Advise requested for speedometer issue and engine performance

I'm a newby. So please forgive my ignorance. I recently bought a 2000 Tacoma TRD 4-by, 2.7L, with a 5 speed transmission. I have discovered that the speedometer is off by about 15mph at highway speed. I replaced the speed sensor sending unit but the problem remains. My question is this: If the speedometer is not accurate, does it effect how the throttle body functions or how the fuel is injected into the engine? Also, I am thinking about upgrading to a better speed registering system. Any recommendations would be appreciated. Oh, the tires are the correct size.

 

Oh by the way, I forgot to mention something important. The old speed sensor gear that I took off the truck had 32 teeth. I replaced it with another 32 teeth unit. I thought the unit was faulty. But it may be that someone in the past put the wrong sending unit in. Can anyone tell me how many teeth should be on the speed sending unit?

 

Toyota dealer should be able to tell by the VIN number.

 

I would look at tire size... does she has stock tires or larger off road tires? I know that with a mechanical speedo tire size effects the speedo and I see no reason why it wont be the same with and electronic speedo either.