Hi i have 2 DSG equipped motors, one is my 1.9tdi VW Caddy and my new vrs.

the question i have is, they both engines rev to the same point but the top speed at 4500rpm in 6th in the caddy is 114, where as the octy is capable of more, therefore what is different, is it gear ratios, diff ratios or somthing else, the answer would help me modify my caddy to cruise at a lower rpm as all my mileage is on 60-70mph roads.

If both DSG's are 6-gear type, only difference would be the final (diff) ratio.

Second possibility might be significant difference of the tyre sizes...

wheel and tyres are both the same (give or take a small %) both run 18's on low profiles.... can the diffs be swapped out? i've come from RWD background so no idea about diffs in fwd cars....

I'd check the gearbox codes or the PR stickers in the service books as this will tell you if the gearboxes are the same or not. At a guess i'd say they are geared slightly different as the caddy would be used to carry more weight than the octavia and they would use the gearbox to allow this.

Just my thoughts on it. Send in the gearbox codes and maybe we could find the technical spec of each box for you.

where can i find these codes?

You should be able to find the gearbox codes on the stickers inside the service book or normally in the boot of the car. Not sure where thye would put one in the caddy though