Shep2112 Posted October 25, 2006 Share Posted October 25, 2006 Hi, has anyone else ever had this problem? With the original factory tires, and with replacement tires matching the original size, my speedometer reports about 5-6MPH above actual speed at 40-45. As I increase speed, say to 75MPH, it's off by 6-7MPH. I've verified this by signs saying 'Your speed is:', pacing with other cars while talking on the phone with them to verify their speed vs mine, and also by getting speeding tickets for lower speeds than I was expecting.. The dealer told me my tires were the wrong size (in between stock and current, I had a set that were one size larger than required, because when I got them changed, they didn't have my size in stock), and that was the problem. I got tired of the wrong speed and being told that, so I eventually bought a new set of the right tires. My speedometer is still reporting wrong. I contacted Subaru through their website and they told me there's a tolerance of 3-5% I think and said basically that me having the wrong speed isn't anything unusual. My question is, what can I do to get this straightened out?! My speed is always incorrect, no matter what my tire inflation is. I'm sick of subtracting 5-6MPH every time I look at the dash. HELP! thanks in advance -Eric Quote Link to comment Share on other sites More sharing options...
ericem Posted October 25, 2006 Share Posted October 25, 2006 You know what, i have a larger size tire on my 93 legacy, and my speedometer is perfect, i used my gps, but mine runs cable. With the sensors i guess there picky, there is probably some way to tune the speedometer, do u have a gps? Its more accurate, and helpful. Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted October 25, 2006 Author Share Posted October 25, 2006 Wow, thanks for the quick reply! I don't have a GPS at the moment, but if HP comes through and replaces my iPAQ phone with another one like they're supposed to, I'll have a GPS soon. Hopefully by next week sometime. But I've checked it with the 'your speed is:' things by on-ramps and school zones, and also checked my wife's Matrix on the same ones, and hers comes up accurate, mine always shows lower than what the speedometer reads. I don't know if I could use a variable resistor or potentiometer or something inline to calibrate the sensor? I'm not sure how many leads there are, etc. Anyone? Anyone? Bueller? Quote Link to comment Share on other sites More sharing options...
WoodsWagon Posted October 25, 2006 Share Posted October 25, 2006 Run bigger tires. I've noticed lots of legacy's 95+ read about 5 under. If you up the tires size, you will bring the car's speed up to what the spedo reads AND you'll be getting more ground clearance. Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted October 25, 2006 Author Share Posted October 25, 2006 that seems like a great idea, except I was told by the dealer the larger tires were causing the error (when I had them).. No one seems to be able to work out the math on this.. Half the world tells me bigger tires equals faster ground speed, others tell me the opposite, all with varying reasons based on how the sensor works, etc.. More clearance would be nice, would it affect my pickup and center of gravity though? Also, I only have about 8,000 miles on these tires, so I don't really want to toss them.. I'd rather just fix the speedometer problem, instead of doing a hack, trying to figure out the size required, etc. Quote Link to comment Share on other sites More sharing options...
WoodsWagon Posted October 25, 2006 Share Posted October 25, 2006 my tires are 20% larger than stock, so they cover more ground per rotation than the origionals. All the spedometer measures is the number of rotations per minute each axel makes. The larger the tire, the lower the spedometer reads realtative to the same ground speed. My spedo reads 10 under, more at higher speeds. Really easy to get in trouble. Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted October 25, 2006 Author Share Posted October 25, 2006 yikes, yeah i'd like to avoid getting into trouble.. haven't had a ticket in 4 or 5 years It just be nice to be in a 65 or 70MPH and know how fast I'm really going. Or stop wondering why those people behind me think I'm going too slow, etc.. So is the sensor like a generator and the fast it goes, the more voltage or current is produced, and therefore the higher the needle goes, or is it something more than that? Quote Link to comment Share on other sites More sharing options...
johnceggleston Posted October 25, 2006 Share Posted October 25, 2006 there have been some recent threads on this, look for VSS or speed sensor. i think nipper posted a rply about how it works. larger tires will make the car travel faster than the current speedo reading, regardless whether or not the speedo is accurate. but the problem is you'll be guessing what size to buy unless you can test drive some one else's tires/wheels. if you can borrow some ones front wheels/tires, and drive with the FWD fuse in just to see how they work. that way you don't have to swap all 4 just to test, assuming it's an a/t. yikes, yeah i'd like to avoid getting into trouble.. haven't had a ticket in 4 or 5 years It just be nice to be in a 65 or 70MPH and know how fast I'm really going. Or stop wondering why those people behind me think I'm going too slow, etc.. So is the sensor like a generator and the fast it goes, the more voltage or current is produced, and therefore the higher the needle goes, or is it something more than that? Quote Link to comment Share on other sites More sharing options...
johnceggleston Posted October 25, 2006 Share Posted October 25, 2006 double post error Quote Link to comment Share on other sites More sharing options...
