Does your tripometer work (if it does, never reset it when the car is moving->it will break the teeth off the gears)? You need to get the tenths of the miles to check the odometer.

I use the mile markers along the highway. The longer the drive the more accuarate. As you pass the mile marker, note the mileage on the tripometer (to the tenth), then drive for several miles, and note the mileage on the tripometer. The difference is error.

For example, as you pass mile marker 112, note the mileage on the tripometer (lets say 14.5), then drive for a while (lets say 100 miles), so when you pass mile marker 222, note the mileage on the tripometer (lets say 124.7). Ok, you traveled 100 miles, but you tripometer read 110.2 miles (124.7-14.5 = 110.2). Your odometer would be 9.3% fast (1-[ 100/110.2]=0.093 or 9.3%).

Paul