delivered the opinion of the Court.
The parties to this action are as follows: Public Service Company of Colorado, plaintiff in the trial court, to whom we will refer as the Company; City and County of Denver, a defendant in the trial court, hereinafter referred to as the city; Thomas G. Currigan, City Auditor and a defendant in the trial court, hereinafter referred to as the Auditor; Frank K. Southworth, Manager of Revenue and a defendant in the trial court, to whom we will refer as the Manager. The gas consumers in the city who are customers of the Company will be referred to as consumers.
The Company brought suit in the district court against the above named defendants to recover moneys allegedly paid in excess of the sums legally due under the provisions of the franchise under which the Company serves consumers of gas within the city. The Auditor filed an answer in which he set forth equitable and legal defenses, which we find unnecessary to incorporate herein in detail. He also filed a counterclaim against the Company to recover for the city certain moneys allegedly due under the franchise by reason of an asserted improper deduction by the Company of $48,694.52 as a set-off against the amount due according to a quarterly report of the Company under the terms of the franchise. This deduction was made by the Company in October 1955, under a claim that it had overpaid the city for the period between August 1952 and December 1953. : The Manager and the city filed an answer in which they supported the claim of the Company. They acknowledged, in their brief “* * * that there are substantial questions of *399law with respect to the matters raised in the litigation and ask the Court to adjudicate the rights of the parties.”
Pursuant to a vote of the taxpaying electors of the city an ordinance granting a gas and electric franchise to the Company became effective February 14, 1947, the provisions thereof which are pertinent to this controversy are as follows:
“Section XIII. As a further consideration of this franchise, and in lieu of all occupancy and license taxes or other taxes on the right to do business, the Company agrees to pay a sum equal to three per cent (3%) on the gross revenue derived from the sale of electrical energy by the Company within the corporate limits of the City and County of Denver, and a sum equal to three per cent (3%) on the gross revenue derived from the sale of gas by the Company within the corporate limits of the City and County of Denver.
“The term ‘gross revenue’ as used herein shall be construed to mean any revenue earned within the City and County of Denver from the sale of electrical energy and gas, after adjustment for the net write-off of uncollectable accounts and corrections of bills theretofore rendered . . .
“The Company shall file with the Manager of Revenue a report of its gross revenue within forty-five days after the close of each quarter in each calendar year. Such report shall contain a statement of the gross revenue classified according to the rate schedules then in force and effect and a statement of any deductions made because of adjustments and corrections as herein provided, together with a computation of the tax to be paid. Coincidentally with the filing of such reports, the Company shall pay to the Manager of Revenue the tax thus computed. Within thirty (30) days from the date filed, or within such additional time as the Manager of Revenue may request, the Manager of Revenue shall investigate the report and determine the accuracy of the amounts reported. However, neither payment of the tax nor a failure to make *400such investigation shall be deemed to estop the City in any way or prevent subsequent investigations by the City and collection of any amount due. If the tax paid is found to be insufficient, the Manager of Revenue shall notify the Company of the deficiency and demand payments of said amount. If the tax paid is determined to be excessive, the Company shall be entitled to a refund of the excess paid.”
The controversy arises by reason of four increases in rates made by wholesalers of gas to the Company for resale to its consumers. These increases in rates were authorized by the Federal Power Commission as a temporary matter. The Public Utility Commission of Colorado authorized the Company to pass the increased cost on to its consumers. This increase in rates charged the consumers by the Company necessarily included the item of increased payments to the city occasioned by the higher “gross revenue” as defined in the franchise. The Federal Power Commission subsequently cancelled the increase in wholesale rates which had been previously authorized and ordered the wholesalers to refund to the Company the exact amount which had been collected thereunder by the wholesalers. The Public Utilities Commission thereupon ordered the Company to refund to its consumers the amount refunded by the wholesalers, and characterized the refund as moneys held in trust by the Company for the benefit of its consumers. The function of the Company under the order of the Public Utilities Commission was to serve as a conduit through which the refunded moneys should flow from the wholesalers to the consumers. A carefully prepared formula was agreed upon, based on sample months and averages rather than on an individual customer by customer charge of individual bills for the period involved. All the expenses of the Company involved in the distribution of the money to the consumers were paid as. a charge upon the trust funds.