Chip Posted October 25, 2006 Share Posted October 25, 2006 The easiest and most foolproof way of validating your speedo is by using the odometer. Go to the nearest highway which has markers every 1/10 mile,(all interstates have 'em), and as you pass by one of the mile markers, set your trip thingy to zero. Drive about 10 miles to the next mile marker and compare your mileage. If your trip odometer says 11 miles, your speedo is reading too fast,(about 10%)...if less, than your speedo is reading too slow. Quote Link to comment Share on other sites More sharing options...
johnceggleston Posted October 25, 2006 Share Posted October 25, 2006 in virginia, the cops claim to catch speeders with airplanes. and in order to accomplish this they have painted big white stripes across the roads all over the state. the 3 stripes are 1/4 mile apart, 1/2 mile from first to third. so when you pass one you just time your self to the next one. then it's just a math problem. MPH = 900 / (qtr. mile seconds) example: 900/15 sec. = 60 mph or 900/12 sec. = 75 mph so come on down to VA and test your speedometer and your luck. my only trouble is that my new cell phone doesn't have a stopwatch, my old one did. The easiest and most foolproof way of validating your speedo is by using the odometer. Go to the nearest highway which has markers every 1/10 mile,(all interstates have 'em), and as you pass by one of the mile markers, set your trip thingy to zero. Drive about 10 miles to the next mile marker and compare your mileage. If your trip odometer says 11 miles, your speedo is reading too fast,(about 10%)...if less, than your speedo is reading too slow. Quote Link to comment Share on other sites More sharing options...
nipper Posted October 27, 2006 Share Posted October 27, 2006 Hi, has anyone else ever had this problem? With the original factory tires, and with replacement tires matching the original size, my speedometer reports about 5-6MPH above actual speed at 40-45. As I increase speed, say to 75MPH, it's off by 6-7MPH. I've verified this by signs saying 'Your speed is:', pacing with other cars while talking on the phone with them to verify their speed vs mine, and also by getting speeding tickets for lower speeds than I was expecting.. The dealer told me my tires were the wrong size (in between stock and current, I had a set that were one size larger than required, because when I got them changed, they didn't have my size in stock), and that was the problem. I got tired of the wrong speed and being told that, so I eventually bought a new set of the right tires. My speedometer is still reporting wrong. I contacted Subaru through their website and they told me there's a tolerance of 3-5% I think and said basically that me having the wrong speed isn't anything unusual. My question is, what can I do to get this straightened out?! My speed is always incorrect, no matter what my tire inflation is. I'm sick of subtracting 5-6MPH every time I look at the dash. HELP! thanks in advance -Eric both my scan gauge and GPS read a differnce, the gps about 4-5 mph, the scaqn gauge 5-6 mph. This is one of those things that go under so what. Depending upon tire inflation and wear this is not unusual. Also the spedos are sometimes calibrated on the safe (low) side. nipper Quote Link to comment Share on other sites More sharing options...
Hodaka Rider Posted October 27, 2006 Share Posted October 27, 2006 Every car I have owned does this. A major car magazine (don't remember which one) commented on this a while back. Here's some theories: 1) Tire sizing can vary something like 3% from what's on the sidewall. If a manufacturer made the speedo 100% accurate with OEM tires, and a consumer got a ticket after putting on different tires that happened to be on the high side of the spec, there could be a lawsuit. 2) There is a conspiracy that the government is involved in. When they changed the speed limits on most roads from 50 to 55, they made all the manufacturers doctor their speedos to read 5 high, so that people were still only going 50. Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted November 1, 2006 Author Share Posted November 1, 2006 Thanks for your replies everyone. I'm really not interested in buying a third set of tires for my car just yet though. I'd rather just correct this inconsistency somehow.. I'll check the odometer via the 10 miles of markers, that sounds like a good idea. I usually only check for a mile or two and obviously that's not going to show much.. Is there any way to recalibrate something, somewhere? Quote Link to comment Share on other sites More sharing options...
cookie Posted November 1, 2006 Share Posted November 1, 2006 Not on modern cars as far as I know. we used to able to calibrate speedos fairly easily with adapters but the ony folks who did it were cops, taxis, and hot rodders. Put a piece of colored tape at each critical mark, a little v will do. 25, 35, 55, 65, etc. I just drive a bit faster than the speedo indicates since a CHP officer told me he only hits folks 10 over because there are so many of them here. Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted November 2, 2006 Author Share Posted November 2, 2006 I'd rather not put stickers on my speedometer Sometimes people are selling clusters on eBay. Is this something that would be corrected with a new speedo, or is the sensor the problem? And is it possible to either combine my speedo and a replacement, or set the 'new' odometer to match mine, if replacing the gauge would fix it? I don't mind doing the math, it's just annoying. And occassionally a passenger might say "Why are you going 80 in a 65MPH zone?" and then I have to say "well at this speed, I'm really doing about 72, my gauge just sucks".. Kinda lame on a car I paid like $26,000 for, and it's always been this way.. Quote Link to comment Share on other sites More sharing options...
Strakes Posted November 3, 2006 Share Posted November 3, 2006 http://www.caranddriver.com/features/1906/speedometer-scandal.html and http://www.caranddriver.com/features/1907/speedometer-scandal.html Quote Link to comment Share on other sites More sharing options...
Shep2112 Posted November 3, 2006 Author Share Posted November 3, 2006 Thanks for the info Strakes.. It seems like mine is still beyond acceptable limits.. The speedo goes to 120MPH on mine, but it's up to at least 7MPH off. I think the tires are supposed to be at 28 and 29 psi, but I keep them at 35 generally.. It's still way over the 'your speed is' things.. Perhaps the sensor is putting out too strong of a signal or the speedo servo is too sensitive? I dunno.. Quote Link to comment Share on other sites More sharing options...
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