From the T5 speedo gear thread above, I need a 7 tooth and 22 tooth for my 4.10 rear end and 245/50-16 tires. However, after being on the dyno rollers Saturday, my speedo is off by 5-10 MPH at 70 MPH, even with a 7/22 combo. How can I be off that far off? I guess I'll have to go with a 7/23 combo.

 

Check your odometer against a measured stretch of highway. I like to use about a 10-mile stretch of relatively straight expressway.

Remember, the odometer is nothing but a turns counter; 1000 turns of the cable = 1 mile. Doesn't get more basic than that. The number of times the cable turns is dependent on the drive shaft rotation, and the speedo gears; nothing else. That's MUCH more basic to the speedo gears than the speedo, which has little flying magnets that sort of "drag" the drum around that the pointer is attached to by induction, and can be affected by the strength of the little magnets, if the little U-shaped piece they're mounted on has been bent a little, and various other uncertainties in the speedo head in addition to the rate at which the cable spins.

If that's right, then you just need to locate another speedo head, that's a little more accurate.