I think pi is involved somewhere.
June 20, 2011 11:55 AM Subscribe
What's the math to figure out my speed and mileage after going to a larger tire?
My speedometer and odometer were very accurate when running a 27.5 inch tire. I now have 29.5 inch tires. How do I figure out my speed and my mileage? I'm real bad with numbers and math.
Regearing my speedometer is not an option at the moment.
Thanks a bunch!
My speedometer and odometer were very accurate when running a 27.5 inch tire. I now have 29.5 inch tires. How do I figure out my speed and my mileage? I'm real bad with numbers and math.
Regearing my speedometer is not an option at the moment.
Thanks a bunch!
Best answer: Assuming you're referring to outside diameters, the circumference of a 29.5" tire is 1.073 times as long as that of a 27.5" tire. Thus you are traveling 1.073 times as fast as your speedometer says, and 1.073 times as far as your odometer says.
posted by jon1270 at 12:00 PM on June 20, 2011 [2 favorites]
posted by jon1270 at 12:00 PM on June 20, 2011 [2 favorites]
Best answer: In layman's terms, what jon1270 is saying is, when your speedo says 70, you're doing (70 * 1.073) ~ 75. And for every 1000 miles your odo shows, you've actually gone 1073.
posted by notsnot at 12:03 PM on June 20, 2011
posted by notsnot at 12:03 PM on June 20, 2011
The reason why pi isn't involved is that both tires are round, so the pi cancels out. If you were switching from a square tire to a round tire, then you would have to worry about pi. So you just divide the diameters to get the answer.
posted by novalis_dt at 12:10 PM on June 20, 2011
posted by novalis_dt at 12:10 PM on June 20, 2011
You could also just find an accurate set of mile makers and go off of that. Around here (Michigan) I'll see "begin measured mile" signs from time to time along the highway. That might be more accurate, since your speedo was probably off to begin with.
posted by pjaust at 12:18 PM on June 20, 2011
posted by pjaust at 12:18 PM on June 20, 2011
I used a GPS that tells me my speed to check how far off my speedometer was after a gearing change on my motorcycle. The GPS can also tell me distance traveled in a trip, which I used to check the odometer.
posted by cjemmott at 12:23 PM on June 20, 2011
posted by cjemmott at 12:23 PM on June 20, 2011
Ah, I thought you were talking about car tires, but you are talking about bicycle tires, right? I was trying to figure out if you had freaky-small tires (29.5 circumference = ~9" diameter) or freaky huge tires (29.5" diameter is somewhere between "pimp" and "monster truck"). So never mind on the rims comment above.
posted by Grither at 12:37 PM on June 20, 2011 [1 favorite]
posted by Grither at 12:37 PM on June 20, 2011 [1 favorite]
Response by poster: Truck tires. I specifically meant to put "measured" and "diameter" in the question. Oops.
And I ditched the square tires after a couple weeks, the dogs didn't like riding in the back.
Thanks everybody!
posted by gally99 at 12:41 PM on June 20, 2011
And I ditched the square tires after a couple weeks, the dogs didn't like riding in the back.
Thanks everybody!
posted by gally99 at 12:41 PM on June 20, 2011
Response by poster: The new tires are on a jeep, so 29.5" is actually on the small side. I hope to be up to 35" in a year or so.
And so far I've yet to find those speedometer test mile markers in California, but it's because of seeing those on trips through Oregon that I know my speedometer was accurate before.
posted by gally99 at 12:46 PM on June 20, 2011
And so far I've yet to find those speedometer test mile markers in California, but it's because of seeing those on trips through Oregon that I know my speedometer was accurate before.
posted by gally99 at 12:46 PM on June 20, 2011
I'd make sure that this is legal in your jurisdiction. If you ever sell the truck, you'd likely have to disclose that the odometer does not reflect the true mileage for the vehicle. This could put off potential buyers and make your truck less valuable. It might also pose an issue with your insurance company.
California interstates do use postmarkers, which are kind of like mile markers, but they are irregular, the numbering is the same in both directions, and the numbers reset at each county line. Still, you could find a couple of these as you drive and check your speedometer that way; it would just involve a little more work compared to the Oregon test zones. A GPS (or a friend with a smartphone) would also do this.
posted by zachlipton at 3:47 PM on June 20, 2011
California interstates do use postmarkers, which are kind of like mile markers, but they are irregular, the numbering is the same in both directions, and the numbers reset at each county line. Still, you could find a couple of these as you drive and check your speedometer that way; it would just involve a little more work compared to the Oregon test zones. A GPS (or a friend with a smartphone) would also do this.
posted by zachlipton at 3:47 PM on June 20, 2011
Response by poster: The total count on my odometer is already inaccurate and the title reflects that.
Thanks for the heads up though!
posted by gally99 at 4:59 PM on June 20, 2011
Thanks for the heads up though!
posted by gally99 at 4:59 PM on June 20, 2011
This thread is closed to new comments.
posted by Grither at 11:57 AM on June 20, 2011