January 7, 1960, the Company filed its claim with the *401Auditor for $249,795.26, asserting that this amount was due it for the reason that an overpayment in that amount had been made under the terms of the franchise for the years 1954 to 1958 — all brought about by the cancellation of the rate increases which were “temporarily” in effect during those years. (The figure above mentioned was the balance allegedly due after the Company had set off the $48,694.52 to which reference has already been made.) June 11, 1962, the Company filed its claim with the Auditor for an additional refund of an alleged similar overpayment of moneys due the city under the franchise for the 1958 to 1960 period, in the amount of $149,406.88.
All pertinent facts were stipulated and the trial court entered judgment upon motion for summary judgment. The trial court denied the Company’s claims as to the $249,795.26 and $149,406.88, and granted the Auditor’s counterclaim against the Company for $48,694.52. The Company is here by writ of error directed to this judgment.
The Company and the city must find justification for their respective positions within the four corners of the franchise, which is the contract between them and by which they are bound. Said document was finalized after lengthy negotiations, action by the city council, and approval by the qualified voters of the city. An examination of this franchise agreement discloses a well prepared and comprehensive statement of the rights, duties and obligations of the parties thereto. The language employed is clear and expresses an intent which is not clouded by ambiguities.
In considering public utilities franchise agreements the courts are bound to give a liberal construction in favor of the municipality in the protection of the public. The general rule in this connection is stated in 63 C.J.S. 605 as follows:
“However, such contracts, while relating to the business interests of the city, are not of a private nature, but are of public concern and amenable to the rules of law *402applicable to public utilities, one of which is that the grant of a public utility franchise is to be liberally construed in favor of the public; * * *”
We now consider the question of whether the Company is entitled, under the terms and conditions of the contract and the facts hereinabove stated, to recover the $399,201.94 represented by the claims filed with the Auditor.
Pursuant to the provisions of the franchise contract hereinabove quoted, the Company “* * * filed a report of its gross revenue within forty-five days after the close of each quarter in each calendar year * * *” for all the years involved in the litigation. Each of these quarterly reports did, “* * * contain a statement of the gross revenue classified according to the rate schedules then in force and effect and a statement of any deductions made because of adjustments and corrections as herein provided together with a computation of the tax to be paid.” (Emphasis supplied.)
In all of these reports there was no mention of any refund received, claimed, or contemplated in the future. The amount paid for each quarter was the correct sum owed to the city under “the rate schedules then in force and effect.” No “adjustment for the net write-off of uncollectable accounts and correction of bills theretofore rendered” is involved for the reason that the Company became a holder of the refund from the wholesalers in trust for the use and benefit of its consumers, and for the further reason that in serving as a conduit for the return of funds to the consumers the process did not involve a “net write-off of uncollectable accounts and correction of bills theretofore rendered.” It is admitted that the consumers received a refund “based on sample months rather than an individual customer by customer charge of individual bills for the period.”
The gross revenue on which the Company paid its franchise assessment was in fact the gross volume the Company actually received during each quarter of the *403period involved. The fact that thereafter the Company served as a conduit for trust funds for the consumer did not increase or decrease the “gross revenue” theretofore received by the Company. Generally the term “gross revenue” means gross receipts of a business before deductions for any purpose except those items specifically exempted. State Tax Commission v. Quebedeaux Chevrolet, 71 Ariz. 280, 226 P. (2d) 549. The refunds from the wholesalers to the Company in trust for the benefit of consumers were not items “specifically exempted” under the franchise agreement. It should require the citation of no authorities to support our conclusion that the rights of parties to a contract clear, complete and unambiguous, should be determined according to the express provisions of said contract. In the instant case we have such a contract, which must be liberally construed to protect the public interest, prepared by experts after long negotiation, approved by city council and the vote of the people. We have no right to change this contract, or, by interpretation of that which is clear, make a new or different contract for the parties.
