Will My Speedometer Read Faster or Slower With Shorter Than Stock Tires
| So recently, I replaced my 2006 Honda S2000 with a 2007 Honda S2000. My new S2K did not come with stock rubber, and the rear tires are 5% larger than stock. The rear stock tires are 245/40ZR17, and what's on the car now are 245/45ZR17 tires. My speedometer reads about 3 mph faster when traveling 70+ mph. From what I understand, the car gets its speedometer reading from the transmission. With this existence the case, are the larger rear tires causing my car to motility faster than what the machine was originally calibrated for? |
| Yes. Bigger tires means more lateral deportation for the same athwart rotation. In your instance, your speedometer should actually read slower than your actual speed. Here'due south a really useful online calculator to see how tire size affects speedometer readings. http://www.miata.net/garage/tirecalc.html | |
| Last edited by shannim on Monday Sep 12, 2011 two:34 pm |
| Admittedly. It's measuring revolutions. With larger tires, each revolution is traveling farther than the auto expects. |
| Quote: My speedometer reads well-nigh 3 mph faster when traveling 70+ mph measured how, exactly? Just, yes. You have a larger tire circumference, so the same amount of revolutions==more distance traveled. Of course, if the gear ratio was inverse between those model years, then you'll accept to effigy that in equally well. |
| This is what I suspected, and that link is really cool. These tires are relatively new, and now I want to alter them to the right size . . . |
| Frennzy wrote: Quote: My speedometer reads about 3 mph faster when traveling 70+ mph measured how, exactly? Merely, yes. You have a larger tire circumference, and then the same corporeality of revolutions==more than distance traveled. Of course, if the gear ratio was inverse between those model years, then you'll accept to effigy that in equally well. Measured with my Garmin GPS. edit: | |
| Final edited by gekigangerv on Mon Sep 12, 2011 2:37 pm |
| Quote: This is what I suspected, and that link is really absurd. These tires are relatively new, and at present I want to change them to the correct size . . . I would guess it would exist cheaper to just get it re-calibrated. |
| gekigangerv wrote: Frennzy wrote: Quote: My speedometer reads nigh iii mph faster when traveling 70+ mph measured how, exactly? Simply, yeah. You lot take a larger tire circumference, then the same amount of revolutions==more distance traveled. Of course, if the gear ratio was changed between those model years, and so yous'll have to figure that in every bit well. Measured with my Garmin GPS. Note that commercial GPS units are not really that accurate for measuring speed. They aren't also accurate in terms of position in the offset place (car units snap to roads most of the time and don't ever report true position). Then, on top of that, they need to do differential equations to mensurate speed (GPS cannot mensurate speed direct) which leads to mathematical errors. |
| shannim wrote: gekigangerv wrote: Frennzy wrote: Quote: My speedometer reads almost iii mph faster when traveling 70+ mph measured how, exactly? But, aye. You lot take a larger tire circumference, then the same amount of revolutions==more distance traveled. Of course, if the gear ratio was inverse between those model years, then y'all'll take to figure that in as well. Measured with my Garmin GPS. Notation that commercial GPS units are not actually that accurate for measuring speed. They aren't too accurate in terms of position in the first place (machine units snap to roads most of the time and don't ever written report true position). And so, on tiptop of that, they need to do differential equations to measure speed... GPS cannot measure out speed directly. While this is undoubtedly correct, the difference in speeds shown past my speedometer and GPS are nigh exactly what the tire calculator predicts that I will see. |
| Yeah, spending hundreds of dollars to 'fix' this 'problem' seems similar a complete waste product. ane. If yous are worried about speeding, err on the side of caution and boilerplate your speedo with your garmin. Compare to whatsoever highway speed detector signs for bonus points. 2. You can hands recalibrate your speedo. (or have a shop practice it, should exist cheap and quick, since they basically just program in the new tire size) iii. GPS is not very authentic for spot-checking speed. |
| And so you've got ane of the better ones. |
| One of my buddies is a cop, I've thought about asking him to clock me to verify. |
