In all cases where the state treasurer is required by law to apportion the moneys in the highway users tax fund to the various counties, if the number of vehicles registered in any of the counties, excluding vehicles registered within the limits of a city or incorporated town within the county, is not available before the state treasurer makes the apportionment provided by law, the state treasurer may estimate the amount to be paid to any county, and may pay to any county a sum not to exceed seventy-five percent of the amount estimated to be due that county, and shall notify the county clerk and recorder in writing that the amount paid is an estimate because registration data is not available.