[cadzilla74]

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.

 

[FloridaRT]

Sounds like you need to buy a Superchips Flashpaq.

Bob

 

[Old Navy]

So you are risking some very expensive repairs out there later with the 10w-30 oil, hope it's synthetic so it will get you through turn in time. There is a reason for the 5w-30 oil and it is a heck of a lot of $$$ to risk with wrong oil viscosity as MDS will not function properly with the heavier oil and will eventually fail due to sludg build up.

MB cars with same system had same problem and Chrysler MDS has also failed with improper oil viscosity. So you might want to reconsider your oil choice. But hey no big deal if you don't we will still love you and the car. :mrgreen:

Well ok who am I fooling, just the car. :Na_Na_Na_Na:

 

[mikeyb]

I'm going to get GPS readings before and after with the new tires. Even with the stock tires the speedo can be off. My Ram with the stock tires is 2mph off at 70mph. Indicates 70 but actually doing 68mph.

MikeyB

 

[cadzilla74]

Old Navy, I'm not using 10w-30 oil. I said I thought my mileage had dropped because MDS was not working because the dealer put in 10w-30 by mistake. After adjusting the mileage based on the incorrect odometer readings because of the different tire size, there is nothing wrong with my MDS. The car has the 5w-20 in it I specify every time I get the oil changed.

Florida RT - the SuperChips would solve the problem. I've got some more pressing needs for liquid assets at the moment so I'm not ready to go that route yet.

mikeyb - I'll be curious to hear if you see results similar to mine. Let me know.

 

[RobAGD]

GPS vs Stock I was also off 2mph at 65-70

With the new shoes I am down to within .5

The Garmin V is an Awsome GPS, too bad Garmin didnt make an updated version of it when they did all thier newer color big memory units :/

-R

 

[Old Navy]

Old Navy, I'm not using 10w-30 oil. I said I thought my mileage had dropped because MDS was not working because the dealer put in 10w-30 by mistake. After adjusting the mileage based on the incorrect odometer readings because of the different tire size, there is nothing wrong with my MDS. The car has the 5w-20 in it I specify every time I get the oil changed.

:not_worth Whew, I am greatly relieved. I guess I just read through too fast and read someting into the statement that was not there in reality. Sorry dude. :not_worth

 

[cadzilla74]

No problem old friend. Hey, it only took me one wisecrack about having the wrong oil to get yer blood boilin' and you back in the game. Nice to see ya again!

 

[Old Navy]

No problem old friend. Hey, it only took me one wisecrack about having the wrong oil to get yer blood boilin' and you back in the game. Nice to see ya again!

Thanks man. I have just been reading/lerking here mostly for a long time now, maybe I should stop, log in and post more often. :thumbs_u: :mrgreen:

 

[MAGNUM8R]

My KFG KDW 255/55/18 tires measure 28.5" I just put the number in the Flashpaq, and all set.
The SC tuner is the way to go!!

 

[nick_danger]

I always did light math in my head to assume that my bigger KDWs were the reason for my lower fuel consumption... I'm not going to adjust it ever - I'll get 6% more miles out of my warranty! LOL 7yr/74,000 mile!

 

[quarky42]

I have started using Google Earth to measure out "known paths" down to two decimal places. (Like most science the least significant number is "guessed" at or approximated if at all possible. There is a way to approximate the 2nd decimal place.)

I usually set the cruise on flat highway for a speed, start my stop watch as soon as the bumper passes a "landmark" (like a sign that I can see on Google Earth) and stop the stop watch as soon as it passes the ending landmark.

For those unfamiliar with it, Google Earth is a free program from http://earth.google.com and is definately worth the download to play with! I am using the patch measurement tool to measure up the distance I travel.

I have compared this method to GPS when my brother-in-law visited and it was dead on perfect within the accuracy limits of my speedometer.

 

[RobAGD]

humm, why not just use a highway marker 1 mile in x seconds ?

-R

 

[67alecto]

According to a tire size calculator, your speedo should be reading 1.466% too slow

http://www.1010tires.com/TireSizeCalculator.asp

 

[cadzilla74]

67alecto, yep I used that link (I was payin' attention to the many threads here before buying the tires) and I expected to only be off by 1.5 or 2 % either way. The GPS saw 5 satellites and had 3 locked on, I gotta figure it was right on. My speedo is definitely reading 4% slower than I'm going. It was probably already reading slower with the stock OEM Conti's. I just didn't do a GPS check before getting the new rubber.