| Semi On wrote: I would guess it would exist cheaper to simply get it re-calibrated. Could just wait until the side by side time the tires need replaced anyways... |
| Or yous can just bulldoze as is and know yous're a few per centum off. |
| Quote: ii. You can easily recalibrate your speedo. (or have a shop do information technology, should be cheap and quick, since they basically just program in the new tire size) that depends. I'm non sure how Honda approaches information technology, just I know when I had my Ram the PCM would only accept settings between the smallest and largest tire they offered from the manufacturing plant, and only then because the Ram offered tire options with different O.D.s. You lot could not go outside of those bounds. If the automobile just had 1 O.D. of tire, you may not take any adaptability at all. |
| Just called i identify. They say that if they tin recalibrate the speedometer, it would price me approximately $350. They would install something where I could suit the displayed speed. |
| I would simply go along in the back of my mind that my speedo reads approximately 10% slow. |
| Yous know your speedo is off and by how much; wait until you need to replace tires before worrying about it once again. |
| Big Wooly Mammoth wrote: You lot know your speedo is off and by how much; wait until you lot need to replace tires earlier worrying about it once again. and if you e'er sell the auto, make sure you lot anticipate the striking in value from marking "non bodily mileage" on the title. |
| Jim Z wrote: Large Wooly Mammoth wrote: You know your speedo is off and past how much; await until you need to supervene upon tires before worrying virtually it once again. and if you ever sell the auto, make sure y'all anticipate the hit in value from mark "not bodily mileage" on the championship. So the speedometer measures rotation in the transmission just the odometer measures rotation at the wheels? Does that make sense? Or are both measured at the manual and just annals if the car is not in neutral or park? |
| Quote: So the speedometer measures rotation in the transmission but the odometer measures rotation at the wheels? no. the human relationship between trans output shaft speed and drive axle speed is fixed. you can't measure the transmission and wheels separately. The trans output shaft speed is bulldoze axle rpm * axle ratio. Quote: Does that make sense? modern cars by and large only use the ABS sensor(southward) to derive the vehicle speed, and the body controller/cluster takes that with tire diameter (rev/mile) to command both the speedometer and odometer. if you change the tire size- say, you put on ones with a larger diameter- the speedometer reads a lower speed than y'all are actually traveling because the larger diameter tires revolve fewer times for a given linear altitude. since the speedo is reading lower, the odometer is recording fewer than the actual traveled miles as well. |
| shannim wrote: And so the speedometer measures rotation in the manual merely the odometer measures rotation at the wheels? Does that make sense? Or are both measured at the transmission and just register if the car is not in neutral or park? You tin can measure both on the "bike" side (rather than the "engine" side) of the manual. I have no idea how information technology *actually* works, but one time you lot're by the clutch there should be a direct correlation between anything spinning and speed / distance traveled. |
| Ah, okay. For some reason I was thinking about the input to the transmission, which is e'er spinning, fifty-fifty in idle, versus the output of the transmission, which stops when in neutral. |
| Jim Z wrote: modern cars generally just use the ABS sensor(south) to derive the vehicle speed, and the body controller/cluster takes that with tire diameter (rev/mile) to control both the speedometer and odometer. if you modify the tire size- say, you put on ones with a larger bore- the speedometer reads a lower speed than you are actually traveling because the larger diameter tires circumduct fewer times for a given linear distance. since the speedo is reading lower, the odometer is recording fewer than the actual traveled miles also. And so let's say I just bought a new truck (2007 F-150), and that truck has ABS. That truck also has wheels that AFAIK were non offered as mill options. Permit's likewise say my GPS reads a steady 63 when the truck reads a steady 60. If you lot had to guess, would that truck demand equipment to fix the speedo, or does it just need some reprogramming? And would a performance shop be cheaper than a dealer to go the reprogramming done? Hypothetically, of course. |
