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  #1  
Old 03-16-2005
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speedometer conversion?

Ok so I want to know the equation to find out my new actual speed.

I went to TRs but it is only useful without a gear change.

I originally had 3.73 tires with 28" tires, now 4.10s and 32" (soon to be 33") tires.

I am not too concened abotu the speed b/c I jsut go with traffic, BUT I am more concerned about knowing an accurate account for mt gas mileage.


So anyone have the equation I can plug and chug?

Thanks
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  #2  
Old 03-16-2005
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the mph are pretty close ..only 1-2 mph off at 70
there is no eqaution..you have to a figure the differance with 28 w 3.73 and the diff 32 w/4.10 and take the average...its not easy to explain...get a GPS its easier...LOL
I'll try to explain better when i get home!..im on lunch and dont have much time!

Last edited by TReff; 03-16-2005 at 12:25 PM.
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  #3  
Old 03-16-2005
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shouldn't matter what kinda gears you got in... the speed snesor is in one of the front wheels I beleive....

if you went from 28" to 32" so the new 32" is 14% bigger than the 28"

70MPH X 14% = 9.8mph

so when you are going 70mph, you are really going 79.8mph


Look at this if you want
http://www.miata.net/garage/tirecalc.html

Last edited by PickupMan92; 03-16-2005 at 12:47 PM.
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  #4  
Old 03-16-2005
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thats true if you were still running 3.73 w/ 32 .
thats what mine was reading on the GPS b4 i changed my gears..
Now it reads 71 @ 70 on the speedo..
ill see if i can find the charts
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Old 03-16-2005
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The speed sensor is either at the rear axle ring gear(early) or the transmission tail housing (late). I don't know for sure when the cutoff year was but I think 2000 was the last year for the axle sensor. If that is the case, then the speed sensor is reading axle speed and doesn't need a correction for the gears, just the tires.
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Old 03-16-2005
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http://www.4wheelparts.com/4wp/asset...gear-ratio.htm
here is a link to the gear ratio guide ...if you figure you were running 28in tire w/ 3.73 =2909 rpm's @ 65mph
now you have 32 w/4.10's @ 65mph =2805 rpm's thats almost a perfect change

Last edited by TReff; 03-16-2005 at 06:07 PM.
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Old 03-16-2005
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sure thats the right link?
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Old 03-16-2005
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damn it...click on the gear ratio guide to bring it up!
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Old 03-16-2005
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html is off or i would put it on the page
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Old 03-16-2005
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Quote:
Originally Posted by TReff
damn it...click on the gear ratio guide to bring it up!

damn kkk'ians.. lol
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Old 03-16-2005
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Quote:
Originally Posted by TReff
http://www.4wheelparts.com/4wp/produ...IRES&man=PCTC#
here is a link to the gear ratio guide ...if you figure you were running 28in tire w/ 3.73 =2909 rpm's @ 65mph
now you have 32 w/4.10's @ 65mph =2805 rpm's thats almost a perfect change
True but his question was on odometer accuracy, not engine RPM.
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.

Last edited by TReff; 03-16-2005 at 06:03 PM.
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  #14  
Old 03-16-2005
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i can tell you with 33 that are worn out (probably 32's now) when i go on a 200 mile trip the odometer isnt even a mile off compaired to the GPS...
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Old 03-16-2005
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Like I said, I don't know if he has VSS from the axle (early) or OSS from the transmission (late).
If the odometer is driven off the trans, then there is not much speedometer/odometer error.
If it's driven off the ring gear, the error for the big tires is 1 - (28/33) = 15% low, meaning that he would be reading about 60 when he's going about 70 actual. His odometer would register 15% fewer miles than actual.
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Old 03-16-2005
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sorry to jump in on this, but i had a quick question.. i saw in another thread a while back about finding tire size, etc.. mine came out to be something like, 31.644.. blah blah.. is that considered a 32?? or, a 31.644.. blah blah.
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Old 03-16-2005
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Quote:
Originally Posted by Strider0O0
sorry to jump in on this, but i had a quick question.. i saw in another thread a while back about finding tire size, etc.. mine came out to be something like, 31.644.. blah blah.. is that considered a 32?? or, a 31.644.. blah blah.
Most "inch size" tires are slightly smaller than the size would lead you to believe. A BFG KO 33 is about 32.8" and a Bridgestone Revo 33 is closer to 32.5".
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Old 03-16-2005
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so they're considered 32s? i have 275/65/16's , they werent' labled bridgestone 32s or anything like that, i was just curious as to what my tire is, inch-wise.
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Old 03-16-2005
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Quote:
Originally Posted by Strider0O0
so they're considered 32s? i have 275/65/16's , they werent' labled bridgestone 32s or anything like that, i was just curious as to what my tire is, inch-wise.
You have p-metric sized tires so they won't be labeled in inches. Acoording to these calculators:

http://www.miata.net/garage/tirecalc.html
http://www.customtruckshowcase.com/p...lculators.html

the diameter of a 275/65R16 tire is 30.1"
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  #20  
Old 03-16-2005
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i just did it on that miata page and got 31.6
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  #21  
Old 03-16-2005
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wow im an idiot, im sorry, my tires are 265/75/16s... sorry
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Old 03-16-2005
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I did it again on both calcualtors and got 30.1".

Are you sure you have 275/65R16?


EDIT: yes 265/75R16 is 31.6, about the same OD as most 32s.

Interestingly, Bridgestone lists the diameter of a P265/75R16 Dueler as 31.0" and a LT265/75R16 Dueler as 31.8" I guess this makes some sort of sense since the tread depth is significantly greater on the LT.

http://www.bridgestonetire.com/tires...productid=1081

Last edited by V8 Level II; 03-16-2005 at 06:55 PM.
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  #23  
Old 03-16-2005
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Quote:
Originally Posted by rwenzing
The speed sensor is either at the rear axle ring gear(early) or the transmission tail housing (late). I don't know for sure when the cutoff year was but I think 2000 was the last year for the axle sensor. If that is the case, then the speed sensor is reading axle speed and doesn't need a correction for the gears, just the tires.

mine's in teh axle, had to replace it
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  #24  
Old 03-16-2005
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Well I do know that when I had 31s still with 3.73 gears at 65 odo indicated, i was actually going 74 (Cop verified)

So I guess there is no equation to use? I know I can guesstimate adn use a GPS but those have error too, unless I use a Trimble unit, or another survey grade. Those hand helds are still only 3 meter accuracy.
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  #25  
Old 03-16-2005
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Quote:
Originally Posted by solodos
mine's in teh axle, had to replace it
If it's in the axle housing, use the formula I gave for speedo correction:

1-(28/33)= 0.15 = 15%

So, your odometer is reading 15% too low with the 33s in palce of 28s and your speedo error should be about 10mph low at 70MPH.

The gear ratio change at the diff doesn't matter because the axle measurement is reading actual wheel RPM. The only thing that changes that is tire size.

[On the 2001+ Rangers, changing the axle ratio does affect the speedometer accuracy because the measurement is taken at the trans output shaft, not the ring gear.]
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