max_stirling Posted June 22, 2006 Share Posted June 22, 2006 I performed a search and didn't find anything relevant. Anyway, I recently got a TomTom Go 910 navi system and I've notice that the GPS unit is measuring 1-2mph less then my speedo reading. Has anyone else noticed this? Is the GPS unit off or is the speedo off? I try to hold the speed steady on a flat road for at least 5 sec before taking the readings. Just curious, but might influence a tire size purchase in the future. I'm considering installing 235/45x17 tires on the stock rims vs 235/40x17 that are closer to stock. A tire calculation site states that the 45 series tire will be about 2mph > than stock @ 60mph, but if my speedo is ~2mph optomistic @ 60mph, then the larger tires will 'correct' the speedo. I would also like a little more sidewall on my tires as well. Any thoughts? Link to comment Share on other sites More sharing options...
Rollin Wes Posted June 22, 2006 Share Posted June 22, 2006 I've noticed my speedometer runs about 1-2mph faster than what those mobile radar trailers show me running when passing them on the road. Link to comment Share on other sites More sharing options...
rallispec Posted June 22, 2006 Share Posted June 22, 2006 my speedo runs 2mph faster than what i measured using a Garmin GPS Link to comment Share on other sites More sharing options...
BDII Posted June 22, 2006 Share Posted June 22, 2006 On WAAS I read 2 MPH slower too. Dont know the answer the to your question though... Need forum help? Private Message legGTLT Link to comment Share on other sites More sharing options...
bosco Posted June 22, 2006 Share Posted June 22, 2006 most vehicles, cars, bikes, read faster so you don't get in trouble. a lot like building in understeer on a car to make it safer for the masses. bosco Stay Stock Stay Happy Link to comment Share on other sites More sharing options...
fweasel Posted June 22, 2006 Share Posted June 22, 2006 My speedo is about 1.7 off at 80MPH vs. my kenwood GPS. I've read at least one other thread on this where nearly every GPS user confirmed the same trend. ignore him, he'll go away. Link to comment Share on other sites More sharing options...
Deer Killer Posted June 22, 2006 Share Posted June 22, 2006 The speedo is fast, putting on 225x45 got me "close enough" to real speed, but still a fraction faster Link to comment Share on other sites More sharing options...
snoman Posted June 22, 2006 Share Posted June 22, 2006 All Subarus are typically calibrated to read 2-3 MPH faster than your actual speed. My 1995 Legacy was the same way. I think this is a good thing... I would much rather have the speedo displaying a faster speed than a slower speed like Chevrolet speedometers do. Link to comment Share on other sites More sharing options...
Scotty Posted June 22, 2006 Share Posted June 22, 2006 By law, the allowable tolerance for a speedo is +0, -3. Link to comment Share on other sites More sharing options...
Jon in CT Posted June 22, 2006 Share Posted June 22, 2006 By law, the allowable tolerance for a speedo is +0, -3.I doubt there is such a law. Link to comment Share on other sites More sharing options...
IwannaSportSedan Posted June 22, 2006 Share Posted June 22, 2006 Jon, I can see where you think that... But I think you ascribe decent common sense to the government. Where there is none... I would not put it past the government one millimeter. It is probably buried in some over-reaching policy by the US DOT. An agency of the Congress' policies have the force of law, or nearly that. Congress need not have voted on it, but by creating and authorizing the DOT, the DOT's policys become an agent of congress act. Link to comment Share on other sites More sharing options...
PGT Posted June 22, 2006 Share Posted June 22, 2006 actually, it seems to be plus or minus 10% at 50 or 60mph, but there's no specific legal stuff I could find. My Garmin shows my speedo is accurate, which is odd, since I'm running 225/45/17's. Link to comment Share on other sites More sharing options...
karlxmas Posted June 22, 2006 Share Posted June 22, 2006 My garmin register faster than indicated. I am also on the oversized tires. Link to comment Share on other sites More sharing options...
LotusSeven Posted June 22, 2006 Share Posted June 22, 2006 On my 05' LGT the "built-in" speedo error seems to be a constant percentage, rather than a MPH "offset". LGT speedo reads 60 mph and Garmin Foretrex 201 = 58.4 mph LGT speedo reads 80 mph and Garmin Foretrex 201 = 77.9 mph LGT speedo reads 95 mph and Garmin Foretrex 201 = 92.5 mph (Normal disclaimers apply: Closed course, professional driver, etc., etc.) :-) Goodyear F1 GS-D3 215/45 ZR17 tires with less than 10K miles Speedo error = + 2.6% Heavy Metal Killed Beethoven Support Classical Music Link to comment Share on other sites More sharing options...
max_stirling Posted June 23, 2006 Author Share Posted June 23, 2006 Thanks for the confirmation everyone, when I do replace my stock tires, I'm going to get the 235/45x17. Though 2.6% difference is not a lot, it does mean that you are travelling 2.6% less then what your odometer is reading as well. Link to comment Share on other sites More sharing options...
