Speedo / Tach / Gauges: 50 Champion speedometer way off

[doubledaddy]

My 50 champion has a large error on the speedometer, when it reads 30 the speed is 25 and when it reads 60 the speed is 52, checked with a GPS speedometer and also by following a car running cruse control at 60, my speedometer said 68. I checked the rear axle, it is a 4:56 and the pinion gear has 17 teeth, part number 520089. I have 195/75/15 tires which measure 26.5 in diameter. This is close to the original 640-15 diameter. I can understand a small error, 1 or 2 MPH but 8 MPH is too much. Is there anything else I need to look at?

Thanks

[studegary]

At that time, up to 10% was a reasonable/acceptable error. You are slightly beyond that. The difference in tires may make up the difference that you observe (beyond acceptable).
There are boxes that can be installed to correct speedometer error or you could just note some reference numbers on your speedometer dial.

[RadioRoy]

Can you find a speedometer driven gear with one more tooth?

[doubledaddy]

I can't tolerate that large an error. I had 670-15 goodyear bias ply tires on it which are about an inch larger in diameter but they were like driving a model T on a dirt road, I have done that in my younger days. It had the current 195/75s on it when I bought it but I wanted the "WIDE WHITEWALL LOOK" and had the tires laying around so I installed them. Big mistake. They did however drop the error to within 3 MPH. A set of 215/75/15 on the rear would just about kill the error but would probably kill what little acceleration the car has.

The pinion for the 4:88 axle would most likely fix the error, it has 18 teeth, part no. 520090. It's available from at least three sources. Never seen a 4:88 but the chassis manual states it's for cars sold for export to mountainous areas. I may try that, it would be the easiest way.

[doubledaddy]

I just found another problem. The 195/75/15 tires measure 25.5 in diameter though the conversion charts state 26.5 in. This makes the tires I have are too small which adds to the speedometer error.