Mopar92
Well-Known Member
According to what I’ve read I need a 32 tooth for my 833 Duster. 27” tires and a 3.23 gear.
Speedometer accuracy
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jbc426
Well-Known Member
I used those charts for my '70 E-Body with a Passon 5 speed. The speedo part of the transmission is essentially the same as an A-833.
The 28" tall rear tires, 3.73 rear end gears equal a 35 tooth driven gear. At freeway speed, the 35 tooth gear spins the speedo 1.21 times too fast compared to my smartphone app for speed etc. I did the math, and choose to be closest tooth count (43) for freeway speed on my speedo, as it has a smaller error at 25 mph ( 1.16) & a larger error at higher speeds. The slope of the speedometer error is at a different angle than that of the slope of the actual speed. They are not parallel in my car.
I understand it can be recalibrated, but can they actually change the slope to match the actual speed slope or do they just cross the slopes at a random moderate speed somewhere in the middle of the slopes? I don't want to pull it out and have it "restored" again to experiment with that at this point.
The reason I wrote this is to see if you are able to compare actual speed to what your speedo is showing you now. It may save you some money, because these gears are not cheap.
I find I can judge slow to moderate speeds without the speedo, but as I get up into freeway speeds it becomes more difficult to be accurate. My car is well insulated and has fresh suspension and high quality shocks, which makes even 100mph seem like a good cruising speed( don't try that at home).
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We were just talking about that in another post.
Speedometer off
Check it out and one astute member posted ~ 1 tooth per 2 mile per hour difference.
72.6-60 is 12.6 which would give you - add 6 or 7 teeth to your 35 making 41 or 42 teeth
Using my calculator and your numbers I get:
(60 x 1.21 = 72.6 Speedometer MPH)
(25 GPS mph x 1.16 = 29 Speedometer MPH)
By my calculations 42 should get you as accurate as possible at fwy speeds but your low speeds will be a bit off
I reread your post and now I'm confused, what are you running right now? and what is your speedometer speed and your GPS speed with that gear?
Close is good enough unless you can find a company that can fine tune your speedo. The trans gears can only get you so close. The speedo can only be so accurate. It's all mechanical. Heck tires as they wear will affect accuracy.
jbc426
Well-Known Member
Sorry. It made sense in my mind, but that's how legends are made.
I called Jamie Passon, and gave him my numbers for tire diameter and gear ratio. He confirmed that I should use a 35 tooth. The same number I come up with on all the charts. Well, some charts don't show the numbers for 3.73 gears.
I ordered a 35 tooth gear from him, and when I was installing it, I found that I had a 35 tooth gear in there already. It looked a tad worn, so I put the new one in. My current speedometer error remained.
Using GPS on my Android phone to check actual speed, I found that my speedo read 29 mph at 25 actual mph, at 70 mph indicated I was at 57 actual mph, at 80 mph indicated I was at 65 mph, at 120 indicated I was at 95 actual. The ratio of error increases as the speed increases.
If I plotted both sets of these numbers on a graph with MPH on both sides of the chart, the actual MPH would split the chart evenly in half right down the middle. Now, when I plot my indicated numbers on the same chart, my indicated speed line would start at the same place on the chart at "0" MPH at the lower left corner and be just above the actual line getting farther and farther away from the line as speed increased. The lines have different slopes and diverge from each other at a certain ratio that I forgot how to calculate from my algebra classes. A picture would work better to explain it, but I don't know how to make one on this PC.
I haven't put the new gear in yet to see what happened, as I had other priorities this weekend including taking a buddy and his son for a ride to show them what a sub 500 hp car versus a nearly 700hp car feels like. I'm helping them build a 1000 to 1200 hp boosted car.
