[Conan_the_Librarian]

The corret wheel and tyre combination should give a rolling radius of 2129.06 mm

Therefore with a standard 18" whell the tyre is 245/45 18
for a 19" wheel it is 245/40 19 for a rolling radius of 2131.88 (0.13% difference)
for a 20" wheel 245/35 20 gives 2134.71 (0.27% difference)

Once you start playing around with cross sections and tyre width it's just a case of keeping as close to stock as possible to not cause any speedometer errors (a potential MOT fail)

However, most speedos read high so they don't fall foul of the construction and use regs with regard to accuracy. A speedometer mat read upto 10% high but may not have any error reading low. manufactures also use this to 'demonstate how economical' your car is. You think your doing 70 and realy your doing 64..........

Your wheel tyre combination is about max for a D2 on the front.
the rolling radius is 2200.68 about 3.36% oversize and probably on the limit of your speedo error. When your speedo says you are doing 70 you may be doing 72.35. However if you speedo is reading high in the first bplace you have just corrected it with the tyre size! Check it against a satnav for accuracy.

The tyres you have (275) are designed for a 9" - 11.5" wheel. A 285/30 20 will need a 9.5" wheel minimum and I think you'll have rubbing issues at the front. At the back you should be able to get an 11" wheel with a proper offset of about 35. I have a 10" rear to go on with an ofset of 55 and i will use 15 or 20mm spacers to make it sit right.

If you want a staggerd set up then to maintain rolling radius parity front to back the 245/35 20 fronts match 285/30 20 rears running on 8.5 - 9.5 wheels front and 9.5 to 11 rears.

265/30 20 and 315/25 30 also match; but will cause your speedo to read high (show 70 when you are doing 68. If you can find them well done. If you can afford them, buy a D3 S8 and stop wasting money on tyres for a D2!!!!!