This study is concerned with investigating experimentally the effects of methanol and Methyl tert-Butyl Ether (MTBE) blending to base gasoline on the performance CO emissions and fuel cost of a spark ignition (SI) engine. The fuel blends were prepared by addition 5, 10, 15, and 20 vol % of methanol and MTBE with a specified amount of base gasoline. The methanol- and MTBE-gasoline blends were designated as M5, M10, M15, M20, and MTBE5, MTBE10, MTBE15, MTBE20, respectively. Experiments were conducted under various engine speeds, spark timings (STs), and compression ratios (CRs). The engine was operated under wide-open throttle conditions. The results of the study showed that the M5 and MTBE10 blends yield the best engine performance in terms of the brake mean effective pressure (bmep), while the M20 and MTBE15 blends are the best performers in terms of brake thermal efficiency (bte). Moreover, M20 and MTBE10 blends give the minimum CO emission values. The economical analysis performed in the study is based on both the current blending fuel prices in Turkey and brake specific fuel consumption (bsfc) of the engine while using gasoline, methanol-gasoline, and MTBE-gasoline blends. It was obtained that, in contrast to the improvement of engine performance, efficiency and CO emissions, methanol and MTBE blends caused increases in fuel costs because of the expensive methanol and MTBE prices in Turkey. Uncertainty analysis was also performed in this study, and it was found that; the uncertainties in the measurement devices do not have noticeable influences on the variations of engine characteristics.