How do tires effect the speedometre?

[Aren Jay]

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[TripleTransAm]

Not sure what the question is, but your description of the speedometer is not the way a speedometer works.

The only way a speedometer would be dead-on accurate in all cases would be to have a 5th wheel somewhere that was always measuring the actual road speeds regardless of tire speeds (you might have seen these on the old 80s Motorweek road tests or on photos of magazine road tests, with the 5th wheel suction cupped onto the rear fender).

So the only alternative is to read shaft speed somewhere along the drivetrain and convert it to an actual road speed to display.

I've never heard of a speedo reading axle speed, but most appear to be reading the transmission shaft speed right at the output end of the transmission (this is the first location where the ratio between road speed and shaft speeds will be locked, since anything before the transmission can change depending on what gear you're in). It's also the location with that fixed ratio closest to the speedo itself, making for a smaller speedo cable.

So since you know the relation between transmission shaft speed and axle shaft speeds (the diff ratio) and you know the ratio between axle speeds and wheel rotation speeds (the wheel/tire dimensions, you know: 2 * pi * r and all that) then you will know exactly what road speed you should be doing when the transmission shaft is rotating at a certain speed. Choose the right speedo gear to translate the shaft speed to a speedo feed for your speedometer, and your speedo tells you how fast you're going.

In the older vehicles, the speedo was driven mechanically by a cable spun off the speedo gear at the back of the tranny... if you changed diff ratio or tire sizes, you had to change speedo gears to correct for the new relationship. Many modern cars will feed the speedo info directly to the PCM and the PCM will then feed the speedo electrically, this allows you to reprogram the PCM for a new diff or tire size ratio.


So, in your case of tires affecting the speedo, they don't actually affect the speedo except to render it incorrect. The transmission shaft speed necessary to obtain 100km/h won't be correct... a taller tire may have you rolling at 105km/h for that same transmission shaft speed, but your speedo will still read 100km/h.

[Aren Jay]

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[TripleTransAm]

Handheld tuner, for one.

[CRUZTAKER]

Well hell....WELCOME BACK TTA!!!
We missed you. I have upgraded to a D80 and completed several courses.
Gage is six, another on the way, and all is good!

As far as the question at hand, I cannot answer in such depth as above, but I do know it's all about rollout, which is directly affected by tire circumfrence.

Smaller tire, higher speedo reading. Bigger tire, lower speedo reading. My wife's Aviator has 20"s. Her speedo shows approx 4 mph less than actual from a stock 17" tire. This cannot be fixed in tuning with her truck. Nor can the lack of tire pressure sensors in the summer tires. This data is stored somewhere other than the EEC.

[TripleTransAm]

We missed you. I have upgraded to a D80 and completed several courses.
Gage is six, another on the way, and all is good!

Nice! You really jumped into the photog thing in a big way! In the past year I've gotten into manual (old) lenses in a big way... the oldest one I've got is a Nikkor (Nikon) 55mm f1.2 (yes, 1.2!) dating from around 1972 or so, factory modified to fit on the newer F-mounts (if you mount a pre-AI lens on many modern Nikons/Fujis, you will tear up the mount).

Add to that one an early '80s 28-50mm f3.5 Nikon AI, a late 70s 50mm f1.2 AI, and on the extreme side a Tamron 500mm f8 mirror reflex lens, and lately a russian Rubinar 1000mm f11 mirror reflex lens <--- if you look out your window and smile, I might just be able to snap a photo of you from here. :)

What started it all was the 500mm but then my father-in-law gave me an old old Sigma 80-200 f3.5 manual lens that I flipped over, when his old Nikon camera died. It was "ball game over" for me... there is "something" about the old Nikon lenses.

Along with those I still have my 12-24 f4 DX Nikon, 17-55 f2.8 DX Nikon, and the 70-200mm f2.8 VR. Tucked away are my 50mm f1.8 AF-D and 24mm f2.8 which I occasionally bring out for car shows (due to filter size) and my first lens the 28-105mm f3.5-4.5.


One of the coolest things that helped my manual photography was when I decided to take the plunge 2 weeks ago and check my camera myself for back focus... I was shocked to see that it did have a small amount of backfocus (ie. autofocus thinks the target is in focus but in reality it misses it by 1-2mm in my case). Went to town on my camera's mirror stoppers (2mm hex adjusters) and WOW! Like I have a brand new camera! I can shoot with lenses wide open now without fear of soft photos... the shallow depth of field was killing me all these years unless I added depth of field by stopping down!!!

You can print out some test charts for backfocus and check for yourself. Way cool adjustment, wish I'd have had the balls to do it years earlier instead of 35000 shots later. :depress:


Gage is six! Wow! And Gage #2 on the way. Way to change the dynamic of the household! LOL! Congrats.

[TripleTransAm]

As far as the question at hand, I cannot answer in such depth as above, but I do know it's all about rollout, which is directly affected by tire circumfrence.

Yup. 1 revolution of the tire will push the car forward the equivalent distance of 1 circumference around the tire (with no wheelspin of course).

