I have a 2012 Fit Sport with auto trans on order. A quick question: Has anyone checked the speedometer reading versus gps on the Sport auto trans? Is there any speedo error compared to gps true speed reading at 60 - 65 mph?

Reason I ask is I might try and see if Discount Tire 2 miles from the dealership I am buying the Fit at will sell me upgraded tires at a steep discount in return for the effectively brand new stock Dunlop 7000s with 2 street miles on them, plus the approx 10 miles that will be on the odometer.

If I can cut a deal like that, I "might" pick the tire size to match speedo to true speed verified by GPS. For example tire sizes gives these results:

185/55/16 = 65 mph
195/50/16 = 64.09 mph (to bring a high speedo reading down)
195/55/16 = 66.17 mph (to take a low speedo reading up)
205/50/16 = 65.16 mph (break even on speedo)

I want an accurate speedo and odometer, and the 195s are a little lighter, and the slightly smaller contact patch vs 205 might give another mpg or so in gas mileage. Any of those tires would be a handling upgrade over stock.

Tires considering are: Kumho Ecsta 4X, General GMAX AS-03, and Conti Extreme contact DWS in that order. If I see any snow at all it will be a light dusting, but do sometimes encounter mud and sand.

I would like better tires than the stock Dunlops if I can cut a "trade in" deal on the new stock tires.

Edit: The car I am trading in is a 2005 Subaru WRX Wagon manual trans with Cobb Sway Bars, Michelin Pilot Sport tires, uppipe, down pipe, and ecu programmed with 71,500 miles. I need an auto trans due to left ankle injury, so am giving up the manual trans, awd, and horsepower, but want decent handling on the Fit. I'll stay with stock rims, stock springs. "Might" do sway bar(s) later.


Thanks for any info.

 

Actually, the 205s are more accurate than the 185s on a Sport... at least, on my Fit Sport MT.

My GPS always showed a slower speed than the speedometer when I had my stock tires (34 vs 35)... but the 205s matched.

 

So your speedometer was reading higher than the GPS true speed, and the 205s fixed that. Interesting. Thanks for response.

Anybody with 2009 - 2012 Sport with auto trans have speedometer versus GPS difference?

 

Picked up the 2012 Sport Fit auto trans today. The speedometer is dead on matched to GPS speed at 50,55, 60, 65 and 70 with stock Dunlop SP 7000 185/55/16 tires.

Dan

 

My tires are 215/40/17 but the speedometer error was 5% vs GPS reading.

 

Maybe your GPS is inaccurate? It calculates speed based on a myriad of variables and averages these. It might be high or low. Think about it; it usually can't calculate closer than a few meters where you are. How's it supposed to calculate with a greater degree of accuracy meters/second (or MPH) than your speedo that's measuring actual revolutions/time.

Go find some mile markers on the interstate and check to see if your odometer is correct. Check your speedometer by running a constant speed for a minute or more and checking the distance traveled and calculating the true MPH. Hint: @60mph you should traverse 1 mile each minute.

Or take it to a speedometer shop or Honda. There must be a way of calibrating the thing using software..

rummage rummage. interestingly there are diagnostics but no mention of calibration. Seems odd.

 

WAAS Enabled GPS provides accuracy better than 1.0 meters laterally and 1.5 meters vertically throughout most of the contiguous United States and large parts of Canada and Alaska, and the "myriad of variables" you mention is called triangulation, once a GPS unit gets a signal from 3 satellites it then becomes an accurate speedometer. EDIT: I would trust any GPS long before I would trust the speedo from any vehicle.

 

what's your tire pressure?

Your diameter is already 1/4" smaller than stock. Add lower tire pressures and you're getting even more differences.

As for the GPS... all I can say is, no matter what, try to verify your basis. I use the GPS, buts that's only after confirming that it was at the point where it's highly accurate and comparing against other devices, namely police radar signs.

 

Chief, WAAS purports 3 meter accuracy. That's +/- 3 meters or within 6 meters (16 feet). How's that work for instantaneous speed calculations. I'd say "all over the place."

How does it measure speed as you increase or decrease altitude (most of the time you're not travelling perfectly horizontal, Karl). It probably measures it along whatever vector you're traveling (and this involves more than 3 satellites to get a fix) which may not be accurate as far as the car goes.

And the calculated speed is only as good as the algorithms programmed. I wouldn't give it the credit you do for accuracy as far as current speed calculation goes when there's a far more simple (ok it does require measuring and calculating) and robust way to check it.

 

no offense, but its +/- 1mph... i do at least +5 to +10 speed limit on highway on the middle lane

is there something that requires you to having such an exacting speedo?

 

4 satellites gets you altitude, and in general I've found that I'm getting 5 satellites most of the time. In those cases, you can measure the velocity vector including altitude. Moreover, 3m accuracy can be really good if you measure across a long distance. Ie drive 1 mile according to the GPS. You've got a variation of 6 m across 1 mile.

 

Edit: also because what I've found is that the errors tend to be a constant displacement rather than a widely varying position, the errors tend not to matter for measuring changes in position.

 

But the speed display is not the average for the last mile. It's a bit shorter.

It measures change in position over time to get an approximation at speed. This will be more inaccurate going around curves in addition to up/down hills.

I'm always amused at how fast they show I'm moving when I'm standing still...

Either way I wouldn't be changing the tire size to get more accuracy from the speedometer. Knowing how far off it is would be useful. It should register high by design; if it were to register low, the car manufacturers would set themselves up for lawsuits. I think they always build in an error margin so it registers high.

 

From what I understand, most GPS units can actually determine velocity independently of position, and generally to a much greater accuracy than by figuring the change in position over time. Some GPS units use the velocity to give greater precision to their position fixes.

Unless a particular GPS had been proven to give inaccurate speed results, presumably because of an error in its programming, I'd trust it to be more than accurate enough for checking a car speedometer. It's probably more accurate than trying to maintain a steady speed through a measured mile, particularly if there are any hills or curves over the mile.

My Fit's speedometer appears to be right on, so far as I can tell. That's true for just about all modern cars, I think. My first car, a 1984 Mazda 626, read quite a bit too high at freeway speeds; IIRC it was approaching 10 MPH off at 75 MPH. It was something of an epiphany for me when I discovered that; it explained why everybody else on the interstates, even the grannies, were passing me like I was standing still.