Brockway Speedometer Calibation

[oldspwr]

I posted this a while back on the Brockway Message Board and figured it may help some folks here...

This post is a little more technical in depth (well actually a lot more...) but may help a few folks out.  I am getting ready to replace the 4.10's in my Dad's 361 with  3.70's to reduce the rpm at 65mph. Once I do that the speedometer will be off so I'll need to correct it.  With the 4.10's in the truck the speedometer is fairly close and I confirmed this with a GPS based on my iPhone.  If your speedometer is way off then this may be a good way to approach it.

Let’s start with the basics...

Based on SAE standards, at 60 mph the speedometer should turn 1000 rpms.  This is a constant value that is true for automotive speedometers, including the ones used in Brockways as manufactured by Stewart Warner.  To determine the correct ratio there are (4) variables we need to consider.  These include the rear axle ratio, the rear tire diameter, the drive to driven gear ratio within the transmission and the ratio of the speedometer adapter.

Rear Axle Ratio

Most guys are familiar with what their rear axle ratio is.  If not, sometimes there is a tag on the rear housing.  In this case we are looking at 3.90’s...

In some cases look you can look at the head of the pinion.  Typically the number of teeth of the ring and pinion is stamped on the head of the pinion.  It this case it's 7 and 38, so there are 7 teeth on the pinion and 38 teeth on the ring gear.  This equates to 38 / 7 = 5.43.

Rear Tire Diameter

For the rear tire diameter, I typically place a straight edge across the top of the tire and measure to the ground.  By doing it this way you will account for the 'load' on the tire.  In reality, the best way is to measure the radius from the center of the wheel to the ground and multiply it by 2, but it is difficult to figure out the exact center (unless you mark your center cap or something like that.)  Regardless measuring the diameter in this manner will get you very close.

Transmission Drive & Driven Gear

Within the transmission, where the speedometer cable or adapter connects to the transmission, there is a set of gears.  One gear is located on the output shaft and this is called the drive gear.  The other gear is what the cable or adapter connects to and this is called the driven gear.  The drive gear typically has between 4 and 6 teeth and the driven gear has between 12 and 15 teeth.  

I realize it is difficult to determine what your drive and driven gears are, but I believe we can work around that.  On the early Brockway chassis records, the Fuller 5 speed transmissions typically had 4 drive teeth and 15 driven teeth.

This is from a ‘63 155T, but it’s the same for my ’55 155W, ’46 260XW, etc...

To determine the gears in the RT910, I went through a Parts Manual I bought off of eBay a number of years ago.  It was for the 1970 Sears Oil  E361T’s, which included my Dad’s truck.  Sure enough on the very last page in the manual I found this...

So for the RT910 the drive gear has 4 teeth and the drive gear has 14 teeth. 
 

More to follow...

[oldspwr]

Speedometer Adapter Ratio

Every Brockway truck that I have purchased had some kind of Stewart Warner adapter on the transmission.  These are typically covered in oil but once you clean them up you can usually find the ratio stamped on them.  This also came from a Brockway manual.  This shows (3) different style speedometer adapters (or what they called ‘correcting adapters’).  I typically see the ‘box type’ on Brockways and the ‘straight type’ on GM cars and trucks.

RL]

If we go back and reference the pic for the 155T, the ratio was 1.155...

So there are 2 options available to correct your speedometer.  The first is to replace the drive and / or driven gears within the transmission and the second is to replace the speedometer adapter.  Years ago when I was working on cars, it was easy to replace the drive and driven gears within the TH350, TH400 and 200-4R and 700-R4 transmissions.  In fact I had a coffee can full gears I collected from the junkyard.  But with the larger Fuller / Eaton / Roadranger transmissions, it’s not really that easy.  And I honestly do now if different gears are available.  So the most practical solution is to replace the speedometer adapter.  To help determine which adapter ratio we need, we need to work through a formula...

Although this is a busy formula, I will work through it to simplify it.  We are going to solve for the speedometer ratio which we know should be 1000 rpms.  

And then we can simplify it to this...

So using my Dad’s truck for reference, we have the following variables:

Rear axle ratio 4.10

Rear Tire Diameter 40.5”

Transmission Drive Gear 4 teeth

Transmission Driven Gear 14 teeth

Speedometer Adapter 1.656

So we’re actually pretty close to 1000.  You have to keep in mind that in some cases it will be difficult to get it to exactly 1000 due to the gear combinations available for the speedometer adapters.  To understand just how close we are, we’ll calculate the percentage...

