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Old 05-12-2014, 08:02 AM   #7
Keith Seymore
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Join Date: May 2009
Location: Motor City
Posts: 9,165
Re: Speedometer ratio

Quote:
Originally Posted by MidLifer View Post
Cheapest is not always best. If this were me I would do the calculation using your final drive ratio and your tire size and buy the right gears for your trans.

Did your truck used to have monster tires on it? That reading sounds way off. The GPSes are right, not doubting those.
I always prefer a speedo adapter when I can get away with it.

There are too many opportunities for error in the calculations, with variation in tire rolling radius vs overall diameter, differences between actual vs published tire data, unknown rear axle ratio and/or perhaps a standard speedo gearset is not available for his monster tire. The ratio calculation and use of an adapter cuts right to the chase.

My last correction (as a result of a trans swap) I was able to find the adapter I needed on one of my shelves.

K
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