i have a scan gauge II and was wondering what the percentage difference is to change in my scan gauge so my mph is correctly shown on my scangauge II. i have 205/50/15 tires.

i came out with 2.8% difference in speed. BUT is this negative or positive? which would basically mean when i am driving at 60 mph, i am actually driving 58.3 mph.

 

That is correct. The speedo is based on the number of revolutions of a stock sized tire to go 1 mile. Your tires are 2.8% smaller than stock and will therefore revolve 2.8% more times to get to a given distance. If the scangauge read an accurate 60 MPH with stock size tires when you were going 60MPH, then now you need to tell it to add 2.8% to get an accurate speed, since a speedo reading of 60 MPH now is only 58.3 actual.

 

That's assuming your speedo was calibrated properly to begin with, which is a pretty big assumption. When I was running stockers, my speedo was fast by about 3%. Going to 185/65 15s from stockers brought the built-in speedo up to the point where it's pretty much right on. I still went out and calibrated the ScanGauge's speedo with a stopwatch.

 

^^^ what he said.

Actually, our xB--the bB in Japan--was originally designed to run on those 185/65x15 tires. The JDM spec was for that size, and most folks agree that our stock OEM size of 185/60 was the cause of the 3% speedo error.

Absolute best way to calibrate your ScanGuage--recommended in the SG owner's booklet--is to set your cruise to say 65 and use a portable GPS to find your true MPH, then keep entering adjustment clicks on the SG until its MPH equals the GPS. (If you REALLY want to nail it, they suggest temporarily changing both SG and GPS to KPH to give you more accuracy--more, smaller units)

Garmin 340 is a great GPS, usually runs about $450, but maybe you could borrow a friend's?

 

And then there's the el-cheapo way: buy a stopwatch, find an uncrowded road where you can drive a rock-steady 60mph, and see how long it takes to go between mile markers. The more the better, obviously. (Less fun than a GPS.)