Scangauge II - Calibrating w/ the correct Speed setting

I finallly got the Scangauge II and had it installed using Detective_Dan's mod. I'm having issue with determining the correct difference in odometer vs actual speed. Since I need to input that for the Speed settings to calibrate the Scangauge II.

I'm currently running 285/85/16 BFG A/T tires (replaced the stock 265/70/16 BFG R/T tires).

Due to the change in tire size, the odometer is a little off and the Scangauge needs to be calibrated to reflect this in the Speed setting. This is done by entering the actual speed you're going at as captured by a GPS or speedometer app vs the odometer reading.

According to this tire calc it shows a MPH difference of nearly 13% (65mph vs 74.49mph) but what I'm seeing on my odometer vs GPS and speedometer app is actually closer to 7% (60mph vs 64mph).

http://www.tacomaworld.com/forum/tirecalc.php?tires=265-70r16-285-85r16

What gives? Which one is correct, my GPS/speedometer app or the tire calc?

 

In my years of Speedometer repair and re-calibrating, the two are married. Also, since I did a bunch of GPS stuff in the latter years, I know the accuracy can vary in a single GPS, depending on a variety of things. I would use tire roll-out calculations for ACTUAL Mileage to determine the Speedo needs. The odometer will fall in line as it depends on the speed input, to add up the mileage. It does that based on the speed signal input , just like the speedo. You should not be able to separate the two calculations at all.

I don't know if anyone else is aware, but I've checked a few Tacomas with factory "everything" and they all say you're going Faster than you are actually going, by a couple of percentage points. This will giva a false MPG reading higher than the actual MPG because you're using a bogus signal. All of them were off the same amount (VERY CLOSE, at least). This could help toyota bolster false MPG capabilities, right? If all of them are off a little?

The Accuracy of speedometers and Odometers are required to be "give or take 2%" to be legal. The accuracy can be changed by tire wear, so this is what they settled on as "close enough" at the federal level.

I don't know about you guys, but I don't trust anyone to give me true info when it comes to stuff like this. I just do the leg work and have an accuracy check done locally, then, if any corrections are required, I work on that. Since I know my Speedo is within legal parameters, I probably won't mess with this one. BUt I also know I'm racking up around 2% more miles on my truck's ODO, so maybe I will eventually feel the need to fix it. Most of our vehicles are off one way or another... If it's reading fast, it raises your MPG, if it's reading slow, it will kill your MPG, but low MPG may not be what's happening at all, but you will be getting lower mileage racked up on the speedo... Is the trade-off worth it? Your call>

 

Thank you for chiming in with your experience.

The truck's speedometer definitely shows a 1-3 mph faster than the ODB II-based speed shown on the Scangauge II (the ODB II speed, I'm guessing, has to be precise, unlike the truck's speedometer which is allowed a 2% margin of error).

Still...That doesn't resolve the issue of whether or not the tire calc is correct and if I should be using the tire calc's data or the GPS/speedometer app data to calibrate the Scangauge II's speed setting.

 

Sorry, missed this thread earlier this year.

A few thoughts :

My experience with my Tacoma has been:
1. When purchased new, odometer was less than 1% error, speedometer was off 2-3 miles per hour, ie real speed of 60, speedometer was 62/63. One theory is manufacturer does this deliberately to avoid legal problems.

2. When about a year old, bought a ScanGauge II, calibrated it by distance multiple times by mile markers to get virtually zero error. Adjusted several times for gallons used compared to pump, and while not recommended, filled to the very top on 2 consecutive fillups at same pump (note - both times immediately drove enough to burn at least 2.5 gallons, which is what I was able to add after pump "clicked off" the first time, to avoid problems with the charcoal canister).

3. Did not really consider time wear over the next 20,000 miles, but should have. When tires replaced at 37K miles, different brand but same size and specs (tire diameter,revolutions per mile) 245/75-16. Only difference was the Michelins tread width was .4 inch wider, overall tire width was the same. Compared to new tires diameter had dropped from 30.5 inches to 29.5. I plan on rechecking ScanGauge every 10k miles and recalibrating distance by mile marker if needed.

The reason for above was a 4% difference in my odometer and the SG on miles at fillup (truck recording more miles than SG).

Conclusions :

With speedometer/odometer/ScanGauge driven by speed sensor (and even older vehicles that were mechanical), accuracy WILL change with tire wear - I see NO way around it, as the speed sensor AFAIK has not way to account for tire wear).

With SG adjusted per above, I have found 2 things that still introduce errors. One is variances in filling up at the pump, ie different pump pressure, how level the vehicle is, maybe even whether someone is using the other side of the pump. The only other variation I have seen is a little error if I change by driving habits substantially - ie normally mostly city, some mixed city/highway, and highway only.

I am very pleased with the ScanGauge, but be aware it takes a little maintenance per above for accuracy.