New BFG KDW's and a little math

Well I really like the BFG KDW2's but I was wondering just how far off the numbers were on my 05 Mag RT since there is no way to set 255-55-18 tire size on my car even with a StarScan. So I got my hands on a Garmin V and with a co-pilot I did some real-world testing to see what's what. Here are the results.

First a compass test. EVIC on and set to compass screen. Garmin showed 91 degrees (almost exactly due east) and Mag said 91 degrees. A few more turns and it was official. The onboard compass is a winner, very nice and accurate.

OK, on to the real stuff. I was a bit worried about speedo being off, also odometer and mpg calculations.

EVIC screen set to digital speed readout. Mag speed 40mph using cruise control, GPS 41.5mph. Mag 60mph using cruise, GPS read 62.4 to 62.5. Mag at 65mph, GPS 67.6. This means the speedo is off by 4% now. If I want to do 70 around here I need to set my cruise to 67.

Ododmeter readings. Stopped the car and reset Trip A odo to 0. Reset GPS distance traveled to 0. At the split-second my Trip A hit 3.7 miles I came to a stop. GPS read 3.95 miles. This means the car is logging 6.33% LESS miles on the odometer.

The odometer reading is also what calculates MPG via EVIC so my mileage is also off by 6.33%. I was worried that my mileage was dropping way off for some reason, even suspected 10w-30 oil was put in by mistake and MDS had quit.

But let's look at the last 2 tanks of gas.

1. 16.88 gallons and odometer read 22,892 - 22,616 since prior fill-up. That comes to 276 miles divided by 16.88 which shows 16.35mpg OUCH.
Add 6.33% to 276 miles and you get REAL distance traveled of 293.5 miles divided by 16.88 which yields 17.39mpg. Much more normal.

2. 15.46 gallons and odometer read 23,173 - 22,892 since prior fill-up. That's 281 miles divided by 15.46 which shows 18.17mpg not bad BUT...
Add 6.33% to 281 miles and you get 299 miles divided by 15.46 and you get 19.34mpg.

3. Add 19.34 and 17.39 and divide by 2 and you get an average of 18.36mpg which is almost EXACTLY the 18.4mpg I observed for the first 22,000 miles of driving this car.

Well there ya have it. Your mileage may vary. But the very minor difference in tire diameter and circumference really adds up.