The argument is made that the claims of the Company should be paid because the “overpayments” were made under a mistake of fact. With this assertion we cannot agree. The Company paid each quarterly installment during the time the increased rates were in effect, with full and detailed knowledge of all existing facts. The most that can be said is that a change in the situation developed at a much later date. Under well established principles of law this is not sufficient to justify relief on the ground of “mistake of fact.” From 40 Am. Jur. § 192, p. 847, we quote the following:
“A mistake, to be available as a ground for recovering money paid, must be as to an existing fact; therefore, where the subjects in relation to which the contract of parties is made are known by them to be of an uncertain and speculative character or value, a mere mistake by them in their estimate of the value is not deemed suffi*404cient to authorize a recovery of the moneys paid on the erroneous estimate. * * *”
There was no error or mistake in the amount reported and paid by the Company, the report and payment was the Company’s voluntary action within the express terms of the franchise; the city did nothing but receive the reports and payments, both the reports and payments were absolutely correct when made (so far as the later refund is concerned), according to “the rate schedules then in force and effect.” The later refund was exactly the amount which the Company received from the wholesalers and left the Company exactly where it was when the reports and payments were made. This refund to the Company did not include the increase to the consumer with relation to the franchise assessment of 3% brought about by the increase in gross revenues. Presumptively, the Company covered all their increased costs of operation in their request for higher rates. They accordingly received from the consumers the amount required as franchise assessment to cover the increase in gross revenues. The refunds had no financial effect upon the Company.
We next consider the question of whether under the counterclaim of the Auditor the city was entitled to recover the $48,694.52 which the Company in October 1955 had set off and deducted from its tax payment for the period between August 1952 and December 1953. Here again the answer is to be found within the terms of the franchise contract. The procedure to be followed by the city to question the correctness of any report and payment by the Company is clearly stated in the franchise as follows:
“Within thirty (30) days from the date filed or within such additional time as the Manager of Revenue may request, the Manager of Revenue shall investigate the report and determine the accuracy of the amounts reported.”
The Manager of Revenue did not within thirty (30) *405days “determine the accuracy of the amounts reported.” He did nothing to question the accuracy of the reports or the amount paid. He has taken the position at all times that the offset was proper. Not until the Auditor was sued and filed his counterclaim was the report and payment challenged. This was more than six years after the report was filed and payment made. The counterclaim must fail under the express terms of the franchise, the granting of which franchise by the city was “the exertion of the proprietary power of the sovereign.” Berman v. City and County of Denver, et al., 120 Colo. 218, 209 P. (2d) 754. In City of Denver, et al., v. Mercantile Trust Company of New York, 201 Fed. 790, the court stated at page 803:
“Again, in the matter of granting authority to street railways to operate lines of roads in the City, the City of Denver was exercising its proprietary, as distinguished from legislative or governmental authority, and in respect to the exercise of its proprietary powers the city is bound by the same rule of equitable estoppel as individuals.”
By the franchise agreement the procedure to be followed by the city in challenging the correctness of reports by the Company was specifically stated, and more than six years elapsed before anyone challenged the correctness of the reports in question, and when objection was made it was made by the Auditor who has no authority to prosecute any claim for and on behalf of the City and County of Denver. The city has never at any time made a claim against the Company for any sum whatever. Conditions necessarily precedent to the right of the city to recover the sum here under discussion have not been performed. The statute of limitations has long since expired and the judgment entered by the trial court on the counterclaim of the Auditor cannot be upheld.
The judgment of the trial court denying the claims of the Company is affirmed; the judgment awarding the *406sum of $48,694.52 in favor of the city and against the Company is reversed with directions to dismiss the counterclaim of the Auditor.
Mr. Justice Hall and Mr. Justice McWilliams dissent.