if i put 390's in the rear end will my speedo be messed up or no?

 

Yes, unless you are replacing a 390 with a 390

 

Im not totally positive, but i dont think it will change your spedo, only if you change your tire hight.

 

im saying that because my nephew just changed his camaro from 273s to 373s and its still exactly the same on his spedo as before.

 

The spedometer will work OK but the speed it shows will be wrong. The reason being, that the speedometer signal comes from the tailshaft of the transmission. Because of the lower rear gears, the transmission tailshaft will be spinning around faster for the same road speed, making the spedometer register a higher speed than actual.

Supposing you started out with 3.30 gears and you changed to 3.90, and you drive along with your spedometer showing 70 MPH, your actual speed will really be 60 MPH. The good thing about this is that your car will accelarate faster, and if you don't recalibrate your spedometer your passengers will think you are driving pretty briskly. Got lots of laughs driving an old Camaro with 4.56 gears on the interstate, with the spedometer showing 100MPH and everybody blowing right by us.

Check your spedometer on the road by using milepost signs and a stopwatch. Drive at a steady 60MPH as indicated by your spedometer. As you pass the first milepost start your stopwatch, as you go past the second milepost stop your stopwatch. If it took axactly 60 seconds your spedometer is correct. If it took more than 60 seconds your spedometer reads fast, if it took less than 60 seconds your spedometer reads slow.