revs per mile, help please!

[KillaKev702]

allrite so I've noticed since i got my tires changed my speedo has been off. Its especially noticeable on the freeway when my speedo says i'm doing 80 when the speed limit is 65 and i'm keeping up with the flow of traffic. So anyway i was wondering if anybody knew how to calculate my revs per mile. I have stock size 245 45 17, but i'm positive my speedo is bad and i'm hoping to fix it with my sct sf3. thanks in advance.

 

[sneaky98gt]

Well, 245 is the width in millimeters. Convert that to inches: 9.646 inches. The 45 is the percentage of sidewall above or below the rim. So 9.646*.45=4.341 inches above the rim. Multiply that by 2 because you have sidewall below the rim as well: = 8.68 total inches of sidewall above and below the rim. Add that to the rim size (which is in inches) to get 25.68 inches as the diameter. Now, circumference of the tire is diameter times pi, or 25.68*3.14, which equals 80.64 inches. Now there are 5280 feet in a mile, times 12 inches per foot, equals 63360 inches per mile. Revolutions would be distance/circumference, or 63360/80.64. I get 785.72 revolutions per mile. Someone correct me if my math is wrong.

If you aren't changing the tire size, I don't know why your speedo would be off. I changed from a 245-45/17 to a 255-40/17, which is actually a shorter tire. Therefore, smaller diameter equals less circumference which equals more revolutions per mile, which would make me speedometer think I'm going faster than I am. But if it's the same size tire, I don't know why you would have any problems.

-Will

 

[HOOCBB]

For some reason, I was thinking that PI*(R*R)=circumference (Pi*(Rsquared)). But I was wrong.

On another note, I have found this calculator to be very helpful, even if it IS on a Miata site!

Tire size calculator

 

[sneaky98gt]

For some reason, I was thinking that PI*(R*R)=circumference. But I was wrong.

On another note, I have found this calculator to be very helpful, even if it IS on a Miata site!

Tire size calculator

Pi*r^2 is area. 2*pi*r, or pi*d is circumference. And that calculator is pretty cool.

-Will

 

[Barnaby]

allrite so I've noticed since i got my tires changed my speedo has been off. Its especially noticeable on the freeway when my speedo says i'm doing 80 when the speed limit is 65 and i'm keeping up with the flow of traffic. So anyway i was wondering if anybody knew how to calculate my revs per mile. I have stock size 245 45 17, but i'm positive my speedo is bad and i'm hoping to fix it with my sct sf3. thanks in advance.

I'm not sure I'd trust "the flow of traffic" as your standard. Before you do anything to your car you can verify a couple of things pretty easily to see if your speedo really is off.
1. Check RPMs vs MPH. At 65 MPH in fourth gear (assuming a manual tranny) you should be at ~2780 RPMs. 80 MPH is ~3420 RPMs. If your speedo is reading 80 and you're only showing 2780 on your tach then you have a problem.
2. Time yourself. The mile markers are set at ~ 1 mile apart. Find a stretch thats reasonably flat and little traffic and set your cruise control at 60 MPH. It should take exactly 1 minute from one mile marker to the next. If it's substantially less than a minute you have a problem.

 

[bboylobo]

i had this problem before when i first got my stang. the person who owned it before me put 3.73 gears and probably had a tune to compensate. well in the end i got the car with no tune and a speedometer that was reading wrong for a while. i have 76000 mile on it but in actuality i prbly only have 68000. anyway i figured it out by comparing rpms to the speed. you can also check if you have aftermarket gears by lifting the rear and spining your tire 1 full spin while making note of how many times your drive shaft turns. if you spin the tire on a stock car, your drive shaft should turn 3.27 times. if you have aftermarket gears like 3.73 or 4.10s, itll spin that many times. the rpms comparison is easier though

 

[Ares]

Find the tire mfg website and look up that tire. Most always list specs including revolutions per mile. A tire size not setup right with computer even if gearing is will throw off speedo a few mph(2-4 maybe). Reading 80 in a 65 sounds more like it's not recognizing a gear correction. Like said flow of traffic is not something to use.

 

[sneaky98gt]

Like said flow of traffic is not something to use.

Seriously. I was on Interstate 40 between Burlington and Durham a couple of months back, going 85 in a 65, and getting my doors blown off on both sides. I mean, what are you supposed to do in a situation like that? Speed like a mofo? Or drive the speed limit and cause a wreck?

But yea, to figure out your speed using the mile posts: 3600/(the seconds it takes you to go one mile). Of course, it would be more accurate the longer distance you traveled, as this would reduce the error in the mile post markings, but just don't forget to take your total time and divide by the number of miles you go.

-Will

 

[Stan Weiss]

I have a couple of calculators on my Web site that might help.

Calculate Speed (MPH - Km/H) given RPM, Rear Gear Ratio and Trans Gear Ratio

Calculate RPM given Speed(MPH), Rear Gear Ratio and Trans Gear Ratio

Automotive Performance Software / Interactive JavaScripts Calculators

Using the mile post on the Interstate is vey simple. At 60 MPH it should take 60 seconds from one post to the next.

 

[KillaKev702]

thanks for all the replies guys, ima go ahead and try using some of these calculators, methods and what not to try and figure it out. wish me luck!