It is the most sensible instinct in accessory buying: start with the kit that covers today's rig, then add sensors if you take on another trailer. Most of the time that works. Sometimes it does not work at all — and the difference is not published anywhere as a single answer.
Here is the short version, before the detail: can you add TPMS sensors later depends entirely on which system you bought. Two models in the range sell extra sensors as a standalone purchase. One is fixed at its kit size the day you buy it. The rest can only grow by buying a bigger kit — which, counterintuitively, is sometimes the cheaper route anyway.
1. Two Numbers People Confuse
Every TPMS has two capacities, and mixing them up is how owners end up disappointed.
The first is the monitor's ceiling — the maximum number of wheel positions the display can hold. That is a design number and it never changes: an S08 takes twelve positions, a 550S takes thirty-six, an 860 takes a hundred and ten.
The second is what came in the box — how many sensors your particular kit shipped with. Buy the six-sensor 550S and you have a display capable of thirty-six positions and six sensors to put in them.
So "can I add sensors" really means: can I close the gap between box contents and monitor ceiling? That depends on whether the seller offers sensors as a separate line item for your model — and in this range, only two models do.
2. Three Different Answers Across the Range
| Model | Monitor ceiling | Extra sensors sold separately? | How you grow |
|---|---|---|---|
| RV01 | 8 wheels | Yes — 116 PSI pair | Add pairs up to 8 |
| S04 | 6 wheels | No | Cannot — fixed at 6 |
| S08 | 12 wheels | Yes — 116 or 217 PSI pairs | Add pairs up to 12 |
| 550S | 36 wheels | No external pair; a 4-pack of internal sensors is listed | Buy a larger kit |
| D550 | 40 wheels | No | Buy a larger kit |
| 860 | 110 wheels | No | Buy a larger kit |
Note the shape of that table. The two models that sit at the bottom and middle of the range — the entry RV system and the heavy-duty truck system — are the ones built for incremental growth. The long-capacity fleet models expect you to size the kit properly at purchase.
3. Models You Can Top Up: RV01 and S08
RV01 — pairs, up to eight wheels
The entry-level four, six and eight-wheel system sells a two-sensor pack rated to 116 PSI. Since the monitor ceiling is eight, an owner who bought the four or six-sensor kit can add a pair and reach the limit. That is genuine, useful expandability at the cheapest end of the range — and it is the model most likely to be bought by someone who is not yet sure how big their setup will get.
S08 — pairs, in either pressure rating
The heavy-duty eight, ten and twelve-wheel system sells pairs in both its pressure ratings: a 116 PSI pair and a 217 PSI pair. Buy the eight-sensor kit today, add a pair later, and you are running ten positions on the same display. The 12-wheel mode is the ceiling.
One caution on the S08: the 116 PSI and 217 PSI pairs are not interchangeable in practice. If you bought the high-pressure version for Load Range G or H tires, you need the matching high-pressure pair — a 116 PSI sensor will not turn your kit into a high-pressure system.
4. The One That Cannot Grow: The S04
The S04 deserves its own section because it is the exception that catches people out, and its name does not make it obvious.
The S04 is a six-wheel system with no expansion path. Its listing offers four variants — the six-sensor kit at either pressure rating, each with or without a signal booster — and no standalone sensors at all. The monitor is built around a six-position layout, and there is nothing in the catalogue that lets you take it to eight.
For the buyer it is aimed at — a six-wheel Class C, a dually plus a two-axle trailer, a motorhome with dual rear wheels — that is not a problem, because six is the right number. It becomes a problem for the owner who reasons "I'll start with six and add two later for the second trailer." On this model, that plan does not have a second step. If you might need eight, the S08 is the model that covers it, and it is the one the range positions directly above the S04.
5. Models Where Growth Means a Bigger Kit
The three long-capacity models — 550S, D550 and 860 — take the opposite approach. Their monitors go to 36, 40 and 110 wheels, and they are sold as a ladder of kits: 6, 8, 10, 12, 14, 16, 22, 36 and upward. Growth is expressed by choosing a higher rung, not by adding sensors to an existing one.
In practice that means if you buy a six-sensor 550S and later need ten, the route is to buy the ten-sensor kit rather than four loose sensors. Worth knowing before you object: the 550S does list a four-pack of internal sensors as a separate item, but no external sensor pair, so that option does not serve an owner who wants to add external positions.
What you actually get for the step up
A larger kit includes a second display. That sounds wasteful if you already own one, and it is the main argument against growing a long-capacity system this way. The counter-argument is that on these models the step between adjacent sizes is small — often less than the price of buying sensors separately would be on the models that do sell them. Which brings us to the arithmetic.
6. The Math That Surprises People
Here is the part that changes how you should buy, and it only shows up if you compare two numbers most people never put side by side: the price of a sensor pair, and the price difference between two adjacent kit sizes.
