Speedometer Pinion Gear

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69_340_GTS said:
Because you have changed the diameter and the circumference.
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EXACTLY..even tho the tire contains steel belts. They are 'radial' belts; not 'bias' belts. That's why a tire mounted on a rim that matches its tread width will always be taller than a tire mounted on a wider rim.
 
CudaFactHackJob said:
Anybody who has ever stretched out a 195X75 tire to an 8" rim after having removed it from 4 1/2" rim knows more about tires than "TireRack.com". Experience is the best teacher.
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I bet they have done it more than once. Not jerking your chain but if you put the same tire on a 3 inch runner, will it get taller? Does the tread stretch?
 
Of course you disagree pishta. But you can't get your tape measure on two points tangent to the tire O.D. no matter what you do. At best you can measure the sidewalls. The diameter is at least 3", maybe 6" away, at the centerline of the tread. So you can't truly measure it. All you're doing is eyeballing it.
 
The tire is 24", but even if it's a hair more what would the difference between 34 or 35 tooth be? 2-3 MPH? I'll report back after installed!
 
Put a level on the top of the tire and measure to the level? You could roll a tire from chalk mark to chalk mark, one complete revolution and measure the track and deduce the circumference as C=2(pi)R, then double the radius. A quarter inch fudge in height will get you a whopping .8 inches more in travel. I think you can get close enough.
(1000 RPM of a speedo cable is 60mph, 60 mph is 88fps. 1000X.8 (.25 inch fudge factor) is 800../12 and you get 66 feet per minute, or 4000 feet per hour), that's less than 1 mph difference if you mismeasured your tire diameter +.25. I think you can get close enough with a tape and a level. Now, tell me how dragsters tires puff up: true radials with no cords inline with the beads? Maybe a different animal....
 
69_340_GTS said:
If the actual (GPS) speed is "30", but the speedometer reads "40", then the speedometer needs to be slowed down. Needs a bigger gear. 30mph÷40mph=0.75. And 29 teeth÷0.75=38.7 teeth. It looks like you need a 39 tooth gear, based on the observed error.
However, with a 225/60-14 (24.6") tire and a 3.23 gear, you should need a 34 tooth gear. Something is fishy here.
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I think there is a pi in there somewhere.
 
Experiment this way: Inflate your rear tires to 50 psi and check your speedometer against gps or a speed indicator. Then drop them to 15 psi and do it again. Do you really think there will be no difference?
 
Hmm... From 50 to 15 psi would flatten the contact patch, lowering the centerline of the axle to the ground, making it act like a shorter tire and its rotation would increase. Run a flat tire on a clutch sure grip and you will burn it up in short order, because the axle on that side is going to rotate faster than the pumped side. Same with mismatched spare on the rear of a SG, dont do it. Put 2 matched tires on the back and run the odd spare on the front. Im not convinced the OD of the street tire changes with rim width, but pressure differences to the extent of an increased contact patch (ie. running flat) would affect a speedometer because the axle is closer to the ground.
 
The bottom line is that all of these sizes and diameters and rim widths and tire pressures and circumferences don't really matter because the op's original calculations are questionable asfaras what his speedo actually read and what the gps actually said. The two weren't perfectly coordinated. If they had been, then it would have been a simple math equation of percentages based on the original number of teeth. On top of that, it's twice as accurate to test at 60 than 30; so we're all left with that disadvantage as well. In spite of all that, I'll stick with my original calculation of 36.
 
pishta said:
Hmm... From 50 to 15 psi would flatten the contact patch, lowering the centerline of the axle to the ground, making it act like a shorter tire and its rotation would increase. Run a flat tire on a clutch sure grip and you will burn it up in short order, because the axle on that side is going to rotate faster than the pumped side. Same with mismatched spare on the rear of a SG, dont do it. Put 2 matched tires on the back and run the odd spare on the front. Im not convinced the OD of the street tire changes with rim width, but pressure differences to the extent of an increased contact patch (ie. running flat) would affect a speedometer because the axle is closer to the ground.
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Are you saying that a steel belted tire can change its circumference even tho the steel isn't stretching?..Because if it's circumference can change at all, then why can't it also be affected by rim width? A wider wheel expands the radius of the sidewall from 90 degrees to the tread to a severe angle; giving the wheel a shorter distance to the ground; as in underinflation.
 
