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94 civic dx all stock 213,xxx MT.

my speedometer reads about 6% too fast for my actual speed compared to my gps. my Odometer however is accurate to about 1.5%. First off, i thought they would be wrong at the same percentage. operating off different sensors?

Would changing from stock tire/wheel size (175/70r13) to a slightly larger one from a donor car (185/65r14) make it more accurate without making it alot harder on the engine load?

 
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yes changing to a larger wheel can potentially throw the speedometer off.

 
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its already off... im trying to bring it closer to the actual speed. to rephrase my question. Will a larger tire increase engine load on a high mileage engine?

 
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I'm surprised that you get different error rates for the speedometer and odometer since they are based on the same sensor. My guess would be that the calculation for the odometer would be more accurate since you probably use a longer period of time to compare distance on the gps to the odometer while speed from the gps to the speedometer was probably done for a shorter time and would have more error. Also from an absolute standpoint, speedometers are not accurate at all speeds but tend to be closest at one speed, for example 60 mph. Other factors also effect accuracy such as initial tire pressure, how long the car has been driven and at what speed, causing the tire to heat and increase tire pressure, etc.
As far as changing tire size to compensate, I would stick with the original size or use one of the tire size calculation programs and stay as close as possible to stock. Also be aware that that two tires listed as the same size may be different due to manufacturing tolerances between different brands. As far as engine load from changing tire size, it would have to be pretty extreme to cause a problem. The tire you are thinking of changing to is 3.6 % different so if you really are 6% off it would help correct the speedometer.