen 3 and change nothing else, and the truck still drives fine and makes the same power. What is wrong is quieter: every system reading vehicle speed off the tone rings now works from a number that is off by a few percent.

What actually changes with a taller tire, and by how much

The starting point matters, because a Gen 2 Raptor and a Gen 3 Raptor do not leave the factory on the same tire. The Gen 2's 315/70R17 BFGoodrich KO2 measures 34.6 inches. The Gen 3's standard-package KO2 is the same size designation but Ford lists it as a 35-inch tire.

• Gen 2, stock 34.6-in KO2 to a 37-in tire: roughly a 7 percent increase in rolling diameter.
• Gen 3, stock 35-in KO2 to a 37-in tire: roughly 5.5 to 6 percent.
• Gen 3 with the factory 37 Performance Package: nothing to calculate, it left the factory calibrated for 37s.

Treat both percentages as directional. Mounted diameter varies by brand and load range, so a 37x12.50R17 from one maker can measure a half inch different from another's.

• Speedometer and odometer. The PCM calculates road speed from tire revolutions per mile using the factory-programmed size. A taller tire covers more ground per revolution, so the truck reads slower than it is actually going, and the gap grows with speed. At an indicated 70 mph on a 37, you are running faster than that, and it throws off trip-computer fuel economy and mileage-based service intervals too.
• Shift points. The 10R80 ten-speed on both generations picks shift points partly from vehicle speed thresholds mapped to stock tire circumference. A taller tire means the same engine RPM produces a faster actual road speed than the TCM thinks it sees, so it holds gears longer or shifts at the wrong point. Small per shift, annoying over a drive cycle, and it can stack on whatever 10R80 harsh-shift or converter-shudder behavior a truck already has.
• Traction control, stability control and ABS. These mostly compare wheel speed sensors against each other, and a uniform size change on all four tires cancels out most of that error. AdvanceTrac (Ford's stability control) does reference an absolute speed signal for some intervention thresholds, carrying the same offset as the speedometer, but nobody has published a number on how much that shifts real-world intervention timing. Treat it as real but secondary to the speedometer and shift-point errors, which are provable and consistent.
• Adaptive cruise. The set speed you hold is derived at least partly from the same wheel-speed signal, alongside radar and other inputs Ford does not publish the weighting for. Treat the practical effect as a possible small drift in indicated hold speed, not a documented failure mode.

None of this is a performance problem. It is a data problem, and it is exactly what a recalibration is built to correct.

Recalibration versus a full tune

These are two different products sold by two different kinds of shops, and the price gap is large enough that you should know which one you are buying.

A dealer tire-size recalibration is a PCM software update entered against your VIN and the new tire spec. A handheld programmer, the same kind sold for power tunes, can do the identical recalibration if you are comfortable flashing your own truck, and several popular units let you dial in the exact tire size without touching anything else. Call around, this is a service labor charge more than a parts charge and it swings by shop.

A 93-Octane ECU Performance Tune runs roughly $700-1,000 for the tune alone. The recommended mod paths for both Gen 2 and Gen 3 bundle the tune with a cold air intake and sometimes an intercooler, which is why a full power-stage package runs several hundred dollars higher than the tune alone. Do not compare a bundle quote to a standalone tune quote and assume you are being overcharged.

Dyno-verified, on one of our own dyno sessions with a 2019 Gen 2 3.5L EcoBoost, back-to-back stock and 93-octane tuned pulls on the same day landed in that 60-90 hp range at the wheels. That is one truck, one dyno, one day, not a controlled study across multiple trucks, and ambient temperature, fuel batch and the dyno itself all move the number. Treat it as directionally correct, not a promise for your specific truck.

Which tuners actually handle the gear ratio math

Not every tune file that claims to support 37s actually recalculates the shift schedule for the new effective gear ratio. Some apply the power maps and leave the tire-size input at stock or a rough guess, leaving you with more power and a transmission still shifting against the wrong speed data.

Ask any tuner these two questions directly:

1. Does the calibration take your exact tire size (37x12.50 versus a generic "37 inch" default) as an input, and does it correct both PCM speed output and TCM shift scheduling from that number.
2. Is the tire-size correction built into the same file as the power tune, or loaded as a separate flash first.

A shop or handheld file that treats tire size as its own input field, and confirms the corrected number back to you, is doing this right. If a shop cannot answer clearly, that is the wrong shop for this job regardless of how good the power tune itself might be.

If you went to 37s on a BFGoodrich KO2 set or the KO3 in the same size and you are not chasing power, skip the tune and get the recalibration done, dealer or handheld set to correction-only mode. Wheel choice does not change this math: a Method 305 NV wheel with correct backspacing and a 37 mounted on it needs the same recalibration a stock wheel would.

Warranty consequences and how Ford detects a tune

A tire-size recalibration by itself is not what voids a claim. It corrects a Ford value to match an approved tire-size change, exactly the kind of correction a dealer will perform for you.

A power tune is different. Any aftermarket calibration writes to the PCM, and Ford's modules can log flash events in a way a dealer scan tool may pick up during a service visit, not only during a powertrain complaint. That history does not necessarily disappear when you flash back to stock. Ford's warranty terms allow a dealer to deny a claim when an aftermarket modification is shown to have caused or contributed to the failure, but whether a specific tune gets flagged, and how a specific dealer handles it, is inconsistent enough that nobody can promise you an outcome. Call your dealer with your VIN before you assume either way.

None of this means never tune. It means know what you are trading. If the truck is out of its powertrain warranty, or you are prepared to self-insure the drivetrain, a tune is a reasonable call. If you are still well inside factory coverage on a Gen 2 with the early cam phaser design, weigh that against the full ranked list of Gen 2 cam phaser fixes before adding a second variable to an engine with a known issue. Still deciding whether 37s are worth it on a Gen 2 at all? Start with the full cost breakdown for that swap.

The short version

• 37s always need a tire-size recalibration, and the error depends on your starting tire: roughly 7 percent on a Gen 2's stock 34.6-inch KO2, roughly 5.5 to 6 percent on a Gen 3's stock 35-inch KO2, none at all on a truck with the factory 37 Performance Package.
• A performance tune is optional and separate. It adds power, it does not by itself fix the tire-size math unless the file specifically corrects for your exact tire size.
• Ask any tuner directly whether the file inputs your exact tire size and corrects both PCM speed output and TCM shift scheduling, not just the power tables.
• A recalibration alone carries little warranty risk. A power tune does, Ford can potentially see the flash history, and it is your call to make with your eyes open, not ours to make for you.