| shannim wrote: Ah, okay. For some reason I was thinking about the input to the transmission, which is always spinning, even in idle, versus the output of the transmission, which stops when the vehicle stops. FTFY. You can be in neutral at 60mph. |
| Jehos wrote: shannim wrote: Ah, okay. For some reason I was thinking about the input to the transmission, which is always spinning, fifty-fifty in idle, versus the output of the transmission, which stops when the vehicle stops. FTFY. Y'all can be in neutral at 60mph. Doh! |
| Jehos wrote: Jim Z wrote: modern cars generally just employ the ABS sensor(s) to derive the vehicle speed, and the body controller/cluster takes that with tire diameter (rev/mile) to control both the speedometer and odometer. if you alter the tire size- say, you put on ones with a larger diameter- the speedometer reads a lower speed than you are actually traveling because the larger bore tires revolve fewer times for a given linear distance. since the speedo is reading lower, the odometer is recording fewer than the actual traveled miles also. So let's say I just bought a new truck Okay. "I just bought a new truck." Quote: (2007 F-150), and that truck has ABS. That truck as well has wheels that AFAIK were not offered as manufacturing plant options. Permit's besides say my GPS reads a steady 63 when the truck reads a steady 60. Okay. "My GPS reads a steady 63 when the truck reads a steady sixty." Quote: If you lot had to guess, would that truck need equipment to fix the speedo, or does it just need some reprogramming? And would a performance shop be cheaper than a dealer to become the reprogramming done? Hypothetically, of course. I don't know, I'm not 100% sure how Ford does it (might want to enquire Drew J.) If the tires on information technology have an outside diameter which corresponds to a tire size they actually offered on the F-150, it might simply exist a setting in the PCM/TCM. If it isn't, you'll probably need a chip of some sort. When I had my Ram, I upgraded to the next size tire up that Dodge offered and just borrowed a DRB-3 to set the new correction. |
| Jim Z wrote: I don't know, I'm not 100% sure how Ford does information technology (might want to ask Drew J.) If the tires on information technology have an outside diameter which corresponds to a tire size they actually offered on the F-150, it might but be a setting in the PCM/TCM. If information technology isn't, you'll probably need a chip of some sort. When I had my Ram, I upgraded to the next size tire up that Dodge offered and merely borrowed a DRB-Three to ready the new correction. They're Ford wheels and what looks like a stock tire size. I'll try doing some more than research. Also, smartass. |
| Figurer: http://www.performanceprobe.com/misc/in ... &page=tire I would just proceed the difference in my head. In fact, I did just keep it in my head - My Subaru Justy took a tire size that was not readily bachelor so I got the closest I could, which was really quite a bit bigger than stock. If I striking a good sized crash-land at speed with some weight in the back, they'd actually rub the upper backside of the mudflaps. |
| shannim wrote: Yes. Bigger tires means more lateral displacement for the same angular rotation. In your case, your speedometer should actually read slower than your bodily speed. Hither's a really useful online computer to see how tire size affects speedometer readings. http://www.miata.net/garage/tirecalc.html This is my favorite. So much so that I have two local copies where I've inverse the default values to friction match my two cars. |
Of course, this is mostly considering I've autocrossed one and even so autocross the other.| Jehos wrote: Jim Z wrote: I don't know, I'm not 100% sure how Ford does information technology (might desire to ask Drew J.) If the tires on it take an exterior diameter which corresponds to a tire size they actually offered on the F-150, information technology might just be a setting in the PCM/TCM. If information technology isn't, you lot'll probably need a chip of some sort. When I had my Ram, I upgraded to the next size tire up that Dodge offered and just borrowed a DRB-III to set the new correction. They're Ford wheels and what looks like a stock tire size. I'll try doing some more enquiry. Also, smartass. had I been thinking on my feet, I would take inserted a couple of screen caps from Police Squad. |
| Go practice some doughnuts in a car park. If you can't reduce the rear rolling diameter by iii% in an hour you don't need an S2000 anyway. |
Source: https://arstechnica.com/civis/viewtopic.php?t=1155451
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