Well, I followed the steps properly, but my speedometer doesn't seem to be accurate. Wondering if I did something wrong somehow, or if this is the way it's supposed to be.
With these new tires, but before having made any changes, the speedometer was showing about 6-7 mph less than I was actually going, for instance it would show 60mph when I was doing 66-67.
(My previous tires were BFG 31x10.50R15 on stock size rims. No known accuracy issues with my PSOM at that time.)
My tires now are Goodyear Duratrac 33x12.50R15, so I got my Revs Per Mile from the manufacturer's website
. The Revs Per Mile is stated as 635, which I plugged in to the formula (RpM) x 108 / 8000, giving me 8.5725, which rounds down to 8.57. I double-checked using the formula on this page
, (RpM) x 0.0135, which also gave me 8.5725.
Connected my ground, changed my PSOM (e 08 6) to 857CAL.
GPS shows I'm doing about 2mph less than the speedometer now says. If the cluster reads 30mph, gps says 28. If the speedometer says 60, gps says 58. The trip odometer shows a discrepancy as well; what my cluster reports as a 16.0-16.1 mile trip, the GPS reports as 15.8.
It *appears* to be accurate between 40 and 50 mph, but not on either side of that range.
Based on my math, using what I in good faith believe to be the correct Revs Per Mile, the conversion constant of 8.57 should be right, which means that what I'm seeing right now should be normal...? Do I leave things as they are, or waste one of my changes to set the conversion constant at the commonly-cited 8.59 (based on BF Goodrich 33's)? If that doesn't work to fix the accuracy, I'll have to waste ANOTHER change to set it back to 8.57, because I at least know that's what it "should" be.
Please advise. I don't want to let this go for too long.