[Carlos Brown]

Wes,

I changed my 4:30 gear out for a 3:90 gear earlier this year and I simply had to change the speedo cable driven gear. The trans-drive/speedo-driven gear combination for the 4:30 was 10/35 teeth respectively. The combination for the 3:90 gear is 10/31. If you are not already aware, the tooth count for the gear combos are indicated on the face of the driven gear. Have fun Wes!

Regards,

Carlos

 

[Carlos Brown]

PeeWee,

I am tearing through all of my old receipts in an attempt to locate that part number. No luck just yet, but I will keep you informed. If memory serves me correct, I believe I ordered a driven gear for a 1987 Cressida w/3:90 and W58. I will post when I come across the info...

Regards,

Carlos

 

[Carlos Brown]

Well Gentlemen,

I posted a reply here earlier in the day on Sunday, February 6th, only to check back later in the day and find it gone... The post was in response to the question "How to determine the proper speedo gear combination," which had been posted by Leslie (that post also disappeared).

Well, the first portion of the response is for informational purposes only - unless you are bored one night and feel like doing some mathematical computations. The primary variables utilized in speedo gear determination is speedo and tire revolutions per mile (RP/Mile), along with tire height and gear ratio. The computation matches 1000 speedo RP/mile to (x) tire RP/Mile. This ratio of course factors in tire height and axle gear ratios. It goes something like this...

Speedo Gear Calculator

1.) Determine Tire Diameter ( 225 x 60 x 14) = 24.63"
2.) Multiply Tire Dia. by pi ( 24.63 x 3.1416 ) = 77.38 tire circumference
3.) Divide one (1) mile into inches or multiply yards by inches (1,760 yards x 36 inches ) 63,360
4.) Divide 63,360 by 77.37 ( 63,360 / 77.38 ) = 818.8 RP/Mile of tire
5.) Multiply tire RP/Mile by Axle Ratio to determine driveshaft RP/Mile ( 818.8 x 4.30 ) = 3,521
6.) Divide by 1000 speedo RP/Mile (3,521 / 1000) = 3.521 (Round this number to 3.5 - this is the ratio you want to achieve).

You will now need to research the available gear ratios for your tranny and determine which combination will work best. My 86 Supra came with a 4.30 rear gear and a 35/10=3.5 speedo gear set. When I went to the 3.90 rear gear I chose the best available speedo set which was the 31/10=3.10. After doing the calculations I found myself at an actual ratio of 3.19, which ends up at a 2.8% error (3.10 / 3.19) x 100 = 97.2 or 2.8%. After correcting for my taller tires (25.1") I recalculate a speedo error of only 1.0%. Supposedly +/- 3.75% is an acceptable error percentage. This equates to 3/8 of one mile error for every ten (10) miles traveled.

I made a more thorough presentation the first time, but hopefully this still helps in your quest to find the proper speedo gear sets. If someone would like, please save this info for others that would like the reference.

Regards,

Carlos

 

[Carlos Brown]

ZZT231,

I ran a few numbers for you and this is what I came up with. Assuming you are running a stock height tire and going to 3.73:1 rear gear, you need to achieve a speedo gear ratio of 3.05. This can be achieved by the following combinations:

1.) 33/11 = 3.0 ratio (2% speedo error)

2.) 34/11 = 3.09 ratio (1% speedo error)

3.) 35/11 = 3.18 ratio (4% speedo error) unacceptable...

4.) 31/10 = 3.10 ratio (1% speedo error)

Now two things! One, keep in mind that a 3.75% error is supposedly acceptable, and most importantly, "try and find a speedo gear that will match the driven gear that is currently being used in your tranny." As an example, I started with a 35/10 for a 4.30 gear and only swapped the 35 tooth speedo gear for a 31 tooth one which mated perfectly with the 10 tooth driven gear. None of that tearing into the tranny mess - just a couple of minutes to complete the whole job and you are going again. If you are not sure which gear you currently have just pull the speedo cable assembly out of the tranny and look on the face of the speedo gear. It will have the two corresponding gear numbers etched on it.

Regards,

Carlos

 

[Carlos Brown]

Dynamic73,

I certainly don't want to give you any bad news, but the 32/11 gear combo will probably have a greater speedo error than you would like. Go through and perform the calculations and see what you come up with.

By my calculations, the 3.90 rear gearset with 24.63" tires will require a 3.19 speedo gear ratio. The 32/11 speedo ratio will produce a speedo ratio of 2.91, which is (2.91/3.19=9%) by my calculation right at 9%. Seems as if you should be closer to a 35/11 or 32/10 for the 3.90 rear gears. Check the math - If you find yourself needing a 35 tooth speedo gear I will send you the one sitting on my desk... PM me!

Regards,

Carlos

 

[Carlos Brown]

Dynamic73,

Well friend, I do not know your Toyota parts guy, so I can't comment on his knowledge about cars, but most of them that I have seen struggle to read the microfiche...

Just keep this in mind, the 4:30 gearset with 24.63" tires requires a 3.5 speedo ratio, and the 3.73 gearset with 24.63" tires requires a 3.05 speedo ratio. It therefore stands to reason that the 3.90 gearset with 24.63" tires will require a speedo ratio somewhere in between the two of these. The 32/11 ratio of 2.91 is outside of this range - something to think about!

Good Luck,

Carlos