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Premium Member
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Discussion Starter #1
I have 3.55 gears with 31's. I'm turnin 2000 rpm's at 65 mph. I have used a few of those calculators to figure out what my actual speed is, one said 68.9 mph the other said 71.1 mph. Which is more accurate? Thanks!!

J
 

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Premium Member
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splitting hairs now are we?


how about you measure the circumference of you're tire, get the overdrive ratio of you're transmission, and do the math, it's not that hard (I've done it before i knew about the "calculators") remember a 31" tire is usually not 31" tall, it's more like 29 or 30" tall.

here's how you would do that... tire height (lets say it's actually 31 inches) then do pie for circumference.... then divide the amount of inches in a mile by that number (97.34) (there are 63360 inches in a mile) so you can figure out how many tire rotations per mile.. (650.5) then multiply that by you're axle ratio to get the driveshaft rotations per mile.. you're axle ratio is 3.55 so 3.55 * 650.5 = 2,309. then multiply that by you're overdrive ratio.. (its 0.71 for an e40d) that gives us 1639 engine revolutions per mile... or at 60mph (1 miles per minute) that would be 1639 revolutions per minute.

so 60mph = 1639RPM. if you want to know what 65mph would be then we'd go 1639/60*65 which is 1775.5rpm. this is of course assuming you have an e40d automatic overdrive transmission and you're tires are a true 31" tall.

so pretty simple, right? just go

exact tire height * pie / 63360 * axle ratio * trans ratio / 60 * desired speed = rpm at desired speed!
 

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Satyr of the Midwest
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Mmm....pie.... :rofl:


There is a lazier route:http://www.grimmjeeper.com/gears.html

Inputting the correct tire size is imperative, doing the calculation manually OR using a script.
 

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Discussion Starter #4
huh?
 

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Discussion Starter #5
I went the lazy route! Thanks!
 

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Super Moderator
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One of the best ways I've heard to get an accurate measurement of your tires is to measure from the center of the tire to the ground. Double it to get the diameter. This takes into account the weight of the vehicle "smooshing" the tire, and is the actual rolling circumference of the tire.

This is a math calculation that eliminates a few steps by putting pi and converting some of the inches to feet, miles per hour, etc....

Speed = (Tire Diameter * Engine RPM) / (Transmission Gear * Final Drive * 336)

So if your tires are actually 30.5" (not true 31s, which they rarely, if ever, are)
Your RPMs are 2000
Your Final Drive is 3.55
Your transmission is in OD (dunno your transmission, but if it's an E4OD, that'd be a 0.71)

So

Speed = (30.5 * 2000) / (0.71 * 3.55 * 336)
Speed = 61000 / 846.888
Speed = 72mph


Just plug the numbers into the blanks and you'll get your speed.
 

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Premium Member
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One of the best ways I've heard to get an accurate measurement of your tires is to measure from the center of the tire to the ground.
Not accurate enough - lord knows where he will eyeball as "center" of his tire, the slight angle that he holds the tape at in all planes not to mention the tape slightly bowing, the play in the little metal clip on the end of the tape, the accuracey of the marked increments themself, the surface condition of whatever ground he is parked on and the PSI of the tire which I'm sure will lessen as he drives. This is all before he takes this measurement and multiplies it by 2 which will degrade his accuracey even more! Ridiculous!
 

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Super Moderator
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And these inaccuracies aren't taken into account when measuring straight across?
There's always going to be inaccuracies, and a margin of error should always be expected.
Inaccuracies aside, from the center down measures the true radius of the tire as it rolls on the ground. Tires naturally bulge on the sides due to the vehicles weight, taking away from it's rolling radius. This inaccuracy is just as much, if not greater, than trying to measure a straight line straight down. It's not THAT hard to measure a straight line. Yeah, you can make it sound as difficult as you want, but ridiculous it isn't.
 

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Am I the only one that can settle for "close enough" around here?
 

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Premium Member
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Am I the only one that can settle for "close enough" around here?
No, my speedometers are all off... my daily driver reads about 5-10% slower than i'm going, but i really don't care.. the only time it's "off" enough to notice is on the freeway, and i know then that 65mph is really about 70. and when i figure my mileage i just multiply my trip counter by 1.1

i just like to over-answer peoples questions whenever possible.
 
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