[PDC]

T-56 Trans in a 1973 car. PO had restored the car with period-incorrect 6k Tach and 100 MPH Speedo. I swapped in a ‘correct’ cluster with 8k Tach and 160 Speedo. All gauges work, but speedo is now reading about 33% fast - at 40 mph the speedo reads 60. That all makes sense since the 160 speedo is ‘about’ 33% more than the previous 100 mph speedo. FWIW - the car read accurate speed with the 100 mph speedo.

Here’s the question - on the American Powertrain website, they offer multiple plastic speedo gears in different colors ranging from 16 to 21 teeth. Their calculator only accounts for tire diameter and ring-gear ratio. There is nothing in their speedo gear calculator to accommodate for gauge calibration. So a 26” tire with a 3.73 ring gear and a T-56 ‘should’ use a 21 tooth Red gear - regardless of whether the car has a 100 or a 160 mph speedo - but clearly that is not the case.

Has only one played with this before and does anyone have some direction on which speedo gear I should be using to get the 160 mph speedo closer to the truth??

Thank You!!

[tom s]

FYI,I have spare gears if you know what color,1/2 price off their website!Let me know what color.Tom

[PDC]

Originally Posted by tom s

FYI,I have spare gears if you know what color,1/2 price off their website!Let me know what color.Tom

It looks the the Red one at 21 teeth has the highest tooth count. I need to pull what’s in there now - but using their online calculator, I’m coming up Red.

[tom s]

Pretty sure I have one.Will check in the AM.Tom

[DogMeister]

60/40=1.50. Whatever speedometer gear you have in it now, multiply that many teeth by 1.5 and that is the gear you need.

[gtospieg]

You may need to pull the tailshaft off and replace the DRIVE gear if you're already at the limit with the DRIVEN gear.

[PDC]

Originally Posted by gtospieg

You may need to pull the tailshaft off and replace the DRIVE gear if you're already at the limit with the DRIVEN gear.

I’m 99% sure I’ve got the 7 tooth Dive Gear and the Yellow 19 tooth Driven Gear. They offer a 21 tooth (Red) Driven gear and a 23 tooth (White) Driven Gear.

I get the gear count math - and that makes complete sense to me. But I am wondering if the way these gears are set up, you actually get more change to overall speedo reading than straight gear math would suggest based on the mechanical internals of a mechanical speedo. I’m basing this on Tremec’s own graph which shows more dramatic change from gear to gear than straight tooth count would suggest. We’ll see…

[tom s]

I have 2 reds and 2 pinks.Tom

[FrankieT/A]

My car has 3.73 gears and uses a pink speedo gear and my speedo is within 2 MPH.

[tom s]

You need to figure in tire size also.A 26in tire to a 29 in tire makes a big change.Tom

[PDC]

Originally Posted by tom s

You need to figure in tire size also.A 26in tire to a 29 in tire makes a big change.Tom

Absolutely. I have run my numbers - tire diameter, final drive, 7-tooth drive gear - through 2 or 3 different online calculators and all of them have me at 20.xx teeth for either the black (20) or red (21) driven gear.

It is interesting that with the ‘incorrect’ gauge cluster, the 100 mph speedometer was within a couple mph of correct. I sourced a very clean 73 cluster with the 8k tach and 160 speedo and she is reading just a touch under 20 mph fast - speedo says 60/62 mph when I am coasting along with traffic on a known 45 mph section of road. The ring gear, tire diameter, drive gear and driven gear are all unchanged. So the 160 mph speedo must be calibrated differently than the 100 speedo.

On one of the websites, they say each tooth on the driven gear is good for (approximately) 6 mph - so 3 extra teeth should give me the 18 mph drop in speed I am looking for.

Car is at the shop for a bit of revision on the final tune and tracking down an oil drip at the dry sump tank, so I asked them to pull the speedo gear when she’s on the rack to see what color / how many teeth I am spinning right now. We’ll get this figured out.

[gtospieg]

Pete Serio rebuilds and restores Speedometers. He would be a good person to ask if there is some difference between the two assemblies that you have. He is a member here.