[NASSTY]

Sambo,
Could be a couple of reasons for the more conservative front offsets by Doug Pearce (DP).

Firstly, Rim size. From what I've seen, a 15" rim with 295 rubber looks fatter than an 18" rim with the same size width tyre. The tyre wall, from what I've seen bulges out more than a lower profile wheel. So the 15" tyres look/are even wider. I'm using 18" wheels and when I compare my wheels with my mates 15" the rubber looks wider even though the tyre size for width is almost the same.

Secondly, Camber. The more camber the more the tyre will tuck under the body. What camber do Harrison’s use?

Thirdly, engineering. In VIC the engineers can be tough. Could be DP wants to factor this in so that the engineer won't ask for extended wheel arches!

Fourthly, DP may want to cover himself for any variances in parts or the Harrison kit itself. I don't own a Harrison so I don't know if this may even be an issue. As the builder it could be that he doesn't want to deal with angry customers whose wheels (for whatever reason) may not fit.

So Rim size, camber, engineering and part variances could be why.

For my kit I used the manufacturers specs and I've had no problems. I'd be giving DP a call and ask him exactly why he's used a different spec. It could even boil down to the look of the wheels inside the arches....

Hope that helps. I know I spent ages searching for the right wheels and tyres and in the end I was relieved when it all worked.

Cheers
(edited for typo's)