In a majority of the Brockway chassis records I looked at, they ranged from  0 to 6% off so we are in the ball park here.  
 

[oldspwr]

But now we can reconfigure the formula to solve for the speedometer adapter we need.  This is helpful if you are making a tire change or a rear axle change.

And again using my Dad’s truck for reference, we can input all of the variables but this time using 3.70 for the gear ratio...

Rear axle ratio 3.70

Rear Tire Diameter 40.5”

Transmission Drive Gear 4 teeth

Transmission Driven Gear 14 teeth

So it looks like I need an adapter with a 1.8995 ratio of something close.  

RATIOS AVAILABLE FOR 8800 SERIES Straight and Offset ADAPTERS

SLOW DOWN RATIOS AVAILABLE    CORRECTS FOR    SPEED UP RATIOS AVAILABLE    CORRECTS FOR

0.9822    1.8% FAST       1    0% CORRECTION
0.9743    2.6% FAST       1.0181    1.8% SLOW
0.9705    3% FAST          1.0263    2.6% SLOW
0.9642    3.7% FAST       1.0303    3% SLOW
0.9615    4% FAST          1.037    3.6% SLOW
0.9578    4.3% FAST       1.04    3.9% SLOW
0.9523    5% FAST          1.0439    4.3% SLOW
0.9457    5.7% FAST       1.05    4.8% SLOW
0.9444    5.8% FAST       1.0574    5.5% SLOW
0.9333    7.1% FAST        1.0588    5.6% SLOW
0.9273    7.8% FAST        1.0714    6.7% SLOW
0.9183    8.8%FAST         1.0784    7.3% SLOW
0.9122    9.6% FAST        1.0888    8.2% SLOW
0.9058    10.3% FAST      1.0961    8.8% SLOW
0.9    11.1% FAST              1.1038    9.5% SLOW
0.8974    11.4% FAST       1.1111    10% SLOW
0.8947    11.7% FAST       1.1143    10.3% SLOW
0.8888    12.5% FAST      1.1176    10.6% SLOW
0.8824    13.3% FAST      1.125    11.2% SLOW
0.8814    13.4% FAST      1.1333    11.8% SLOW
0.8797    13.6% FAST      1.1344    11.9% SLOW
0.8769    14% FAST         1.1368    12.1% SLOW
0.8684    15.1% FAST       1.1404    12.4% SLOW
0.8667    15.3% FAST      1.1515    13.4% SLOW
0.8653    15.5% FAST      1.1538    13.4% SLOW
0.8626    15.9% FAST      1.1555    13.5% SLOW
0.8571    16.6% FAST       1.159    13.8% SLOW
0.85    17.6% FAST           1.1667    14.3% SLOW
0.8462    18.1% FAST       1.1765    15% SLOW
0.84    19% FAST             1.1818    15.4% SLOW
0.8333    20% FAST        1.1905    16% SLOW
0.8319    20.2% FAST      1.2    16.7% SLOW
0.8241    21.3% FAST       1.202    16.8% SLOW
0.821    21.7% FAST         1.2133    17.6% SLOW
0.8105    23.3% FAST      1.2179    17.9% SLOW
0.8095    23.5% FAST     1.2338    19% SLOW
0.8    25% FAST               1.2353    19.1% SLOW
0.7894    26.6% FAST      1.25    20% SLOW
0.7785    28.4% FAST     1.2666    21.1% SLOW
0.7778    28.5% FAST     1.2844    22.1% SLOW
0.7764    28.7% FAST     1.2857    22.2% SLOW
0.7754    28.9% FAST     1.2878    22.4% SLOW
0.7719    29.5% FAST     1.2896    22.5% SLOW
0.7714    29.6% FAST     1.2955    22.8% SLOW
0.7692    30% FAST       1.2963    22.9% SLOW
0.7647    30.7% FAST    1.3    23.1% SLOW
0.7555    32.3% FAST    1.3077    23.5% SLOW
0.7511    33.1% FAST      1.3235    24.4% SLOW
0.75    33.3% FAST         1.3314    24.9% SLOW
0.7466    33.9% FAST    1.3333    25% SLOW
0.7444    34.3% FAST    1.3394    25.3% SLOW
0.7352    36% FAST       1.3434    25.6% SLOW
