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Speedometer calibration for larger tires

  • Thread starter chris ratay traveler
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kurienp

kurienp

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Newbury, Berkshire, UK
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T6 Ocean 204 4Motion
Looking to convert my 18" alloys, 255/45r18 (27x10) to 17"Steel, All Terrain 225/65r17 (28.5x8.9).

I'm wondering if the speedo will be more accurate, or will I need to adjust the speedo. What are people using for speedo adjustment?

I found this one from GTV-Vans: https://www.gtv-van.com/en/speedometer-calibration-kit.html?id=251321552&quantity=1

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I would personally just measure the speed diff using a GPS speedo on the phone and keep the difference in mind when driving. Currently with 235/55/17 - I know that the van speedo reads ~3mph less than the GPS speed ... and there is an ~10% error margin in the speed cameras here in the UK.

The speedo will never be accurate as the wear on the tyre will also cause to give a different value.

Unless the change is massive, I would not bother with the device.

Measure the difference between reported and true speed with a GPS app on the phone and then decide if it is worth 545 Euros.
 
Velma's Dad

Velma's Dad

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I would personally just measure the speed diff using a GPS speedo on the phone and keep the difference in mind when driving. Currently with 235/55/17 - I know that the van speedo reads ~3mph less than the GPS speed ... and there is an ~10% error margin in the speed cameras here in the UK.
I'd probably do the same... however I see OP is planning to fit a wheel/tyre combo that will cause speedo to under-read by nearly 6% (versus the original calib).

The UK norm for speed prosecutions is speed limit + 10% + 2 mph (ie 68mph in a 60 limit) so a substantially under-reading speedo is not ideal unless you keep it constantly in mind. Or as you say, at least check calib with new tyres against a GPS app so you know exactly how you're rolling.
 
Cath&Stef

Cath&Stef

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Horsham, Sussex
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T6.1 Beach camper 150
I see VW dealers now offer 275/35/20 as an altenative fitment on Transporters and they claim no speedo adjustment is necesary. Current our van is fitted with 275/40/20, I chose 40 profile for a tad more comfort.
After testing on a level dual carriage way against GPS on Google maps, I figure the speedo reads about 8% (err on side of caution when compared to the standard 16" wheels) under.

To stay legal, I figure a speedo reading of 27, 37, 47, 56, 65 approximates to 30, 40, 50, 60, 70 mph respectively, bit less at the lower end, bit more at the higher end.
 

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