Reading the Sidewall

The markings on a tyre like 235/65R17 98H decode as: 235 — width in mm; 65 — profile (sidewall height as a % of width); R — radial construction; 17 — rim diameter in inches; 98 — the load index (a coded maximum weight per tyre); H — the speed rating. (A leading letter, where present, is the service class — P = passenger-car, a US convention; LT = light truck. Most tyres sold in Kenya are Euro-metric and carry no leading letter.)

Two of these are safety ceilings, not fitment trivia. The speed rating first, briefly: it's the sustained speed the construction tolerates — a budget replacement below the vehicle's rating is a quiet downgrade of your highway margin.

The load index deserves the detail, because it's the one that ends in a blowout.

What the index is. It's a coded number, not a weight — 98 does not mean 98 kg. Each index maps to a specific maximum load in kilograms on a standardised chart, and the relationship isn't linear, so you look it up, you don't calculate it:

How to know the load your tyres can bear. Read the index → find the kilograms per tyre → multiply by four:

• our 98 = 750 kg per tyre, so four tyres carry 4 × 750 = 3,000 kg;
• that total must exceed the vehicle's loaded weight — the gross vehicle weight (GVW ) with margin to spare. A vehicle whose GVW sits near 3,000 kg is riding on its tyres' limit when fully loaded, which is no margin at all.

The rule this creates: you may fit a higher load index than the vehicle's specification asks for (it's in the owner's manual and on the door-pillar label), never a lower one. A replacement one or two indices down looks identical and quietly lowers the ceiling.

What the blowout is. Past its rated load, the tyre carries more than its casing was built for, so it flexes harder on every rotation — and flex is heat (Sub-topic 3). Even on perfect tread, the heat climbs past what the structure can take and it lets go suddenly: a load-and-heat blowout, not a slow wear-out. This is the tyre's half of Module 7's overloading — and why an overloaded pickup on good tyres still blows one on the Mombasa road on a hot afternoon.

Replacements should match the vehicle's specification — and critically, match across an axle (Sub-topic 4).