On the models that sell sensor pairs, moving up one kit size at purchase costs less than buying the same two sensors afterwards. On the RV01, going from the six-sensor kit to the eight-sensor kit at checkout adds a smaller amount than the two-sensor pack sells for on its own. The same pattern holds on the S08 between its eight and ten-sensor kits, where the step is roughly half the cost of a high-pressure sensor pair.
Which produces a conclusion that feels backwards: if you think you might need the next size up within a year or two, buy it now. Waiting to "add sensors later" is not the cheaper path — it is the more expensive one, and on the models with no sensor listing it is not a path at all.
7. Deciding Before You Buy
Five questions, in the order that settles it:
- How many wheels will the rig have in three years? Not today — include the trailer you are thinking about buying.
- What is the monitor ceiling of the model you are looking at? Six, eight, twelve, thirty-six, forty, a hundred and ten.
- Does that model sell extra sensors? Only the RV01 and S08 do, and only as pairs.
- If not, what does the next kit size up cost at checkout? On most models it is a small step — and cheaper than retrofitting would be.
- If your rig is fixed at six wheels forever, does expansion even matter? For a dedicated six-wheel Class C, the S04's fixed ceiling is a feature, not a limit.
| Your situation | Best route |
|---|---|
| Six wheels now, maybe eight later, tight budget | RV01 or S08 — buy the pair later, or take the bigger kit now |
| Six wheels, permanently | S04 — fixed ceiling is fine, and it is purpose-built for six |
| Eight to twelve wheels on a heavy rig | S08 — pairs available in both pressure ratings |
| Growing toward a fleet or multi-trailer operation | 550S, D550 or 860 — buy the kit size you will need |
| You rotate trailers and expect to keep adding | 860 — 110-wheel ceiling and stored trailer profiles |
GRUNDIG S08 Heavy-Duty TPMS
- 8, 10 and 12-wheel modes on one display
- Extra sensor pairs sold separately — 116 PSI and 217 PSI
- Up to 217 PSI (15.0 bar) for Load Range G, H and J tires
- Stated 40 m (131 ft) open-air range
- IP67 external sensors, anti-theft hex nuts
- 5" HD display with 650 mAh built-in battery
Working out how many wheels you actually need in the first place is the other half of this decision — our guide to counting wheel positions covers that, and the trailer TPMS buying guide covers how capacity and pressure rating narrow the field. If you are still choosing between the two closest models in the range, our 550S vs D550 comparison covers that, and the booster sizing guide covers what you need on top of the kit.
8. Frequently Asked Questions
Can I add sensors to a TPMS I already bought?
On the RV01 and S08, yes — both list two-sensor packs as separate purchases, so you can add positions up to the monitor's ceiling of eight and twelve wheels respectively. On the 550S, D550 and 860 there is no standalone external sensor pair listed; growth on those models means buying a larger kit. It is worth asking the seller directly if your situation is unusual.
Is the S04 expandable to eight wheels?
No. The S04 is a six-wheel system with a six-position monitor, and its listing offers no standalone sensors. That is a design ceiling rather than a stock problem. If you may need eight positions, the S08 covers that range with eight, ten and twelve-wheel modes.
Is it cheaper to add sensors later or buy the bigger kit now?
On the models that sell sensor pairs, buying the next kit size up at checkout usually costs less than buying the same two sensors afterwards — the step between adjacent kit sizes is smaller than the price of a pair. So if you expect to need the extra positions, buying them at purchase is both simpler and cheaper.
Can I upgrade a standard 116 PSI kit to the high-pressure version later?
No. The pressure rating is set by the kit you buy, not by the sensors you add. A 116 PSI sensor will not convert a standard system into a 217 PSI one. If your next trailer may run higher pressures, start with the high-pressure kit rather than planning to upgrade.
Do extra sensors need reprogramming at a dealer?
No. New sensors pair to the display, and on the multi-axle systems you can reassign which physical position maps to which figure from the menu — useful after a tire rotation as well. The limitation on expansion in this range is whether the sensors are sold, not whether the system can accept them.
9. The Bottom Line
Expansion is a feature of specific models, not a property of TPMS in general. Two systems in this range — the entry RV01 and the heavy-duty S08 — are genuinely growable with sensor pairs. One, the S04, is fixed at six wheels for its whole life. The long-capacity fleet models grow by stepping to a bigger kit, and on those the step up costs less than retrofitting would.
So the question is not "can I add sensors later" in the abstract. It is "how many wheels will I have in three years, and does this model's ceiling and parts list get me there" — a two-minute check at checkout that saves buying a system twice.
Disclaimer: Sensor availability, kit configurations and monitor capacities quoted here reflect the manufacturer's product listings at the time of writing and may change; where a model lists no standalone sensors, this article says so rather than asserting they cannot be sourced. Confirm the current configuration with the seller before ordering. A TPMS is a monitoring aid and does not replace regular manual pressure checks.