recently i have noticed that when i am speeding up, still on first or second speed, the rpms will get as high as 6000, but there is no way i'm doing that much. Also at high speeds, i noticed that the MPH are off by about 10 mph. Is there any way to fix this? Any help would be gladly appreciated. Thanks

1990 Chevy Lumina
160,000
3.1 L V6

 

wrong size tires for the mph possibly, or wrong gear ratio, and ive heard of stock tachs being off by as much as 1000- 2500 rpms.