Im trying to say that a steel belted radial (DOT spec) can't change its circumference, but it can change its shape. Im thinking a tire that has a flat on the bottom will run faster than a round tire with the same circumference. I think the 60-75 series sidewall can take up most rim width variance without affecting the belted OD of the tire. I have to quote the same "Tire Rack" site again as it has the most on-point info that I could find. "...Additionally, a tire transitions from an unloaded to loaded state (your reducing tire pressure would do same thing) as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate..." These guys are in the biz. Im sure they know more about it than me.
 
Looking at a cross section of a tire as a trapezoid, it's easy to visualize the side legs (sidewall) as two radii that change the distance between the tread and the rim, based on their angle.
 
Hope I don't hijack but there should be a simple answer to this and couldn't find a thread for it. I changed out my gears from 2.71 to 3.55 and swapped out my Speedo gear with the correct, according to chart, RED 36 tooth (26" tire) and indexing on the "cover".
I cannot get the Speedo to read at all. The cable looks to be seating in the gear assembly properly and I tried feeding it in more with some needle nose just to be sure. Still not reading. Removed the cable again and noticed that I can spin the gear shaft with my fingers. Seems to me that if the gear was engaged I shouldn't be able to do this however it looks to be seated in the housing the correct amount. Any ideas? LA360 904 trans.
 
Yep. Make sure that the gear housing is indexed correctly in the transmission case.
You need to line up the number of gear teeth on the gear housing with the dot at 6:00 on the transmission case.
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Does that mean Yep it should not spin freely? It is indexed properly 32-38. Just a thought... I did not match up gear diameter to the old 27 tooth. Is it possible to have a red gear for a Jeep or different trans and different smaller diameter?
 
Ok. Found the issue. I pulled the gear and it is indeed RED however there is also a red 26 tooth (see below). D'Oh! Guess I should have done better homework. Found this info from a member no longer on FABO. Maybe it will help someone else however I cannot verify the accuracy.........

"TOOTH COLOR PN#

24 WHITE 3410024
25 BLUE 3410025
26 RED 2538926
27 WHITE 52068149
28 BLUE 52068150

29 BLACK 52068151
30 YELLOW 52068152
31 GREEN 52068153
32 BLACK 52068154
33 YELLOW 52068155
34 GREEN 52068156
35 ORANGE 52068157
36 RED 52068158
37 WHITE 52068159
38 BLUE 52068060
39 GREEN 52068061
40 ORANGE 52068062
41 RED 2538941
42 WHITE 2538942
43 BLUE 2538943
44 BLACK 2538944
45 Yellow 2538945


Speedometer Pinion Gear Chart
Source: 1972 Dodge FSM
Speedometer Pinion Gear Chart
All Car Lines - All Transmissions
Number of Teeth on Pinion Gear Listed Under Each Axle Ratio.

Tire

Size 2.45 2.71 2.76 2.93 2.94 3.21 3.23 3.54 3.55 3.91 4.10

24.06 27 29 30 32 32 35 35 38 38 42 44

24.84 26 29 29 31 31 34 34 37 37 41 43

25.15 26 28 29 31 31 33 34 37 37 41 43

25.47 25 28 28 30 30 33 33 36 36 40 42

26.01 25 27 28 30 30 32 33 36 36 39 41

26.71 24 27 27 29 29 32 32 35 35 39 40

27.34 24 26 26 28 28 31 31 34 34 37 39

27.01 24 27 27 29 29 31 32 35 35 38 40

27.71 24 26 27 28 28 31 31 34 34 38 39

28.34 26 26 28 28 30 30 33 33 37 38

28.65 25 26 27 27 30 30 33 33 36 38

28.88 25 25 27 27 29 29 32 32 35 37

24.29 26 28 29 31 31 34 34 37 37 41 43

24.78 25 28 28 30 30 33 33 36 36 40 42

25.41 25 27 28 30 30 32 33 36 36 39 41

26.97 24 26 26 28 28 31 31 34 34 37 39

23.82 25 28 28 30 30 33 33 36 37 40 42

24.24 25 28 28 30 30 33 33 36 36 40 42

24.78 25 27 28 29 29 32 32 35 35 39 41


Note: Pinions with less than 26 teeth require a different adapter than the one used for all other pinions.
Back to the tire page.
Tire Diameter Page

