How to determine what speedometer gears?

[DanR]

I am building a new T5 and have hit a snag!!

Need to determine what gears that best fit my Alpine (s).

If I have a 4:22 rear gear and run P195 55 15 tires how do I determine the correct selection of speedometer gears for the T5

Found this Link: https://www.tremec.com/calculadora.php

Now I need to answer the questions .....

NOTE: The T5 being built has a "yellow" drive gear

 

[260Alpine]

Dan, Maybe this will help. 4.22 will make for a granny 1st gear. I've also used gear ratio adapters before too.

https://duckduckgo.com/?q=t5+speedometer+gear+tooth+count+to+rear+ratio&iax=videos&ia=videos&iai=https://www.youtube.com/watch?v=fMEe5SySuxA

 

[260Alpine]

Speedometer gear calculator:

https://www.tciauto.com/speedometer-gear-calculator

 

[husky drvr]

I am building a new T5 and have hit a snag!!

Need to determine what gears that best fit my Alpine (s).

If I have a 4:22 rear gear and run P195 55 15 tires how do I determine the correct selection of speedometer gears for the T5

Found this Link: https://www.tremec.com/calculadora.php

Now I need to answer the questions .....

NOTE: The T5 being built has a "yellow" drive gear

Dan, what speedometer head revs/mile and what T-5 gear ratios ( OD .63 or .72)?

Is this T-5 the one that was behind the GM engine? Do you have the tag number?

 

[husky drvr]

Dan,

Doing the math, it appears you need a 6 tooth DRIVE gear mated to a 21 tooth DRIVEN gear. If the variables I used are correct.

Ford and GM T-5's use different speedo gears as far as I can tell.

 

[DanR]

Dan, what speedometer head revs/mile and what T-5 gear ratios ( OD .63 or .72)?

Don, It is a T5 1352 238 that had the 3:35 1st gear, But I have found for rebuild, a 1st gear set of the 2:95 to 1, 2nd 1.94, 3rd 1.34 and 5th has .63

Currently has a "yellow" drive gear installed now. The yellow has 7 teeth. Looking for the correct driven gear or even replacement for the drive gear mated with the correct driven gear for my Green1.

AND WHILE AT IT I might as well figure the correct gears for a 3:89 rear with same tires and wheels.

My wheels have P195 55 15 tires, 7" wide and stand 22" tall, rear axle ratio is 4:22 to 1.

Trying to figure the gear for the speedo cable (color/number of teeth).

 

[260Alpine]

Dan, You need a different input shaft for the 2.95 first and can't mix NWC and WC parts. Those gear ratios would be better for the 4.22 rear.

 

[DanR]

I understand not to mix. The input shaft will be corrected for the 2:95.

The shop doing the rebuild has built them for me before. But, I will recheck to clarify!

 

[husky drvr]

Don, It is a T5 1352 238 that had the 3:35 1st gear, But I have found for rebuild, a 1st gear set of the 2:95 to 1, 2nd 1.94, 3rd 1.34 and 5th has .63

Currently has a "yellow" drive gear installed now. The yellow has 7 teeth. Looking for the correct driven gear or even replacement for the drive gear mated with the correct driven gear for my Green1.

AND WHILE AT IT I might as well figure the correct gears for a 3:89 rear with same tires and wheels.

My wheels have P195 55 15 tires, 7" wide and stand 22" tall, rear axle ratio is 4:22 to 1.

Trying to figure the gear for the speedo cable (color/number of teeth).

Dan,

Still presuming your using the stock speedo with 1020 revs per mile.

Your reported tire height at 22 inches is more than 50 revs per mile greater than what most speed calculators use for your tire size. About 7% faster.

removed extraneous information

 

[260Alpine]

Don, I get 23.44" for the diameter of 195-55-15.

 

[husky drvr]

Don, I get 23.44" for the diameter of 195-55-15.

Jim,

I also used 23.44 ( a calculated number ) until in the #6 post, Dan stated his tires are 22 inches tall. Actual measurements take precedence. It seems the 7 inch rims he is using make quite a difference as compared what seems the standard 6 inch measuring rim dimensions.

 

[DanR]

I am really confused!!

When asked for the height of my wheels (tires) I measured 22"

Then the 23.44" for the diameter ? Height vs diameter?

And I am utilizing stock speedometer.

 

[MikeH]

I am really confused!!

When asked for the height of my wheels (tires) I measured 22"

Then the 23.44" for the diameter ? Height vs diameter?

And I am utilizing stock speedometer.

I think if you go to a tire site and use their tire calculator, https://tiresize.com/calculator/ plug in the tire size, it will show the diameter. Actually measuring the height on the vehicle could vary as husky_drvr said based on rim width or tire pressure.

 

[Barry]

Jim,

I also used 23.44 ( a calculated number ) until in the #6 post, Dan stated his tires are 22 inches tall. Actual measurements take precedence. It seems the 7 inch rims he is using make quite a difference as compared what seems the standard 6 inch measuring rim dimensions.

Don,

Rim width affects tire cross section width, but it has almost no affect on the diameter of a radial tire.

 

[MikeH]

Googling around it looks like the diameter would not be affected (or only by an insignificant amount) by rim width with a radial tire. I did find this, don’t know if it would help or not.

http://www.rimsntires.com/specspro.jsp

 

[Bill Eisinger]

Hey Dan- just make your life easy and get a GPS spedo like I'm doing with my project!

 

[Barry]

This thread is a very good example of why not to use measured tire diameter for anything except possibly appearance. The simple (and correct) method is to go to the tire manufacturers website and look up the tire data with emphasis on revolutions / mile which is what matters for gearing and speedometer calculations.

 

[Barry]

Posted as information

Dan and I had a long discussion about getting a cable driven Series Alpine "1020" speedometer to read correctly with a T-5 transmission, 4.22 rear gears and 195/55-15 rear tires. The gist of that discussion was:

To get a SA "1020" speedometer to read 60 MPH at an actual 60 MPH, the transmission cable should revolve 1,020 times / mile.

The revolutions / mile values for 195/55-15 passenger radials range from 885 to 890 with a typical value of 888 revolutions / mile.

With a 4.22:1 rear gear, 888 tire revolutions / mile = 3,747 transmission tailshaft revolutions / mile.

With a 7 tooth "yellow" drive gear:

A 26 tooth driven gear will result in the speedometer cable turning 1009 revolutions / mile and a "SA 1020" speedometer should read 59.34 MPH at an actual 60 MPH.

A 25 tooth driven gear will result in the speedometer cable turning 1049 revolutions / mile and a "SA 1020" speedometer should read 61.72 MPH at an actual 60 MPH.

A 27 tooth driven gear will result in the speedometer cable turning 971 revolutions / mile and a "SA 1020" speedometer should read 57.18 MPH at an actual 60 MPH.
​

IMO, it is doubtful that a 50+ year old "SA 1020" speedometer will read correctly. I would start with a 26 tooth driven gear and be prepared to change the driven gear to what ever tooth count is necessary get the "SA 1020" speedometer to read close to 60 MPH at an actual 60 MPH. FWIW, the calculated difference between driven gears is 2.3 MPH per tooth.

With a 3.89:1 rear gear and a 7 tooth yellow drive gear, I would start with a 24 tooth driven gear and adjust accordingly.
​

Hope this is helpful to someone.