No, no, no! the speedo is just fine and dandy. Just as I
posted, different tires, different gear ratio. The 5 mph
error is pretty consistent over all of my average driving
speeds. It works at 65, works at 50, works at 35 and even
down to 15 or 20. All run pretty close to 5 mph off according
to the GPS. When I'm in a speed zone all I do is keep it about
5 under and it's close enough. Had the local copper shoot me a
few times with his speed trap in a 20 mph school zone and he
said I was within a mile per hour every time. I was holding
it at 15 indicated at the time.
The reading on the head should be fairly consistent just as it
would be if you changed the speedo gear in the transmission or
used an ratio adapter off the transmission for a correction.
Percentage wise....yep. the difference percentage wise of
5/15=.33% vs 5/50=.10%, etc. So....as Jim posted, I don't
think you can use the percentage as a constant for the distance.
If I'm indicating 55 mph and travel for one hour my odometer
will read 55 miles traveled. In reality I've traveled 60 miles.
So at this speed, I've traveled 5 miles further on my tank of
fuel than my calculations would indicate using a direct reading.
So using the additional miles traveled with a correction means
that the mileage that I got on what I thought was a 154 mile
trip in reality was closer to 170 miles which means my
mileage for the day was more like 14.5 mpg.
And this would account for the drop in mileage around the time
I rebuilt the engine. Just prior to that I had change tire
sizes and installed the 4.10 gear. Then just last summer I
changed tire sizes again, each time calculating my mileage
without a correction. And thinking my mileage had gotten
worse then it was when when I first got the truck.
Thanks for figuring that out for me guys.
I'm happy with 14.5 given the other problems that winter
driving can impose on fuel mileage.
dg
Last edited by Denny Graham; 02/15/2016 6:06 PM.
