Duff Diff?

[Nippon Tuck]

I recently took 17inch alloys off of my Bongo and changed them for 15 inch alloys.
The 17 inchers were 225 55 all round, however, the 15inchers have 215 65 rear and 195 65 front.

Since changing them (I think) I have noticed a droning sound coming at about 50+MPH

I understand the sound could be the DIff on its way out. could it have anything to do with the change of wheel ratio?
When it gets up to speed it sounds like an aeroplane!!!

[francophile1947]

Those wheels are over 4% different in circumference, so, if you have 4WD, you may have damaged your viscous coupling. If it's a 2WD, it shouldn't cause any problems.

[Nippon Tuck]

It is a 4WD. I had no problem when I went from 15 inch to 17 inch before.
Is the Viscous Coupling a big job. Have I done myself a terminal and wallet whacking mischief?

[Simon Jones]

As long as all 4 wheels are same rolling circumference then change of size won't matter. It's only when front and rear differ that the VC in the transfer box gets knackered. You could try disconnecting the front driveshaft and if the noise goes away you know you'll need to look out for a 2nd hand unit from a breaker. I don't recall anyone changing one before.

[Northern Bongolow]

the tyre sizes you say you have on have a 27mm circumference difference so will kill something very quickly.
you need 195 70 15 on the front and 205 65 15 on the rear to be under 1mm difference.
if you fit 195 70 front and 215 65 rears you may just get away with it . as thats 1.7 mm difference.
best to fit all four at 205 or 215 on a four wheel drive, then you can rotate for even wear and just carry one spare that fits all four corners.
check with the insurer though
http://wzeu.search-results.com/r?t=p&d= ... ecalc.html

[Nippon Tuck]

You guys are GOOOOOOOOD!
I changed the fronts to 215/65/15 and it made the world of difference.
No noise.


CHEERS.