Steve
The speedo accuracy will change with the tires sizes.
There was quite a bit of discussion on the list a while
back on calculating what gear you need. There are
only so may speed gears available (16 thru 20). If
they can't correct the error, you'll have to pull the
trans and change the speed drive gear in the trans.
I saw on TV (Trucks)_last week a device that connects between
the speedo cable and trans to adjust the speedo when
changing tires. They changed from stock wheels on a
F250 to Super swampers 38 inchers. Perhaps this is
an answer for all of us. For now , if you still have the
stock gear (15 tooth). try a 17. It should drop the
reading about 7 mph at 60. Of course this brings about
is issue of how to determine when you are actually
going 60 mph. Also will the odometer record 1 mile
correctly. The TigersUnited goes into how to do this.
The quickest way is to find a flat stretch of interstate
and drive at indicated 60 mph for at least 5 miles. Do
the math. For one mile in 54 sec /60 sec = 60 mph/ ? mph.
Cross multiply and divide (3600/54 = 66.67 mph). So if
you are doing 1 mile in 54 secs as indicated by the mile
posts you are actually going 67 mph. Works the other
way too. If you take 62 secs for 1 mile your area
actually doing 58 mph (3600/62). You odometer will be
off accordingly.
Dave Johnson
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