Hook me up with some speedo gears!

[///Matt]

hey all. I just got a refurb speedo in the car, and the (known wrong) speedo gear of 26 teeth read faster than GPS speed. I ordered the”right” speedo gear of 33 teeth (28” tire diameter, 3.55 rear, on my 727) and it reads slower than GPS speed. I don’t know where to blame for the “right” gear reading wrong, other than probably a slightly off speedo cal. I’m not removing it for re-cal.

SO: who’s got a spare 30 and 31 tooth laying around that I can borrow? I really don’t want to but 2 more $60 gears to figure out which one works for me. I can either borrow yours the return, or buy the one from you that works for me... or I can trade you some of my 26,33, and 36 tooth gears that I have. (PO threw in the 36 saying it was the “right” gear, but it’s way not.)

 

[R/T Boy 67]

With your car at,as driven condition, with correct tire pressure etc. go for a drive. Divide your speedo indicated speed, by the actual gps speed. Multiply the number of teeth on your speedo gear by that number. That is the number of teeth you need. Example. Indicated speed is 60, actual speed is 50. 60 ÷50 is 1.2. If you have a 30 tooth pinion you would need a 36 tooth pinion. Try this at different speeds, in this case the results should all be 1.2. If not the speedo needs to be recalibrated

 

[beanhead]

If you've already done this I apologize, but before you replace your gear did you clock the mounting piece properly? It can be set in three different positions I believe and if you don't get the right one it won't work correctly

 

[///Matt]

If you've already done this I apologize, but before you replace your gear did you clock the mounting piece properly? It can be set in three different positions I believe and if you don't get the right one it won't work correctly

I did clock the mount properly. As I understand, if you pick the wrong mount clocking, one won’t fit and the other won’t turn the speedo... I think.

I did the math as R/T Boy suggested and it appears to be a pretty consistent .86 ratio, so instead of the 33 I put in, or the 26 that came out, I need a 28 tooth pinion.

I have a spare new speedo cable that I’m going to install first, just to be safe... but probably looking to get a 28 and sell 3 other pinions.

 

[hunt2elk]

Just curious, when you are going down the road and your needle is at 60, what is your actual speed? You can check it with a gps or your phone. I am fairly certain I have 28 and 29 gears that I would trade you for your 33 & 36, but with your setup I believe you should be in the mid to high 30's.

 

[///Matt]

Just curious, when you are going down the road and your needle is at 60, what is your actual speed? You can check it with a gps or your phone. I am fairly certain I have 28 and 29 gears that I would trade you for your 33 & 36, but with your setup I believe you should be in the mid to high 30's.

All the calculations and charts say i’m supposed to be using the 33 I put in... but with the 33t, at 60 indicated I’m running about 70 GPsS (roughly).

I took a 26 out of the trans, which was never replaced after the PO put a 3.55 rear in it. He gave me the 36t with the car, but whoever told him is was the gear he needed wasn’t doing him favors.

With the 26t in the car, speedo was reading higher than GPS speed by about the same margin that 33t is doing the opposite of.

 

[hunt2elk]

Something just doesn't seem right. On my Road Runner with 275/60/15 - 28" tall tires and 3.55 gears, I am running a 37 tooth speedo gear. 60mph actual speed reads 59 mph on my dash.
I've never heard of using the mathematical formula stated above, but I'm sure it will work. So if I take your 33 and multiply it by 1.14 (which is 100-86) it comes up at 37.62 teeth. And that is right about where my car is at.

 

[///Matt]

Something just doesn't seem right. On my Road Runner with 275/60/15 - 28" tall tires and 3.55 gears, I am running a 37 tooth speedo gear. 60mph actual speed reads 59 mph on my dash.
I've never heard of using the mathematical formula stated above, but I'm sure it will work. So if I take your 33 and multiply it by 1.14 (which is 100-86) it comes up at 37.62 teeth. And that is right about where my car is at.

That’s all the fun of magnetically driven speedometers, haha... yours is also a 727 with normal gear sets in it? I have the same tires and same final drive as you, and if I stick a 37t in mine it’ll read 50 when I’m actually going 65, lol.

I believe that in your case, your magnet is a bit overpowered, causing you to need a bigger gear than mopar would tell you, and mine (even though just refurbished) is a bit underpowered and requires a smaller gear to get the right needle sweep.

But I know that my speedo cable is old and doesn’t fully and properly attach the the back of the speedo, so I’m going to throw a new one in, in the hopes that maybe it’s just not “fully” engaged and maybe the new cable will speed up my gauge. If it doesn’t work, I’m down to trade you.

 

[///Matt]

So with tire sized at 28”, and 3.55 gears, we should be at 33t pinions... but yours apparently runs like it has 3.91 gears or a 25” tire, and mine runs like it has 2.94 gears or a 32” tire.

I’m half tempted to just install a GPS electro-mechanical speedo conversion drive.... but they’re like $350 so no.

 

[Officer Higgins]

With your car at,as driven condition, with correct tire pressure etc. go for a drive. Divide your speedo indicated speed, by the actual gps speed. Multiply the number of teeth on your speedo gear by that number. That is the number of teeth you need. Example. Indicated speed is 60, actual speed is 50. 60 ÷50 is 1.2. If you have a 30 tooth pinion you would need a 36 tooth pinion. Try this at different speeds, in this case the results should all be 1.2. If not the speedo needs to be recalibrated

That's some great calculations and using your noodle work. Good thing your mopar guy otherwise i might have to try to demean and diminish you out of pure jealousy, to make myself fell better lol...great calculations math never was my strong point.

 

[Officer Higgins]

You are considering 350 bucks for gps speedometer setup but wanted to borrow someone's speedometer gears because they cost 60 bucks.

 

[///Matt]

You are considering 350 bucks for gps speedometer setup but wanted to borrow someone's speedometer gears because they cost 60 bucks.

I’m tempted to spend the 350 because then the speedo will always be trying to show the right speed, whether I change transmission, differential, or tire configuration... but its too expensive, so I’m not.

The difference is that (in a perfect world) it’d be money spent on a sure thing, instead of money spent playing trial and error.

But again, not serious about buying it. Just want to figure out the right pinion gear and get it in there without buying 3 or 4 of them that will then sit in a drawer.

 

[hunt2elk]

Just checked my stash. I have an extra 28, 29, and 31.

 

[R/T Boy 67]

The proper way to set up the speedo, is to gear for the odometer, and then recalibrate the speedo, if incorrect. A lot of people don't do it that way, myself included. My odometer has been around a few times, so I don't care.

 

[///Matt]

The proper way to set up the speedo, is to gear for the odometer, and then recalibrate the speedo, if incorrect. A lot of people don't do it that way, myself included. My odometer has been around a few times, so I don't care.

Yeah... it’s just too much work and money to pull the speedo and send it off for a good re-cal. Plus, in today’s world of “you probaby have to ship it somewhere” for the cal, the calibration is at risk anyway with transit jostling and magnetic interference encounters along the way. And since the speedo matters a lot more to Johnny Law than the already-exempt odometer... I’ll just get close with gearing. Ha

 

[R/T Boy 67]

I agree. All that matters to most of us is that the speedo is correct. If the magnet is too strong in the speedo, and it makes the gearing seem off from all the charts, then so be it. I believe the speedo cable is designed to rotate at 1000 revolutions per mile for the odometer to be correct. If those revolutions happen in one minute then the speedo should read 60 mph. I feel better now.