New Speedo , will it fix my dodgy fuel cosump on the Info centre??

Dear peeps,
Has anyone of you had a new speedo fitted to rectify any electrical problems on the gta??, and does the miles always get zero'd??, my dealer is ahving to do this as the mpg on the info centre is comp[letely out, i mean seriousley out by about 1/3rd of what it should ???