Honestly, I've tried looking everywhere on the web and here for ages now!! Can't find the answer and I should've gone to bed ages ago!

I'm trying to establish exactly what size and dimensions my 17" Zeniths are...

I've established (I hope!) they are 112x5, but what is the offset?!

Am I right in thinking the rim width is 7J, whereas the 18" is 7.5J...?

I've found some 17" wheels I quite like the look of which are also 112x5, with tyres being 225/45/17 91W. States they have an offset of 49 and rim width of 7J, so will they be okay?

Thanks so much for reading

You'll be fine with that size.

The 18's are 7.5j et51 so you'll have plenty of room

Ok thanks SNH.

I've done a bit more searching and have confirmed everything is the same apart from the offset. My current offset is 54 on the 17" Zenith, the new alloy has an offset of 49, 5mm difference seems quite a lot, will this cause any issues...?

Nope, bags of room. I run 8.5x18 et25's so you have nothing to worry about

I'm trying to establish exactly what size and dimensions my 17" Zeniths are...

7 inches wide, ET 54.