0.7333    36.3% FAST    1.36    26.5% SLOW
0.7222    38.4% FAST    1.3636    26.7% SLOW
0.72    68.8% FAST        1.3846    27.8% SLOW
0.7143    38% FAST        1.3889    28% SLOW
0.6947    43.9% FATS    1.4    28.6% SLOW
0.6944    44% FAST       1.4394    3.05% SLOW
0.6923    44.4% FAST    1.44    30.6% SLOW
0.6862    45.7% FAST    1.4444    30.8% SLOW
0.6842    46.1% FAST    1.4571    31.4% SLOW
0.68    47% FAST           1.4615    31.6% slow
0.647    48.3% FAST     1.4706    32% slow
0.668    49.7% FAST     1.4835    33.6% slow
0.6666    50% FAST      1.497    33.2% slow
0.6617    51.1% FAST      1.5    33.4% slow
0.6561    52.5% FAST    1.5111    33.9% slow
0.65    53.8% FAST        1.5241    34.4%slow
0.6481    54.2% FAST    1.5385    35% slow
0.6471    54.5% FAST    1.5429    35.2% slow
0.6302    58.6% FAST    1.5455    35.3% slow
0.625    60% FAST          1.5866    37% slow
0.619    61.5% FAST        1.6    37.5% slow
0.6049    65.3% FAST    1.6154    38.1% slow
0.6037    65.6% FAST    1.6531    39.6% slow
0.6    66.6% FAST           1.6564    39.7% slow
0.5952    68% FAST        1.6667    40% slow
0.5929    68.6% FAST    1.68    40.5% slow
0.5882    70% FAST       1.6863    40.8% slow
0.5833    71.4% FAST    1.7    41.2% slow
0.5789    72.7% FAST    1.7143    41.7% slow
0.5777    73% FAST       1.7273    42.2% slow
0.5709    75.1% FAST    1.7307    42.3% slow
0.5701    75.3% FAST    1.7515    42.9% slow
0.5625    77.7% FAST    1.7538    43% slow
0.5607    78.3% FAST    1.7778    43.8% slow
0.5555    80% FAST       1.7831    44% slow
0.5392    85.4% FAST    1.8    44.5% slow
0.5357    86.6% FAST     1.8545    46.1% slow
0.5321    87.9% FAST     1.8667    46.5% slow
0.5294    88.8% FAST    1.879    46.8% slow
0.52    92.3% FAST        1.8889    47.1% slow
0.5185    92.8% FAST    1.9231    48% slow
0.5131    94.8% FAST    1.9286    48.2% slow
0.5108    95.7% FAST    1.9487    48.7% slow
0.5102    96% FAST       1.9576    48.9% slow
0.5032    98.7% FAST    1.96    49% slow
0.5018    93.3% FAST    1.987    49.7% slow
0.5    100% FAST            1.993    49.8% slow
0.4887    104.6% FAST   2    50% slow
0.4859    106% FAST      2.0462    51.1% slow
0.4825    107.2% FAST    2.0606    51.4% slow
0.4761    110% FAST        2.0727    51.7% slow
0.4681    113.6% FAST     2.1    52.4% slow
0.4667    114.2% FAST    2.1361    53.2% slow
0.4621    116.3% FAST     2.1429    53.3% slow
0.4561    119.2% FAST     2.1636    53.8% slow
0.4503    122% FAST       2.1923    54.4% slow
0.45    122.2% FAST         2.2208    55.02% slow
0.4444    125% FAST        2.2222    55% slow
0.4427    125.8% FAST     2.25    55.6% slow
0.4343    130.3% FAST     2.2587    55.7% slow
0.4313    131.8% FAST      2.303    56.6% slow
0.4286    133.3% FAST     2.3182    56.9% slow
0.4186    138.8% FAST     2.3333    57.1% slow
0.4135    141.8% FAST      2.3884    58.1% slow
0.4105    143.5% FAST      2.4182    58.6% slow
0.4    150% FAST               2.4359    58.9% slow
0.3961    152.4% FAST      2.5    60% slow
0.3882    157.5% FAST      2.5245    60.4% slow
0.386    159.9% FAST       2.5758    61.2% slow
0.3746    166.9% FAST     2.5999    61.4% slow
2.6694    62.5% slow
 

More to follow...