Let's do speedo and rear gear math!

Here we go. I have a 1964 Dodge that has a '63 Big block 727 transmission. The rear end has been updated with 3:73 gears. Rear tire diameter is 26.5". Picture attached is from 63 service manual. Need to know what diameter an original 700 X 14 tire would be and which gear to use, to get speedo close. I have had NO luck trying to get a ratio adapter for this car. The output shaft has 8 teeth. Need to factor in the difference in tire size and no correct choice for the new gear ratio. Thanks!

Approximately 205/70/14 25.30 diameter.

What I'd do is get the exact dia from "rollout" then go to the MP chart. then you'd have your dia and 3.73 ratio. someone here posted the chart that goes to the tenths of tooth count, I'd find it or see if someone will repost it. IE if says you need a 26.2 tooth pinion for your dia then find a 26 tooth pinion and if you need a 26.7 tooth pinion then find a 27 tooth pinion & if you have an early trans then yes you'll likely need the adapter

You cannot use later pinion math for early pinion choices. The number of teeth on the output shaft is different and will throw the math off if you try to extrapolate from later pinion calculators.

good clarification. OP, with that early trans & pinion selection being limited (as you know) you might try evilbay or the guys at www.slantsix.org or FABO and somebody has to be selling those adapters

You shouldn't have a problem getting a ratio adapter,

http://imperialservices.net/Bparts.html

You'd need a 23-tooth pinion and there ain't no such thing. There are inline ratio adapters that can be installed in the speedo cable.

I have tried to contact Imperial Services without much luck. I'll keep everyone posted...

Imperial Services has been sold, all in transition now. Today I talked to the new owner and a ratio adapter for this car is coming, 4-6 weeks out. Built to the .67 reduction ratio required!

how did someone figure .67 ratio reducer? what pinion are you using now? i don't think the math is right.

A 700 x 14 tire should be about 25.5 inches diameter

R.

Here's what we did. Drove the car with a GPS showing the actual speed. At actual 40 mph, speedometer reads 60. At 50 mph speedo reads 75, and at actual speed of 60 mph speedo reads 90. In each example, divide the actual mph by the reading of the speedometer. In this case, it works out to a .67 reduction to match the speedometer to the GPS. Having a ratio reducer built since there is no factory gear available to correct the difference.
This is an early (63 BB 727) transmission. They are different from the '66 up units. Has an 8 tooth drive gear and a 16 tooth gear on cable now, car updated with 3:73 rear gear and tire at 26.5" diameter. This should work. It may be a little pricey, but they are the only game in town for the 62-65 trans.

Ran into the same problem with a 63 Sport Fury running a 65 BB trans with a 64 valve body. Car has 3.91s

Just got the biggest gear I could get. Damn was it pricey

Small tires and high (numeric) axle ratios generate speedo cable speeds that can't be corrected with the available 16-21 tooth early pinions. The .67 reduction allows the user to alter the normal math to choose the correct pinion.

As I stated earlier, the OP's combo would require a 23-tooth pinion which was never available so the .67 reduction would call for a 15-tooth pinion which, also, was never available; so a 16-tooth would be the closest.

That's why an inline reducer is more practical, they allow for a wide variety of reduction ratios.

john, i'm very familiar with the ratio reducers. i use one in my '65 coronet. it just seems to me that .67 is very steep, even with a 16 or 17 tooth pinion.

i'll assume the 16 tooth pinion was with a 2.76 rear gear. 2.76 divided by 3.73 (new gear) is .74. my take is a .74% reduction. o'well, what do i know?

Well, doing math with the GPS and the speedometer, this takes the actual rear tire size into the formula. You don't even have to figure it in, which is what you may be missing with the .74 (2:76 + 3:73) number. The difference between the orig 7.00 X 14 versus the new diameter tire. These were the readings driving the car:

GPS mph divided by speedometer reading = reduction required
40 divided by 60 = .66666667
50 divided by 75 = .66666667
60 divided by 90 = .66666667

.67 should put it spot on...

i trust the math, not the gps.

Did you show your math?

Problem is, the math only guarantees an accurate odometer not an accurate speedometer. Many speedo errors are in the head itself.

GPS factors in new tire size without having to measure diameter and calculate to the original 7.00 X 14 tire, which the 'math' does not do.

Not exactly, what the math does not take into account, because it is an unknown, is the EFFECTIVE diameter of the old and new tire. Effective diameter is the actual diameter with the load of the vehicle on it, not the listed diameter specification.

And that is why the math is wrong. Problem solving is a [censored] when you don't know how the formula works.

FWIW, finally got around to checking my original factory reduction unit and it's .80 reduction.

I dont have GPS but I had assumed it was high tech dead on accurate, ain't so ???. What John said I would get the odo correct with the correct pinion then after that if the speedo ain't correct then it would need to come out & go to a speedometer specialty repair shop. I use 5 mile markers in a row on the interstate to check the odo and those portable lit up digital roadside MPH signs the city sets out on occaision to check the speedo

Free version gives you actual mph, paid version gives you actual miles plus a wealth of other features. I have on IPhone, likely Android version. Invaluable for syncing speedo/odometer.

Actually to my way of thinking the GPS is MORE ACCURATE than the math because you're never quite sure how many ACTUAL revolutions the tire will require per mile. You can quote rolling radius all you want, but it has to vary some and that's a dynamic process. Think of drag slicks.

We use GPS to measure movement on civil structures, and that is well below inch accuracy. It's repeatable, as well.

R.

When Lacking a GPS I would drive the car on the Interstate from mile marker to mile marker. Needs to be over 25 miles though, the more miles the better the accuracy.

I divide the miles traveled on the odometer and the actual miles travel by the mile marker signs to get a percentage. If it is 20 percent too low I get a speedometer gear with 20 percent less teeth to speed it up.

That being said, John is right about speedometer heads being off. My method works for calibrating the Odometer with the hopes the speedometer will be right afterwards.

Try this http://vexer.com/automotive-tools/speed-rpm-calculator

If the actual odometer head is off, the GPS method still factors that in and corrects it. Along with the effective tire diameter and all variables, so it seems. I don't see the GPS being inaccurate, so I'm confident the .67 ratio adapter is the answer. Thanks for all the responses!