So I know you can put the outback strut and spring assembly on a legacy L. What I was wondering is what happens you go from the r14 tires to the r15 will my speedometer not read out right and if it doesn't read out right what can I do to fix it?

205/70-15 (stock outback tire size) on my 96 Legacy L made my speedometer read 68 mph 72. Makes it read slower. Should give an idea of about how far it'll be off.

And as far as a fix, only thing i can think of is swapping to an outback trans that's speedometer is geared for those tires

Its an electronic speed signal so it can be corrected with an electrical speedometer corrector. There are various different types out there if you search google.

 

Another way is to swap in an Outback transmission, which will have the added benefit of better gearing to help turn the larger tires.