The DOE national average diesel fuel price increased 3.8% when compared to the fourth quarter last year. We continue our work towards cost effective, industry leading fuel economy while at the same time meaningfully contributing to environmental improvements.
The driver market remains challenging, yet we are succeeding in attracting and retaining high performing driving associates. We continue to benefit by leveraging our model and the advantages it provides us with our driving associates. As a result, our driver turnover has trended favorably and is well below what we understand to be the industry average.
In July of 2012, Congress passed a bill which included a requirement that the Federal Motor Carrier Safety Administration (FMCSA) issue a rule mandating that all motor carriers engaged in interstate commerce must use electronic on-board recorders (EOBRs) to monitor compliance with hours-of-service regulations. All trucking companies will be required to adopt EOBRs by no later than July 2015. Our entire tractor fleet has been fully equipped with EOBRs for over two years. We believe this mandate will allow the carriers that have already adopted EOBRs to be more competitive in recruiting and retaining driving associates.
Our average tractor count for the quarter increased 5.4%. We invested $34.9 million of net capital expenditures in the fourth quarter, as our tractor fleet remains one of the most modern fleets in the industry with an average age of 1.9 years. Included in the fourth quarter net capital expenditures is a real estate investment in a property in Phoenix, Arizona that will serve as our headquarters. Our gain on sale decreased from $2.5 million in the fourth quarter of 2011 to $2.0 million in the fourth quarter of 2012, as the used equipment market was less robust.We have returned $155.2 million to our shareholders in the form of quarterly dividends and stock repurchases over the twenty-four-month period ending December 31, 2012. We ended the quarter with $5.7 million of cash, $80.0 million of borrowing under our unsecured revolving credit agreement, and $490.2 million of shareholders' equity.