Cert For Larger Tyres.. Speedo Correction?

[3vilc]

Now i'm sure we all know you need/should have, a cert to run larger than stock tyres. Im assuming that this is due to speedo innaccuracy on diesels. (because the government no doubt thinks we are fitting larger tyres just to make road users last longer..but thats a whole nother thread in itself ).
So what i'm really asking is, how would one go about correcting the speedo on a MU? I am yet to buy one so not sure how the speedo is setup. If its electronic not too hard..but i guess cable would require messing with the drive gear somehow.
Although my local certifier is pretty good so would probably pass it anyway. (Looking at lifting etc as well so need a cert anyway so thats not an issue)

[mudgrip4]

I know for the bighorns (87-92) the instrument shop quoted about $90 3-4 years ago to fit a little adapter unit into the actual cable, with correct reading then coming out of it into the speedo. Also know of a mate who simply took the speedo needle off his navarra, rotated it just a little and then refitted it - reads good now..

Mike.

[kykin]

u could use this www.miata.net/garage/tirecalc.html
and work out how far off ur speedo is, very helpfull, but a lil more involved searching to get ur measurments right with tyres measuer in inches... in aus we can go up 2 sizes and not get in much striffe as far as im aware.

[3vilc]

In NZ it states any increase in tyre circumferance 5% over the stock tyre size must be certified (which on a 4wd, is virtually anything over the average 235/75R15, roughly a 30" tyre). Im not yet sure weather they are more concerned about the speedo being inaccurate or just to check the tyre spec is appropriate for the rim (ie. someone trying to run a 33x12.5 on a stock 7" wide rim). Either way im not really sure it would be a problem, the tyres im thinking of the manufactures specs match the rims i want. Was really just curious as to weather the MU uses an electronic speed sensor or cable drive speedo, as electronic is fairly simple to modify.

[geeves]

Belive it or not when I had to certify my truck he told me all I had to do to certify the speedo was to but sticky lables on the dial with the correct speeds Oddometer didnt matter. Check this with your certifyer though things change and so do interpretations. There is a place around that makes drive adapters so any speedo can work in any setup but not cheap. Let me know if you need details

[roga]

The Mu has a cable driven speedo Damn it

[geeves]

The reason for cert on bigger tyres is 4 things.
1 Is the new tyre rim setup putting too much strain on the suspension
2 the bigger tyres are heavier and have far greater rotating inertia Do the brakes still work. (the test is to accelerate to 100kph and stop 3 times in 2 minutes )
3 the center of gravity is raised so is the whole thing still stable.
Lucky last do the tyres rub on anything under normal use. With a 30mm suspension lift mine is ok on 31x10.5 but would need to define normal use for 32s They dont touch on road but do in the rough stuff

[3vilc]

Pretty sure it did just state 'during normal road use' when i was reading the lvvt website the other day. so unless you can find a road where you can get full artic it should be ok. looking at running a 32x11.5 on a 15x8 rim with the appropriate offset, with at least a suspension lift so should be all good.
Suppose yeah its all upto the cert places interpretations, some may not care, some may be happy with speed marks on the dash, some may want it fully recalibrated