Looking for some wisdom from those who have any experience...I have an idea as to what's correct, but I admittedly know nearly nothing on the subject. I've been working to educate myself on this before asking you all by reading posts on CO and information elsewhere. It sounds like many people have found the spec. tire dimensions to cause inaccuracies with the speedometer and I have seen Joe mention in a post or two it's best to use the rolling radius in the calculation (if I understood correctly). I've a photo of the trans serial and gears when I took the housing off if anyone was curious.

Transmission information: I believe I have a 71 Buick TH400 that came with a press fit 21-tooth metal drive gear and an orange 35-tooth driven gear. I was told when purchased it was a recently rebuilt trans from a drag Olds so I'm assuming it must have had quite a rear end gear and tire combination based on the drive and driven gears.
My set up: Rear tires - Nitto 450 275/50/17r (Spec. diameter is 27.83", but I've measured the tires with car weight at just around ~27.25"), rear gear is 3.23, and the new drive gear to be 15-tooth (once I get the old gear off...)

The TCI calculator gave the following:
@15-tooth, 3.23, 27.25" = 35.82251143352061
@15-tooth, 3.23, 27.83" = 35.07594094730279

Using the nearest whole number and tire diameter under load the TCI calculator says I should be using a 36-tooth driven gear. Is this correct? Or is the rep correct to say 35-tooth?

 

The 35 tooth will probably be closer to ideal. I’m guessing the tires will “grow” in diameter ever so slightly at highway speeds. In my opinion, either gear would probably have an acceptable margin of error, but the 35 would most likely be more accurate.

 

Thanks Matt! Looks like I was mistaking loaded diameter with rolling diameter.

 

The simplest, most accurate way to select a gear cuts through much of the estimation...

Old school -- hold the speed at 60 mph; use a stopwatch to time between Interstate mile markers. Multiply the number of teeth by the number of seconds from the stopwatch and divide by 60. The result is the correct number of teeth.

New school -- hold the speed steady; use GPS to determine actual speed. Multiply the number of teeth by the GPS speed and divide by the steady speed. The result is the correct number of teeth.

 

Thank you Gary! I'll be sure to do this as well once complete to double check that the 35-tooth is as the best choice.

 

The problem with this is that you get into tooth counts that don't exist. I would just go off of the chart and online calculators to get the drive gear and driven gears that will work for the application.
With the 3.23 start with an 18 tooth drive gear and a 44 tooth driven should be what you need.

 

Thanks Randy. I had referenced this chart in the beginning and did ask transmission rep about going with an 18 tooth and he said 100% go with the 15. His certainty had steered me away from the CSM chart...

I would imagine the right combination of either accomplish the same end goal? Is there a benefit of going with one drive/driven combination vs the other (15/35 vs 18/44)?

 

I think the higher tooth count would be a bit more forgiving (not change as much as the tires wear). No speedo is perfect, but I think I would go with the factory on this one. It worked for me when I had to change mine up as I had a 20 tooth in my TH400 when I got the car and had to switch it up.

 

The factory chart is probably accurate if you convert the older tire measurements to modern. Tire diameter doesn’t change, just the unit of measurement. If I remember correctly, a G78 is about 28 inches tall, so are 275/60R15 tires. As long as your tire diameter measurement is accurate, the speedo chart in the service manual or the TCI speedo will work.

Also, speedometers aren’t “x miles per hour off”, speedometer inaccuracy is a percentage. If a speedometer reads 50 and your actually going 45, your speedo isn’t 5 mph off, it reads 10% low. If your speedo reads 100, then your actually going 90mph (10% of 100 is 90mph).

 

Very good point!

 

Yes I believe the G78-15 had a diameter of 28" and it appears that the Nitto also rounded up to 28.0" when calculating/providing the tire revolutions per mile (747). Nice to know I can use the CSM for these tires.