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

    <documentType>4</documentType><periodOfReport>2004-01-15</periodOfReport>
    <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</value></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>750</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>COMMON STOCK (ESPP)</value><footnoteId id="F1"/></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>87.6804</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding><nonDerivativeHolding><securityTitle><value>COMMON STOCK (SIP)</value><footnoteId id="F2"/></securityTitle><postTransactionAmounts><sharesOwnedFollowingTransaction><value>981.765</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeHolding></nonDerivativeTable><derivativeTable><derivativeHolding><securityTitle><value>STOCK OPTION</value><footnoteId id="F3"/></securityTitle><conversionOrExercisePrice><value>27.83</value></conversionOrExercisePrice><exerciseDate><value>2003-01-01</value></exerciseDate><expirationDate><value>2010-01-25</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value></underlyingSecurityTitle><underlyingSecurityShares><value>4061</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>4061</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>STOCK OPTION</value><footnoteId id="F3"/></securityTitle><conversionOrExercisePrice><value>34.42</value></conversionOrExercisePrice><exerciseDate><value>2004-01-01</value></exerciseDate><expirationDate><value>2011-01-24</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value></underlyingSecurityTitle><underlyingSecurityShares><value>12910</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>12910</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>STOCK OPTION</value><footnoteId id="F3"/></securityTitle><conversionOrExercisePrice><value>42.31</value></conversionOrExercisePrice><exerciseDate><value>2005-01-01</value></exerciseDate><expirationDate><value>2012-01-23</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value></underlyingSecurityTitle><underlyingSecurityShares><value>10737</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10737</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeHolding><securityTitle><value>STOCK OPTION</value><footnoteId id="F3"/></securityTitle><conversionOrExercisePrice><value>47.10</value></conversionOrExercisePrice><exerciseDate><value>2006-01-01</value></exerciseDate><expirationDate><value>2013-01-15</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value></underlyingSecurityTitle><underlyingSecurityShares><value>9837</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>9837</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeHolding><derivativeTransaction><securityTitle><value>STOCK OPTION</value><footnoteId id="F3"/></securityTitle><conversionOrExercisePrice><value>85.43</value></conversionOrExercisePrice><transactionDate><value>2004-01-15</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F3"/></transactionCoding><transactionAmounts><transactionShares><value>5774</value></transactionShares><transactionPricePerShare><footnoteId id="F3"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2007-01-01</value></exerciseDate><expirationDate><value>2014-01-15</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value></underlyingSecurityTitle><underlyingSecurityShares><value>5774</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>5774</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Shares held in the PACCAR Inc Employee Stock Purchase Plan (ESPP).</footnote><footnote id="F2">Shares held in PACCAR Savings Investment Plan (SIP).</footnote><footnote id="F3">Option to buy awarded under PACCAR Long Term Incentive Plan.</footnote></footnotes><remarks/><ownerSignature><signatureName>Kenneth R. Gangl</signatureName><signatureDate>2004-01-16</signatureDate></ownerSignature></ownershipDocument>

