Mini 4WD Setting Gear Set: AR (15456) vs FM-A (15516)
Item 15456 and item 15516 solve the same problem in different boxes: both are Tamiya "Setting Gear Set" bundles that give you a choice of five final-drive ratios - 3.5:1, 3.7:1, 4:1, 4.2:1 and 5:1 - instead of forcing you to buy five separate single-ratio packs. The catch is that they are not interchangeable. 15456 is named for the AR chassis but Tamiya also lists it for Super-II, VS and VZ; 15516 is named for FM-A and also covers Super X and Super XX. Picking the wrong one wastes money on a set that will not sit in your gearbox, so this is a buyer's guide first and a tuning guide second.
What both sets actually give you
The core idea is identical: instead of committing to one ratio at the counter, you get a small parts tray covering the full official range for single-shaft chassis - 3.5:1 for outright top speed, up through 3.7:1 and 4:1, to 4.2:1 and 5:1 for acceleration and hill-climb out of technical sections. That range only applies to single-shaft/prop-shaft chassis (AR, FM-A, VZ, Super-II, Super X/XX, VS and others); the twin-shaft "PRO" family (MS, MA, ME) tops out at three ratios sold as separate single-ratio packs, not a bundled set like these two.
Lower numeric ratios spin the wheel faster per motor turn (more top speed, less torque); higher numeric ratios multiply torque more per wheel turn (more acceleration and climbing force, lower top speed). That relationship is simple arithmetic and holds regardless of which set you buy - the real decision is which chassis you own and, if you could run either, what each box actually contains.
15456: Setting Gear Set for AR Chassis
The Setting Gear Set (for AR Chassis), item 15456, is built specifically for the AR chassis's gearbox and prop-shaft layout. It bundles a 1.4mm hollow propeller shaft and a 620 ball bearing alongside the five-ratio gear selection, so a single box covers both the ratio choice and three of the small parts you would otherwise buy separately — a 1.4mm hollow lightweight propeller shaft, a gear shaft and a 620 ball bearing to run a lightweight, low-friction drivetrain. Typical retail sits around US$12.50-15 depending on the shop, though Tamiya does not publish one fixed global price.
15516: Setting Gear Set for FM-A Chassis
The Setting Gear Set (for FM-A Chassis), item 15516, covers a wider chassis family out of the box: FM-A, Super X and Super XX. Contents run slightly deeper than 15456's — as well as the gears, Tamiya lists a fluorine-coated gear shaft, a carbon-reinforced 8T pinion, a propeller shaft and bearings. Tamiya does not publish the exact shaft or bearing sizes for this set the way it does for 15456. Tamiya USA lists it at roughly US$9.80, cheaper than 15456 at most retailers despite covering more chassis and including the extra pinion and crown gear pair.
Choosing between them
For most builders the decision is already made by the chassis in front of you: AR, Super-II, VS and VZ take 15456; FM-A, Super X and Super XX take 15516, and the two are not interchangeable — the gear trains are cut for different gearboxes. The rules angle is narrower than the fit angle: the Official Race Regulations only require that "all running gears must be installed according to the set gear ratios", and the Stock Class rules add that "drive gears must be used in combinations permitted by the regulations" and that permitted pinion gears depend on the chassis. If you are choosing a chassis and a ratio strategy at the same time rather than retrofitting one you already own, weigh the fuller parts tray and lower price of 15516 against 15456's narrower, AR-only fit. Neither set changes which ratio wins on your track - that still comes down to how much straight you have to build speed against how many technical sections need recovery torque, the same trade-off the wider gear ratio chart walks through in more depth.
Class rules and gear-matching
Tamiya itself publishes two rulesets — the Official Race Regulations and the Stock Class Regulations — and neither locks the gear ratio: both allow any of the set ratios, provided the running gears are installed to one of them and, in Stock Class, the pinion is one permitted for that chassis. Locked-ratio “box stock” classes do exist, but they are organiser-defined rather than Tamiya's, so check the event's own sheet. A bundled set is a convenience, not a requirement — nothing in either Tamiya document says the gears have to come from one box. Confirm current event rules before race day regardless - a local BMAX or club night can still run its own restrictions on top of Tamiya's base regulation text.
Frequently Asked Questions
Can I use 15516 on an AR chassis if I run out of 15456?
No. 15516 is built for FM-A, Super X and Super XX; its gears are cut for the FM-A, Super X and Super XX gearboxes, not AR's, so it is a fit problem first. On the rules side, the Official Regulations only require that running gears be installed to a set gear ratio, and Stock Class adds that permitted pinion gears depend on the chassis — so check the ratio combination you end up with, not just the box it came from.
Which set gives more top speed?
Neither - both bundle the same five ratios (3.5:1 through 5:1), so top speed depends on which ratio you install and gear it against, plus your motor, tyre diameter and battery, not on which box the gears came from.
Is 15516 really cheaper despite covering more chassis?
Retailer pricing at the time of writing puts 15516 around US$9.80 against roughly US$12.50-15 for 15456, but Tamiya does not fix a single global retail price for either item - check current pricing before buying, particularly if ordering from outside your home region.
Once you know which set fits your chassis, the gear set compatibility guide and motor and gear ratio matching article cover how to pick the ratio itself once the box is open. See also the AR chassis tuning guide and FM-A chassis tuning guide for the rest of each platform's setup.