Gearhead makes a good point. A speedo that only shows 7 miles travelled over 10 true miles will give you a 30% error in gas mileage when you calculate it. Easy enough to figure out. Find a spot on a highway that has those small little signs with mile and 1/10 miles numbers on them. Actual road speed doesn't matter here. Catch the odometer reading at one sign and when you pass the next sign that reads one exact mile difference then glance at your odometer. Probably won't be exact but within 1/2 a number as it rolls into view will tell you the speedo gear is close enough. Anything over a 1/2 number can be corrected by a speedo gear change at the tranny end of the cable.