The main point I tried to make to them is that either car is deadly in the wrong hands. I think they got this point as I took off from the house all smooth and slow. I lulled them into a conversation with each other long enough to distract them as I approached the entrance of a long freeway on-ramp at about 10 mph. Just when they least expected it, I suddenly blasted 500 cubic inches of Mopar muscle through 3 gears to redline. They never saw it coming, and its safe to say I put them right in shock, especially when I told them there were two more gears available after that. What are friends for after all.
This started in a want ad but is good info and should be shared. I will move the posts here when I can get to my laptop.
Got it done. Discuss away!
Posts are accurate. In late '60's we serviced city police cars at our dealership. There were complaints about speedo inaccuracy, so we sent cars to a speedo shop that could run the cars on rollers and check their accuracy. This proved that the speedo's were as accurate as the factory could make them. Typically a mechanical speedo when tested from 0-70mph will show accuracy/ inaccuracy in various points of the range. Typically we would try for accuracy at 65-70mph and would change drive gear 1-2 teeth to attain.
Here in Colorado the speed on the interstate is 75 MPH most people do 80 to 85. Was in CA last fall, and in the immortal words of Sammy Hagar "I can't Drive 55"
pishta
I know I'm right....
Police cars had calibrated speedos. Very few other cars did. I was reading an AMC highway patrol website (150 mph 401 Javelins!!!) and there was a story about a guy passing a Mopar Highway patrol car that was doing 51 in a 60, well the cop lit him up for speeding and it was dismissed when they found the cops speedo was 9 mph fast! My old rule when looking for speedo gears at yards was teeth / 10 was pretty close to rear end ratio. 41 tooth was a 4.10 rear, 27 was a 2.76 with stock tires.
diymirage
HP@idle > hondaHP@redline
growing up in the Netherlands they have this "correction" that comes of the top of every speeding ticket
i dont recall the exact calculation but for sale of argument, lets just call it 3 MPH
meaning if you got clocked doing 77 in a 35, they would only write you up for doing 74, hereby forever settling the speedo calibration issue
(not that i would ever admit this to you guys, but i actually scooted by with getting to keep my license, which i would have lost if it wasnt for the correction)
69_340_GTS
Well-Known Member
It isn't the gear's fault, your speedometer itself is not working properly.
I have a question... I saw a post where someone had numerous gears for sale and the 40+ tooth gear seemed twice the diameter as the 28-30 tooth gears. if the center-line of the gear is the same for each gear how can the lower number of teeth gear fit into the trans and mate to the output shaft gear or visa versa? is there a unique insert that is needed for certain gears?
68gtxman
I used to reMember
The speedo gear holder at the transmission is indexed. It must be turned to one of four (?) positions dependent plastic speedo gear. I haven’t checked the book on this for a while, so look into the FSM for details.
69_340_GTS
Well-Known Member
The index marks on the gear housing have the number of teeth (a range) right next to them. Pretty straightforward.
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That makes total sense. Thanks
jbc426
Well-Known Member
I suspected it as the culprit, and sought out advice from a local expert at a respected speedometer shop. He said that if I have a 3 mph indicated error at 25 mph that by the nature of this specific speedometer's design(1970 E-Body Ralley gauge), that error should be roughly 6 mph at 50 and 9 mph at 75 mph. However, my speedo error rate increases slightly more than that.
He also said that by adjusting the airgap between the spinning magnet and some other part that actually drives the speedo needle is how they calibrate them. I'll try to put the gear in on Friday, see what happens and report back.
When these cars were built most came with F 70 14 bias ply tires. Most of the time on acceleration The tires were going 60 and the car was going 30. I can remember when passing another car you would have to go the oncoming lane get it straight. And then plant the throttle hoping your quarter panels didn't get flattened out from the fish tailing as you were going bye. You always had the tire smoke to hide the plate number if you hit when passing
I really never cared what the speedo said. It was the radar gun that was my worry. and they usually left you go a under 10 over and still do.
I really never cared what the speedo said. It was the radar gun that was my worry. and they usually left you go a under 10 over and still do.
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