P155/80R13 22.76 B60-14 21.74 H60-14 24.26
P165/60R15 22.80 B60-15 22.74 H60-15 25.26
P165/70R13 22.09 B70-14 23.03 H70-14 25.97
P165/80R13 23.39 B70-15 24.03 H70-15 26.97
P175/70R13 22.65 B78-14 24.06 H78-14 27.34
P175/80R13 24.02 B78-15 25.06 H78-15 28.34
P185/70R13 23.20 C60-14 22.34 J60-14 24.50
P185/70R14 24.20 C60-15 23.34 J70-14 26.25
P185/75R14 24.93 C70-14 23.73 J78-14 27.65
P185/80R13 24.65 C70-15 24.73 L60-14 24.68
P195/60R14 23.21 C78-14 24.84 L60-15 25.68
P195/60R15 24.21 C78-15 25.84 L70-14 26.46
P195/70R13 23.75 D60-14 22.58 L78-14 27.88
P195/70R14 24.75 D60-15 23.58 M60-14 25.22
P195/75R14 25.52 D70-14 24.01 M60-15 26.22
P205/60R13 22.69 D70-15 25.01 M70-14 27.09
P205/70R14 25.30 D78-14 25.15 M70-15 28.09
P205/75R14 26.11 D78-15 26.15 M78-14 28.59
P205/75R15 27.11 E60-14 22.82 M78-15 29.59
P215/60R14 24.16 E60-15 23.82 N60-14 25.76
P215/65R15 26.00 E70-14 24.29 N60-15 26.76
P215/70R14 25.85 E70-15 25.29 N70-14 27.72
P215/75R14 26.70 E78-14 25.47 N70-15 28.72
P215/75R15 27.70 E78-15 26.47 N78-14 29.29
P225/60R15 25.63 F60-14 23.24 N78-15 30.29
P225/70R14 26.40 F60-15 24.24
P225/70R15 27.40 F70-14 24.78
P225/75R15 28.29 F70-15 25.78
P235/60R14 25.10 F78-14 26.01
P235/60R15 26.10 F78-15 27.01
P235/70R15 27.40 G60-14 23.78
P235/75R15 28.88 G60-15 24.78
P245/50R16 25.65 G70-14 25.41
P245/60R14 25.57 G70-15 26.41
P245/60R15 26.57 G78-14 26.71
P255/50R16 26.04 G78-15 27.71
P255/60R15 27.05
P255/70R15 29.06
P275/60R15 27.98
Formula
To determine the diameter of a P metric tire use this formula. Width/25.4 = width in inches. Width in inches x aspect ratio, expressed as a percentage = height of sidewall. (Height of sidewall x 2) + rim diameter = tire height.

Decoding the tire information
Let's decode an example tire, a P185/70R14. The P stands for P metric. The 185 is the width of the tire in millimeters. The 70 is the aspect ratio of the tire, in this case 70 % . The R means it is a radial, you might see a B, for bias ply on occasion. the 14 is the rim diameter.
Example formula

Let's run the formula using our example tire. 185/25.4 = 7.28. 7.28 x 70% = 5.10. (5.10 x 2) + 14 = 24.2. Our chart calls out the size as being 24.2, sucess.

Exceptions, caveats, and disclaimers
The sizes given are not all inclusive so you might need to run the formula.

The stated diameters may not correspond to an actual measurement as not all tire manufacturers adhere to this formula, precisely, case in point BFG tires tend to be wider than the designator would lead you to believe. It will get you close enough to do the job."
 
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