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Hello everyone,
I bought my 1987 Trans Am last year and have been working at making it roadworthy ever since. My last major issue before I start to drive it is repairing the speedometer.
Currently, the speedometer reads below my actual speed. I'm not sure by how much. Now, a speedometer that reads below the speed you are going, should need a driven gear with less teeth, correct? I heard more teeth will slow it down.
This is where the trouble begins. My differential ratio is 3.27, my Tire Diameter in inches is 26, and my drive gear is 17 tooth. I put that through one of the calculators online, and it told me I needed a 43 tooth driven gear, instead of my 39 tooth. Not only is this not possible, because it won't fit in my housing, but wouldn't adding more teeth to the driven gear, thus slowing it down, just make my problem even worse?
The next suggestion I had was to change the drive gear by dropping the teeth count from 17 to 15. Wouldn't doing this also slow down my speedometer? If my speedometer is running slow, why is my Calculator suggesting I make it slower? My final, most important piece of information is that my driven gear housing is electrical, NOT mechanical. Does this change the calculations that need to be made?
Thanks to anyone who can help me figure this out This is my driven gear housing. My speedometer is electrical, not mechanical. this is a photo of my 17 tooth drive gear.
Not sure what numerical system you're in, that there's 17 teeth on that drive gear. I see 7.
You don't really need an "online calculator" for something as simple as speedo gears. Grade-school arithmetic, not even "math". Easy enough to just do it yourself.
The speedo cable or sending unit of that type, turns 1000 turns per mile when set up properly. (NOT 1001, as you sometimes see: a brief moment of thought will reveal that there is NO POSSIBLE gear train within the speedo that can do tenths of a mile, if that were the case; rather, the speedo turns over to the next mile on the 1001st rev of the cable)
Measure your rear tire radius under loaded conditions with a tape measure, from the center of the wheel to the ground, and at the normal inflation pressure you use. DO NOT try to "calculate" based on its "trade" size or measure its "diameter". I'll use 13½" for this example butt once you get the ACTUAL measurement by this method, plug it into the same process. Use your actual tooth count for the gear ratio, not "3.27"; these numbers are 41 & 9 for your gears.
A mile is 5280 feet. This is 63,360 inches.
The tire rolls out 2 × pi x tire radius, per revolution. For the 13½" example, this is 84.823". The tire therefore rolls 63,360 ÷ 84.23 times per mile, which is about 746.97 times.
The drive shaft rotates your gear ratio times that many times, per mile. 746.97 × 41 ÷ 9 is 3402.8 revolutions. (your engine would turn this many revs in a mile in 4th gear if you had a manual transmission; which is obviously also, this many RPMs at 60 mph)
Your speedo gears then, need to be in the ratio of 3.4028:1, to get from 3402.8 to 1000. Obviously you're limited to whole numbers of teeth (no fractional teeth on a gear), in combinations you can actually buy. Best driven gear you can do with that 7-tooth drive gear then, would be 7 × 3402.8; this would be 23.94. A 24-tooth driven would give you an error slightly less than .2%, in the direction of the speedo reading low. This is PLENTY close enough, as tread wear and seasonal variations in tire pressure will create larger uncertainties than that, even if all else is perfectly equal. Keep in mind that since you have a pink drive gear, your driven has to be compatible with that; they make different thicknesses of gears, which won't mesh right if not compatible.
Since you have the VSS, your driven gear will be a little disc with a D hole in it and teeth around the edge. That style of gear was used for only a very few years and only in a few kinds of cars, so there never were very many of them. AFAIK they're no longer available new, and I don't know whether anybody even tries to maintain an assortment of used ones. You may find yourself out of luck.
How many teeth does your current driven gear have? What is your tire radius when measured correctly, as described above? How far off is your speedo, in %? (like, 2 mph at 30, 4 mph at 60, 6 mph at 90; this would be about 6.667% error)
Last edited by sofakingdom; 06-25-2024 at 12:38 PM.
Not sure what numerical system you're in, that there's 17 teeth on that drive gear. I see 7.
This is really interesting because this implies that I have a 39 tooth driven working with my 7 tooth drive, instead if a 17, which I'm supposed to have according to my tire diameter and diff ratio. This would explain why my speedometer is going slow, not fast. Thus replacing the 7 tooth drive gear with the 15 tooth I have will bring me to a more accurate speedo, according to speedometer calculators. My only question is how did you know the tooth count? Pretty much all drive gears only show 6 or 7 lines on the wall.
So to summarize:
A compatible driven gear for this 7 tooth drive would be roughly 22.
A Compatible drive gear to work with this driven gear would be 15 tooth.
15 tooth drive and 39 tooth driven will bring me closest to my desired ratio, given that my diff ratio is 3.27, and my tire diameter is 26, correct?
Is a 7 tooth drive and a 39 driven even physically compatible? If not, it can't possibly be in my vehicle, working, can it?
I counted those little "ridge"-looking things, aka teeth. Starting at the left. One, two, three, ...
How many teeth does your current driven gear have? What is your tire radius when measured correctly, as described above? How far off is your speedo, in %? (like, 2 mph at 30, 4 mph at 60, 6 mph at 90; this would be about 6.667% error)
I have a 39 tooth driven gear, tire radius of roughly 13- I can't check at the moment- and I haven't driven the vehicle in months so I can't check the speedometer yet. I'm going to get new tires this week and take the vehicle to a transmission shop to have the 15 tooth drive gear put in. I don't mind if my speedometer is slightly off, as long as it works well. Right now it doesn't.
Just read the sticky threads. Follow the speedo gear charts. They make speedos right. The calculators get you close. No need for 10 paragraphs; the info is posted.
06-25-2024, 10:34 AM
