Oversized tyres and speedo calibaration

Marks 4WD adapters (Melb) do these little black boxes that intercepts speedo signal from gearbox.
This allows you the abilty to recailbrate speedo to suit different tyre size,normal tyres during week, 35's on week end, flick a switch to suit, speedo is near right.
These units are available for cruisers,not sure about the GU
Shane

Hi Brad,
What size tyres are you wanting to put on ?

Pezza

Are you sure the tyres have a larger overall diameter? They shouldn't, unless you want to upset the balance of the vehicle. The speedo will only be affected if the difference is considerable. Remember that speedos are up to 10% out anyhow. Therefore yours will likely be moved towards the true speed with bigger tyres. Check with a GPS and calculate the % error and use this as a reference when driving and take care if you fit really big tyres!!

Have used the Jaycar kit here Brad as have several sets of wheels, pretty cheap and works well now !

Actually my current GPS does such a great job of Speed display - better than speedo at sub 20kmh- that I moved the unit to alter the tacho count.

Robin Miller

You may not have to recalibrate the speedo, with 33inch tyres my GU speedo is almost dead accurate (102km/hr by GPS at indicated 100km/hr) where it would previously be at 95 for indicated 100.

As previously mentioned Marks 4wd Adaptors make a speedo match box to electronically recalibrate the speedo. Im not sure of prices, suppliers...ect.

Im running 285's and my speedo is out according to GPS by 10kph...

you can buy units, or you can go to places like Ringwood Instruments who will recalibrate it for you for a yillion $... :(

Our GU with 285/75/16 shows GPS 102kmh at an indicated 100kmh by the speedo. Highway roadside checks shows distance under now by about 50m per 1000m . ie trip meter out now by about 5% under actual.

look here for a lot of info
http://www.carbibles.com/tyre_bible.html

Or here is even better gives direct comparisons as to size, offset and all.
http://www.rims-n-tires.com/rt_specs.jsp