(a) In general. Where an election is made under section 4(a) of the Act interest is computed under section 6601 of the Code (or corresponding provisions of prior law) on any increase in tax attributable to such election for each taxable year involved for the period from the last date prescribed for payment of the tax for such year (determined without regard to any extensions of time for filing the return) through the date preceding the date on which the election is made. Where the election under section 4(a) of the Act results in a decrease in tax for any year to which the election applies, interest is computed in accordance with section 6611 of the Code (or corresponding provisions of prior law) from the date of overpayment through the date preceding the date on which the election is made. Where there is a net increase in tax as a result of the election under section 4(a) of the Act, no interest shall be imposed on any underpayment (and no interest shall be paid on any overpayment) attributable to the dealer reserve income adjustment for any year to which the election applies for the period commencing with the date such election is made and ending on the date prescribed for filing the return (determined without regard to extensions of time) for the taxable year in which the election is made. This rule applies regardless of whether the election under section 4(b) of the Act is made. If there is no net increase in tax, interest on any underpayment or overpayment attributable to the dealer reserve income adjustment for any taxable year to which the election applies for the period commencing with the date of the election shall be determined in accordance with §§ 301.6601-1 and 301.6611-1 of this chapter (Regulations on Procedure and Administration).

(b) Installment period—(1) Where payment is not accelerated. If the election under section 4(b) of the Act is made to pay the net increase in tax in installments, no interest will be imposed on such net increase in tax for the period beginning with the due date fixed under section 4(c) of the Act for the first installment payment and ending with the date fixed under such section for the last installment payment unless payment of the unpaid installments is accelerated under other provisions of the Act. See subsections (f) and (g) of section 4 of the Act.

(2) Where payment is accelerated. Where payment of the unpaid installments is accelerated because of the termination of the installment privilege, interest will be computed under section 6601 of the Code on the entire unpaid net increase in tax for the applicable period set forth below:

(i) In the case of acceleration under section 4(f) of the Act for reasons other than nonpayment of an installment, from the date of the notice and demand for payment of the unpaid tax to the date of payment; or

(ii) In the case of acceleration under section 4(g) of the Act for nonpayment of an installment, from the date fixed for payment of the installment to the date of payment.