Speedometer gear

After completing my B Body,, total restro,, My speedometer is reading about 10 mph slow.. I pretty sure I've got 355 gears,,, 15'' tires,, about what gear do I need to put in the transmission,,, Speedometer was completely rebuild.. Thank you for any help..

http://www.moparts.org/Tech/tech/pages/speedo.html. You need to know the actual height of your tires.

Does the odometer count of the miles with road markers correctly?

10 mph slow at what speed? 40 ? 60 ? 100?
Errors in accuracy are directly proportional to speed as they are the result of an incorrect ratio.

The link Stumpy posted will help. Telling us 15" tires isn't enough, the diameter of the tire is the key. A super thin sidewall 195-50-15 tire will surely be shorter than a 235-75-15.

A speedometer that reads too slow needs a speedo-pinion gear that is smaller. This will spin faster.

The opposite is also true.

In many cases, a single tooth difference can be worth 2-3 mph of change.

I have a variety of used speedometer gears that I've used to correct errors. They are easily found on ebay for under $25.

Depends on what year B body as well.

there is no one chart that covers all years.

Ball and trunnion transmissions will use a different gear than later transmissions, so a 64 B will NOT use the same gear as a 68 B for example.

Then the even later transmissions with the plastic pinion adapter will use a third type of gear, but I forget when that went into effect.

None of the three different gear styles interchange, though you can swap the second and third style as an assembly with the adapter and speedo cable.

I would measure rollout in a parking lot, the distance starting with a rear valve stem at 6 O'clock then rolling it straight forward till the stem is back at 6 O'clock (~8ft) then divide Pi into that number for the exact diameter. With that number & your gear ratio then pick the correct pinion teeth number from the chart CNXT posted. this will get your ODO correct & if the speedo magnetic gismo is working correct, the speedo will also be dead on (you did say it was rebuilt). what you could do right now is either use GPS if you have it or go 5 mile markers on the interstate & see if the ODO has the same error % that the speedo does & if so then with the correct tooth count swapped in the trans you should be good.

I've never measured "roll-out" but I have heard the word. Thanks, Robert for that easy explanation!

I have found this smart phone app an invaluable tool.

Hello John, you here? What does the odometer do?

Using GPS to check my speed,, it shows 10 MPH slow according to GPS,, Up to 12 or more difference at higher speeds.. Thanks

the odo or the speedo? Frankenduster, Kunkel put me on to rollout as opposed to just measureing from the underside of a piece of stock on top of the tire down to the pavement for dia.

I said odometer, not speedometer. They work in completely different ways.

Looks like the best thing I can do is see what gear I have in the transmission first,, when I do I'll repost for your option,, I'll get back to you when I check what I have now.. and the tire diameter,, The info I got here helps a lot,,, By the way the millage works great.. Thanks

Does that mean the odometer is counting the miles correctly?

Answer to 71 Bird,,, yes it works good... Thanks

Ok, finally. Then if the odometer works as it should, that means you have the correct gear in your trans, and the speedo needs worked on. Who redid the speedometer.? The odometer is purely mechanical with gears, and the speedometer is magnetic. Not many people have the equipment to mag and demagnetize the speedometer to make it right. You can either get it repaired, or keep playing with the trans gears until it reads right, but then the odometer will either be adding miles up that you haven't driven, or not registering enough miles. My car is the same way.

He says it works but not that he tested it using the hiway mile markers. So it may be off as badly as the speedo.

I asked several times if he tested it with highway signs. That will get to the root of the problem. Either the speedo is wrong, or the trans gear is. Like pulling hens teeth.

I didn't check it on highway signs,,, I will when the weather breaks,,, dry roads, no salt..

Ok, that will tell the tale. And as a side one tooth on the trans gear is worth about 2 MPH.

Hi way signs to measure distance or speed to calibrate speedometers today are as buggy whips and curb feelers.


Download a speedometer app or use a gps navigation system. See posting from several days back.

Agreed, that app works great

Hiway signs work just fine if you haven't spent the big bucks for a smart phone.

In my state the painted stripes next to the mile markers are dead accurate...the airborne cops use the stripes to issue speeding tickets so they have to be accurate.

For the price of a tank of fuel there are millions of smart phones for sale. Obama even gives them away.

Many highways in Calyfornua no longer have mile markers.

Before smart phones, I too used mile markers and a stop watch, that cost me more than a used smart phone costs today. The smart phone has proven more convenient for me and accurate and I ain't required to find no highway to check my speedo or odometer.

Could help OP with his issue.

I recently went thru a speedo accuracy issue with wife's 55 T Bird in which I had swapped in an AOD 4 speed auto transmission. I not only had to sort out mph speedo, but too an odometer accuracy problem. Could have done it with mile markers and a stop watch,,,,but would have wasted a few additional hours doing so over using this app.

60 mph, how long to go 1 mile=

120 mph, how long to go 1 mile=

90 mph, how long to go 1 mile=

To check the speedo by mile markers you don't need a stop watch. Just see if the odometer goes one mile from marker to marker. Just do it during normal driving takes no extra time at all. Don't make it seem harder than it is. Yes your app will do it too but, for those of us that don't need or care for the fancy phone and apps, we can still do it the quick old fashion way.

As an ex--fireman for the Chicago, Burlington and Quincy Railroad on the head locomotives of very high speed passenger trains such as the California Zephyr as a young man, trains sometimes running over 100 mph(speed limit 87 mph) we were always aware of our speeds.

Using either our RR certified pocket watches(or just introduced in the early 60's, Bulova Acutron wristwatches), we(Engineer or Fireman) were always train speed aware, and often would time our trains using mileposts and watches to check out the accuracy of the diesel locomotive speedometers, as accuracy was paramount.

Using this primitive technology, we would be close enough for government work,,,however if eyeballing your watch or the mile post you always were subject to a 'fudge' factor of several mph being common.

Using modern technology utilizing GPS either via a smart phone app or a Garmin type navigation device you are dead nuts on,,,,and far simpler. Calculate the answers to my 3 questions on mph and get back to me for further information, most who attempt will not have the correct answers.

It is impossible to accurately determine looking at an odometer that when the 10th mile indicator is exactly the same as it was when you started you measurement, however 5 or 10 mile measurement average will dial it in more accurately.


My point being that a GPS device is mandatory if you wish indisputable accuracy, if government work standards are sufficient and you have the time,,,mileposts and wristwatches will certainly do the job.

As for me I would prefer to have purchased and installed the correct speedometer gear on the first attempt.

The reason for suggesting to OP that a GPS device may help him in his quest.

The purpose of this forum is to help our fellow members in offering up opinions,,,,as to the most expeditious solution to problem that is presented

When I proposed a smart phone app to perhaps help him out, never occurred to me thst one should first ax if he had access to a smart phone.

Tough Crowd!

We're the best. yes the mile markers are as accurate as an non electronic system can be but agreed I would borrow a smart phone or something similar. I catch Hell for this. I unhook the wire to disable the ringer at night.

You can calculate exactly what speedometer pinion gear you need if you know the tire size and axle gears. I went through this exercise when I upgraded my 65 Barracuda to LBP wheels and have a write-up about it.

Tire Upgrades

What size tires do you have on the back?