Mk3 festa speedo is purely mechanical, this can be modded, speedy cables charge around £50 to do this, simply send them your wheel size, gearing and diff etc, they will sort you out, or by keeping the profile, lower profile tyres will help everything and keep the gearing nearer,
www.kabamus.com/garage/gears.html
is a good site, simply input you tyres size etc and find a near equivalent overall size, say a 195-50-15 in place of a 165-13 etc.
tabetha