Speedometer reads low?

I had my fiance pace me in her brand new 2007 Tacoma and it appears that my speedo is reading about 5mph lower than my actual speed. When my speedo said 65mph, she was doing 70mph.

I bought this car used and I have always thought that the original owner blew up the rearend and replaced it with one from a parts car because it has surface rust on it while the rest of the car has none (bought it from a dealer so I couldn't ask him). Well now I am thinking it may have been a 3.23 rear insted of a 3.42 rear. Which literally blows my mind because this car will burn out 275 BFG KDW's all the way through 3rd gear!!!!

Speaking of tire size, the fronts are 245/40/18 and the rears are 275/35/18 which I don't believe would cause this speedo inaccuracy.

What are your thoughts and what do I need to check. I will look at the tag on the diff cover and get that number for ya'll.

Thanks,
Eddie

I was looking around and saw that the 16" non z-rated tires that came on the Camaro were about .5-.7 inches larger in diameter than the tires currently on my car. Do you think that difference thrown in with general speedometer inaccuracy could be the reason my speedo reads high?

Most likely you are tuned for a different gear than what the car has in it. Read the RPO codes to figure out what stock gear you had,
GU2 Axle, Rear 2.73 Ratio, GU5 Axle, Rear 3.23 Ratio, GU6 Axle, Rear 3.42Ratio. Then an easy way to get a ball park of what gear you have in it now try this, put a mark on the tire and turn the driveshaft one revolution. If it's about 3-1/4 turns, you've got 3.23's, 3-1/2, then 3.42's, 3-3/4, then 3.73's .

My car has the GU6 code for 3.42 gears and apparently it did have the Z-rated tire option so my current tires are close to the stock tire size.

Thoughts?

It sounds like, if indeed the rearend was replaced, that the gears in it are a lower ratio, possibly 3.23, and you are tuned for 3.42. Try doing the driveshaft method that i mentioned in post 3 and see how it comes out