Auto Trans Correct Speedo Gear?

[Robert R.]

My '73 is equipped with THM 400 and a 3.08 rear end (AO). I also have new 225/70-15 tires on it. The issue is that the speedo registers a lower speed than actual speed. We tested by driving at a steady rate and having a second car follow. With cell phones, we called out our speed to each other. The difference at one point was 8mph (32mph on speedo while actual at 40mph).

We looked at the side plastic gear and it is a white 36 tooth gear. To make the speedo more accurate, would it be better to go to the 35 or 34 tooth.

FYI - the previous owner had 235/60x15 on it when I bought it. And he may have changed the gear.

What would you suggest to resolve this?
Thanks in advance for your response.
Bob Ricchio

[Joe L.]

Re: Auto Trans Correct Speedo Gear?

My '73 is equipped with THM 400 and a 3.08 rear end (AO). I also have new 225/70-15 tires on it. The issue is that the speedo registers a lower speed than actual speed. We tested by driving at a steady rate and having a second car follow. With cell phones, we called out our speed to each other. The difference at one point was 8mph (32mph on speedo while actual at 40mph).

We looked at the side plastic gear and it is a white 36 tooth gear. To make the speedo more accurate, would it be better to go to the 35 or 34 tooth.

FYI - the previous owner had 235/60x15 on it when I bought it. And he may have changed the gear.

What would you suggest to resolve this?
Thanks in advance for your response.
Bob Ricchio

Bob-----


The 225-70-15 has an OD very similar to the original GR70-15 installed on your car. According to GM, the correct speedometer gear for your trans and 3.08:1 rear gear ratio is a 35 tooth PINK gear of GM #9780628 which is still available from GM.

However, in a case like this, you might want to experiment with a couple of different gears if you want to get the speedometer accuracy as close as possible. There are on-line calculators for this but you're always left with the uncertainty of exactly what the tire OD is. This affects ultimate accuracy.

By the way, to get your exact, true speed, you are much better off to use a portable GPS unit. If you don't have one, perhaps you could borrow one for a few hours. This will provide you with a VERY accurate true speed. Doing it the way you're doing it assumes that the speedometer in the other car is accurate. It may not be.

[Robert R.]

Re: Auto Trans Correct Speedo Gear?

Joe,
Thanks for your input. Your previous advice got me the tires on the car now. I did talk with Paragon and they also suggested the 35 tooth gear which they said has been changed to Orange. One is being sent out now.

BTW -those Firehawk PV41s on the car are fantastic. It is like a totally different car with these on the corners.

Bob

[Robert R.]

Re: Auto Trans Correct Speedo Gear?

Joe,
As a followup to replacing the speedo gear, today we installed the 35 tooth gear and did use a GPS to check the speed readout accuracy. We are now going 5MPH faster than the speedo needle. We drove at 30, 40 and 50 with the unit. Before the gear change (36 tooth), we estimated 8-10 faster.
Would you recomend going to a 34 tooth gear or leave well enough alone?

Or could there be something else to check? ..the speedometer itself?
Your comments, suggestions are welcome.
Bob

[Joe L.]

Re: Auto Trans Correct Speedo Gear?

Joe,
As a followup to replacing the speedo gear, today we installed the 35 tooth gear and did use a GPS to check the speed readout accuracy. We are now going 5MPH faster than the speedo needle. We drove at 30, 40 and 50 with the unit. Before the gear change (36 tooth), we estimated 8-10 faster.
Would you recomend going to a 34 tooth gear or leave well enough alone?

Or could there be something else to check? ..the speedometer itself?
Your comments, suggestions are welcome.
Bob

Bob-----


Yes, armed with that information, I'd go with the 34 tooth gear. It should get you as close to accurate as you're going to get. Due to the limited number of gear combinations available, factory speedometer error in the 2-3% range is fairly normal. "Certified" speedometers (like those used on police cars) use a specially calibrated adapter to "fine tune" the speedometer accuracy and reduce the normal error range. Still, there is some degree of inaccuracy even in these speedometers.

[Robert R.]

Re: Auto Trans Correct Speedo Gear?

Dave and Joe,
With the 35 tooth gear we are close but still off by 5MPH. Your comment of changing the driven gear makes me wonder if at some time if the trans may have had some work done to it. Curriously, the parts chart for these gears shows '69-'72 years for the 34 tooth piece.
Would there have been internal gear changes on the THM400 from '72 to '73?
Ultimately, having the car registering a close to accurate speed is the goal.
I will let you know what happens next.

Bob

[Robert R.]

Re: Auto Trans Correct Speedo Gear?

Thanks Dave,
The process I have been taking has been to put things back to published specs in hopes of getting it close (tire size that is the same original GR70s, speedo gear, etc). That was the reason for the trans drive gear question. But you bring up a point that I did not think to address; the 35 year old speedometer. It may needed to be looked at.

Thanks again for your input.
Bob