We have overriden the default mapping by adding:

    <indexConfigurations>
     <defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
    	<fieldMap type="Sitecore.ContentSearch.SolrProvider.SolrFieldMap, Sitecore.ContentSearch.SolrProvider">
    		<fieldNames hint="raw:AddFieldByFieldName">
                  <field fieldName="vehiclename" returnType="string" indexType="string">
                    <analyzer type="Sitecore.ContentSearch.LuceneProvider.Analyzers.LowerCaseKeywordAnalyzer, Sitecore.ContentSearch.LuceneProvider" />
                  </field>
                </fieldNames>
           </fieldMap>
     </defaultSolrIndexConfiguration>
    </indexConfigurations>

However, while querying on this field, it is case sensitive.
Kindly help