Speedometer accuracy

Legally and for IVA, speedos must not under-read and must be accurate to within no more than 10% over-reading.
Most over-read 5-10% to give a safety margin to allow for tyre sizes etc. (and flatter the car's performance)
My Hawk with Alfa 12v transmission, Veglia Speedo and 29/61 R TB15 Michelins, gives spot-on speed measurements throughout the range on SatNav and phone app testing.
In other words, there is no safety margin in a speed limit, the speedo means what it says.
Also any tyre with a larger rolling circumference will cause the speedo to under-read ... illegal/IVA fail.
Similarly adverse gear ratios/final drive changes will have the same effect.

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Originally Posted by StruanR

Similarly adverse gear ratios/final drive changes will have the same effect.

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This would depend on how the speedo is picking up its "input". For me, with a 166 engine and gearbox I will have to have the speedo sensor "watching" the bolts fly past on one of the CV joints, so only tyre size as any impact to accuracy. But changing tyre sizes has a surprisingly big impact. On one of my day cars I took the tyre width up one "step" but kept the profile the same, and all "slack" in the speedo reading vanished .. it's now spot on. Definitely something to keep an eye on if your looking to change tyre sizes.

Quote:

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Originally Posted by StruanR

Legally and for IVA, speedos must not under-read and must be accurate to within no more than 10% over-reading.
Most over-read 5-10% to give a safety margin to allow for tyre sizes etc. (and flatter the car's performance)
My Hawk with Alfa 12v transmission, Veglia Speedo and 29/61 R TB15 Michelins, gives spot-on speed measurements throughout the range on SatNav and phone app testing.
In other words, there is no safety margin in a speed limit, the speedo means what it says.
Also any tyre with a larger rolling circumference will cause the speedo to under-read ... illegal/IVA fail.
Similarly adverse gear ratios/final drive changes will have the same effect.

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The same wheel/tyre combination and Veglia speedo, but with Ferrari 308 transmission and gearbox, speedo over-reads nearly 10%.
36mph shows 40mph, 74mph shows 80 mph.
Still legal but the other end of the scale.