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.

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.

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.

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. I was able to find a good chart of the different ratios available on this site...

https://speedometercablesusa.com/gear_box_adapters.html

Right now 1.8889 looks like it will be the closest ratio I can find so I may go this this one. I would rather have the speedometer spin a percent or so fast instead of slow, so I may decide to use a different ratio...