Originally Posted by
deraltekluge
How did you know? The tachometer and the speedometer are effectively geared together. If it took 2500 rpm to produce an indicated speed of 60 mph to begin with, it'd take 2500 rpm to produce an indicated speed of 60 mph no matter what tires you put on the car. By the way, a rule of thumb for changing the size of tires on cars: To go to wider tires, increase the section width by 10 mm, reduce the aspect ratio by 10 points, increase the wheel diameter by 1 inch, and you'll maintain about the same actual tire size and gearing. E.g., my car came with 175/70R13 tires, and I replaced them with 185/60R14. There are calculators available on-line
http://www.google.com/search?hl=en&n...or&btnG=Search
You are correct that a car will always give the same speed per rpm in a given gear if the speedometer is hooked into the transmission. Though in my case, the speedometer cable is hooked into the front wheel's grease cap. Its a crazy old VW design. So, the speed reading is independent from the drive train.
I guess you could argue that the speedometer was also affected by the tire size. As the expected distance traveled per rotation of the tire has changed.