<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>3/A</documentType><periodOfReport>2002-02-01</periodOfReport><dateOfOriginalSubmission>2002-02-06</dateOfOriginalSubmission>
    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000075362</issuerCik>
        <issuerName>PACCAR INC</issuerName>
        <issuerTradingSymbol>PCAR</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001191908</rptOwnerCik>
            <rptOwnerName>GANGL KENNETH R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>777 106TH  AVENUE NE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>BELLEVUE</rptOwnerCity>
            <rptOwnerState>WA</rptOwnerState>
            <rptOwnerZipCode>98004</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>VP, FINANCIAL SERVICES</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeHolding><securityTitle><value>COMMON STOCK (ESPP)</value><footnoteId id="F1"/></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>87.7</value><footnoteId id="F2"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><footnotes><footnote id="F1">Shares held in PACCAR Inc Employee Stock Purchase Plan (ESPP).</footnote><footnote id="F2">These shares were omitted from the Reporting Person's original Form 3 and also were omitted from ten Forms 4 filed by the Reporting Person after his original Form 3 was filed.  Shares adjusted to reflect 50% stock dividend of 5/28/2002; there were no other transactions with respect to these ESPP shares.</footnote></footnotes><remarks>Fractional shares rounded to nearest 1/10.</remarks><ownerSignature><signatureName>Kenneth R. Gangl</signatureName><signatureDate>2003-10-30</signatureDate></ownerSignature></ownershipDocument>

