Hay folks,

My front passenger alloy has a slight buckle so I bought a lighty used replacement off of ebay with no buckles or kurb damage only a few storage marks. The problem is the I've just noticed I've bought a rear one. There's for some strange reason a very small difference in the size from the front alloy the the rear alloy. The front is 9J X 20 (49mm on the inside) and the back is 10J X 20 (46mm on the inside), The pic shows what I mean.

I know the tyres are different sizes, Front 245/40/20 and the rear 275/35/20 but why is there's a difference in the alloy?

I ask because I have this "rear" alloy to replace the buckled front one but worried that the 3mm difference will cause problems.

You'll also note the TPM sensor is still a Ford part.

 

I would be more worried about the fact that the rear rim is 1 inch wider than the front. The tire, even if its the same tire size, will not be the same from left to right when on a different rim width. It will bulge differently and flex differently. The car's steering will different turning left or right, and I'm willing to bet it will pull one way or the other. Sounds like a really bad idea.

It's likely the offset from the wheel center to the mounting surface is different as well, which changes the scrub radius (the distance from the centerline of the tire to the point where the steering axis meets the ground) and that will mess up the steering.

The 3mm you're looking at is inconsequential. It's there because the rear wheel is 1" wider and so has a slightly different design.

 

Hay Delta-V,

Thanks for that info, I can be such an idiot sometimes, I didn't check the width only the the rim . And you're right about the rear mounting surface is different. I've just washed the car and I noitced the rear alloys have more of a indent where the nuts go in.

I've never known of alloys being different sizes on the same car only the tyres. Oh well an another life lesson learned.

I guess this means I'll have to buy other one and make sure I get a front one this time.

 

Yup, that's what the 9 and ten mean in "9J X 20" and "10J X 20".
Pretty common on high powered rear wheel drive cars, since the rear needs more traction. Some even have a taller wheel in the back, too (My Z06 is 17x9.5 in the front and 18x10.5 in the back).