Is a Speedo mod needed

Hi John. The larger the tyre diameter the Slower the speedo will read. Best to obtain/borrow a GPS and check your speed accurately against the vehicle speedo. This will show any error. Bob.

Speedo will read about 4-5km slow. 95 km/hr indicated will be 100 km/hr true

viz

Go here and type in your sizes It will tell you the difference


http://www.rims-n-tires.com/rt_specs.jsp




You can buy a correction kit from Jaycar for $50 or a ready made one from Marks Adapters (google It) for $150.

Mate with a toyota it will correct it to accurate. They always read fast by 5 to 10%.
Cheers Pete

John,

go straight to the tyre size calculator on this site and use the tyre comparison section:

ExplorOz Tyre Size Comparison

Just realise that the 285/75's will increase your fuel consumption and reduce the noticable power from your engine. I changed my 265/75 16's for 285/75 16's and only lasted one day before I swapped them back - mine is an 80 series diesel with gas injection. It could barely accelerate when doing 110.... any slight hill would slow it down... Yours might be a lot different with the Safari Turbo.
There was a 4km/hr difference in speedo between the tyres but the speedo was only out by 2km/hr of actual speed on the gps with the bigger tyres (if this makes sense...)
Drew

When I checked out going from 235/75/15 (29") to 235/85/16 (32") on the Paj, the requirement was that any change more than 10% requires a recalibration.

Check with your state registration authority.

Did a GPS check after the change and I was only under reading by 4%, i.e. 100 on the dial is 104 over the ground. Speedo was over reading in the first place.
So haven't worried, just sit on 95 by the dial in a 100 zone and pass every one. :o)
PS - when the tyres are down in tread, i.e. tyre is 20mm smaller in diameter, difference is only 2%.

i went fron 265/75/16 to 285/75/16 on the GU and the Spedo is 100% accruate now. checked this with 2 Navmans and Ozi Exporer, strangly, all 4 speed measures came in at 100km/h

Johny Boy - If you look in members file swap I placed and others have placed a Tyre Size calculator that will show multiple tyre sizes with speedo readings.

If you can not find email me and I will send you both spreadsheets, and anyone who wants a copy.

You can put up to 5 tyre sizes in at a time. Cheers Tony

I went from 70 profile to 75 on the old prado. The speedo was out already from new by 4klm per hour so with the 70 series on it I was showing 110 on the speedo but the GPS showing 106, when I put the 75 profile on, 110 on the speedo, 110 on the GPS so it was spot on. You might find your speedo the same. My wifes Toyota is out by 6k, showing 110 on speedo, gps showing 104. They are allowed to be out this way but not the other way around....Regards Steve M