Ric Posted June 23, 2006 Share Posted June 23, 2006 Thanks for the confirmation everyone, when I do replace my stock tires, I'm going to get the 235/45x17. Though 2.6% difference is not a lot, it does mean that you are travelling 2.6% less then what your odometer is reading as well. Not necessarily. All car manufacturers aim to have the speedo read a little high. That doesn't mean the odometer will be high by the same amount. See if you can check it over an exact known distance. Link to comment Share on other sites More sharing options...
skylimey Posted June 23, 2006 Share Posted June 23, 2006 Well as an unscientific data point, I drive Pasadena to Turlock pretty frequently, and in my Q45a it was 301 miles, in the Mercury rental it was 301 miles, in my LGT it is 310 miles. So something is a bit off, but other than running out of warranty sooner, there's no real downside. From Legacy to SVX to SVX to Q45a to LegacyGT Link to comment Share on other sites More sharing options...
ilh Posted June 23, 2006 Share Posted June 23, 2006 I'll add my data. According to my Garmin eTrex Legend, my speedo reads about 1.5mph too high. This is with stock tires (when they were new). I have confirmed this by timing a measured mile. --Lee Link to comment Share on other sites More sharing options...
Scooby Fan Posted June 23, 2006 Share Posted June 23, 2006 Some "New age" speedos are adjustable to correct these errors. My brother drives a Turbo PT Cruiser (I'm embarrassed to be related to him:lol: ) and he experienced the same problems you are discussing. Her just took it to his Chrysler dealer and had it adjusted to agree with the GPS. Anyone know if that is possible with Subarus?? BTW there are NO speedos in any car that are totally acurate...if you have one, it is a fluke. Top end cars are the worst...Porsche, Ferrari etc:confused: ....some are out 20%.....Road & Track did a big article on this some years ago. Link to comment Share on other sites More sharing options...
LotusSeven Posted June 23, 2006 Share Posted June 23, 2006 Thanks for the confirmation everyone, when I do replace my stock tires, I'm going to get the 235/45x17. Though 2.6% difference is not a lot, it does mean that you are travelling 2.6% less then what your odometer is reading as well. Not necessarily, the odometer is a separate digital instrument. Even though (on my LGT) the speedo reads 2.6% high, the odometer seems to be off only about 1% (high also). This will presumably get worse as the tires wear and the wheels turn more revolutions per actual mile. I think the speedo calibration is probably "designed in" but the odo error is likely just normal production variability. Heavy Metal Killed Beethoven Support Classical Music Link to comment Share on other sites More sharing options...
Deer Killer Posted June 23, 2006 Share Posted June 23, 2006 Some "New age" speedos are adjustable to correct these errors. My brother drives a Turbo PT Cruiser (I'm embarrassed to be related to him:lol: ) and he experienced the same problems you are discussing. Her just took it to his Chrysler dealer and had it adjusted to agree with the GPS. Anyone know if that is possible with Subarus?? BTW there are NO speedos in any car that are totally acurate...if you have one, it is a fluke. Top end cars are the worst...Porsche, Ferrari etc:confused: ....some are out 20%.....Road & Track did a big article on this some years ago. Ours is adjustable, good luck getting a dealership to do it. I think most think of their SSM as a fancy paperweight. Link to comment Share on other sites More sharing options...
ebpda9 Posted June 23, 2006 Share Posted June 23, 2006 my speedometer is dead on with the hp ipaq navi system Link to comment Share on other sites More sharing options...
recuomo Posted June 23, 2006 Share Posted June 23, 2006 Same issue with my '05 LGT 5EAT based on my 2 Garmin GPS. Exactly 2% error over 100 miles. I have checked it on 4 trips NY to VA. Link to comment Share on other sites More sharing options...
Garandman Posted June 24, 2006 Share Posted June 24, 2006 I performed a search and didn't find anything relevant. Anyway, I recently got a TomTom Go 910 navi system and I've notice that the GPS unit is measuring 1-2mph less then my speedo reading. Has anyone else noticed this? Is the GPS unit off or is the speedo off? If the GPS was off in steady speed your location would be off as well. It is buffered. One thing I do find "off" is max speed - it seems to use too short a gate. I don't think our Chevy Express really tops out at 149 mph..... We have GPS in all our company vehicles and the salespeople have them as well. The Subaru is typical, about 2-3 mph slow at 75 mph. Nissan Quest about the same. A Honda Accord V6 is the closest, almost dead on at 75 mph. A Chevy van is 2-3 mph slow. A Ford van (replacement tires) is more like 4 mph off at 75 and always has been. Who Dares Wins スバル Link to comment Share on other sites More sharing options...
sarcomax Posted June 24, 2006 Share Posted June 24, 2006 You go to turlock on purpose? Link to comment Share on other sites More sharing options...
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