So if you get the circumference of the tires surface, you'll know how far the car is pushed for 1 axle revolution. If you know your diff ratio, you'll know how many axle shaft revolutions occur for 1 transmission shaft revolution. Same goes for the transmission gears.

2 * pi * r is the circumference of the wheel. So as long as everything else remains the same (diff gear), the speedo error is the ratio of the two radii.

Ratio of circumferences = (2 * pi * r1) / (2 * pi * r2) and the 2 *pi cancels out in the above so you end up with r1/r2 as the ratio. (the radius being the distance from center of wheel to the edge of the tire, which is sometimes hard to measure, so you can just compare diameters 2*r1/2*r2 or d1/d2 and get the same result (ie. only cancel out the pi).

[MM03MOK]

... the oldest one I've got is a Nikkor (Nikon) 55mm f1.2 (yes, 1.2!) dating from around 1972 or so, factory modified to fit on the newer F-mounts (if you mount a pre-AI lens on many modern Nikons/Fujis, you will tear up the mount).

:wave: /Steve! I have that same lens! It came on the used F2 I bought in 1977. Interesting to know you can have the mount modified.

[Aren Jay]

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[CRUZTAKER]

We so need to leave this fellows thread alone and start a new one.:D
Those were the days...hijacking threads with photog crap.;)



Along with those I still have my 12-24 f4 DX Nikon....

Would this be for sale....:thinker:

I have an 80-200 f/5.6 on ebay right now.

I really want a 10-20 something for my D80.

[TripleTransAm]

:wave: /Steve! I have that same lens! It came on the used F2 I bought in 1977. Interesting to know you can have the mount modified.

Yep. I don't have the sites handy but there is one guy that offers the service for a small fee. It involves cutting away some of the mount at the back, displacing the small "ears" forward. You can tell an AI'ed lens by the duplicate set of F-numbering at the lens mount, and the "ears" are hollow (to allow later cameras so view the F-numbering through the prism). Solid-ears are usually a sign of a non-AI lens, and is certain death for most of the modern Nikons.

It (like the later 50mm version) is a cool lens that makes ultra sharp "nice" photos when stopped down a bit (2.8+) but wide open it has this special glow that many like. It's a very good performer. Lack of ED coating makes for a different colour rendition but leaves it prone to internal flaring if not aimed properly.

[TripleTransAm]

Would this be for sale....:thinker:

Nope. 2 weeks ago it might have been. :) But then I figured out this back-focusing thing and it was enough to take this lens from "meh" status to "now we're talking!".

It's nowhere near my favourite lens (mostly because at 12mm it really smears the image at the outer edges of the frame) and I use it only when I really need the wide angle coverage in small spaces. I resort to a Photoshop plugin called "lens corrector" or something like that, which applies a user-definable "squeeze" at just the right places to make the image less odd-looking at the edges. If it wasn't for the focus-tweak and this plugin, I'd have been pretty disappointed in the lens, for the $$.

The 17-55 2.8 on the other hand... a real workhorse. Worthy of its reputation (and I'm appreciating it even more now that I can accurately focus and get crisp shots at wide open 2.8).

As for hijacking the thread... jeez, there ain't much more that can be said about speedometer error with non-stock tires. :cool4:
(except to say that if it's traction-control equipped, his Marauder is probably not running so well above 40 mph unless the front/rear size ratio was maintained).

[TripleTransAm]

Expect at 100,000 miles it would read 101500.

Looks Like I'm going to be buy a set of 255/55 rears this spring.

Check your math.

Taller tires have a greater circumference, no? So every revolution takes you 1.528 % farther (not 1.015%) so if you were to indicate 100000 km travelled on the odometer with these taller tires, in reality you would have travelled 101528 km by the time the odometer had gone 100000km. Same for your speed... 100km/h shown on the speedo is actually 101.528 km/h as per the radar.

Is this enough of a drift from the stock tire size to freak out the TC without finding the corresponding front tire size to maintain that front/rear ratio? Dunno. Good luck.

By the way, no such thing as free energy in this universe. You get 1.528% more distance travelled per axle revolution, but you also get 1.528% less force being generated at the tire contact patch for the same given axle torque. It's probably the equivalent of dropping from a 3.55:1 rear axle ratio to a 3.50:1...

[ctrlraven]

Stock size 245/55 has a diameter of 28.6", a circumference of 89.9" and revs per mile is 705.

Plus zero size 255/55 has a diameter of 29.0", a circumference of 91.2" and revs per mile is 694.

The difference on the speedometer at 60mph would be 1 mph. A taller tire is going to rotate slower. I run 255/55 and just adjust my Revs Per Mile to 695 which is close enough.

[TripleTransAm]

The difference on the speedometer at 60mph would be 1 mph. A taller tire is going to rotate slower.

A taller tire at an indicated 60mph is going to rotate at the exactly the same speed as the shorter one. It's the actual road speed that will vary.

[Aren Jay]

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[TripleTransAm]

More distance lower odometre readings and gets you there faster.

But takes slightly longer to accelerate to that speed. (mind what I said about the torque multiplication!)