Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap.
So I cant get a speedo gear? I am not looking for 100% accuracy, just close, maybe 3-5 MPH off, Could I just go ask for a speedo gear for a 75 maverick with 3.73's and a C4? Or is it not that easy?
The car is about done and this is the only part I have, and I am kinda low on cash, I was hoping I would be able to pick up a speedo gear at ford for like 20 bucks and be close...
You have tons of money - put a T-5 in it, and then I can help!
I'm following along - my problem is that I'm not familiar enough with the C4's to know what their drive gears are like - and if they even mesh up with the cable out of the STang. Does it have the speedo hooked up and it's just not accurate? Or is it not even hooked up? Another variable is your tire height. How tall is the tire? Or what's the size of it - we can calculate height from that. The '92 Vert came stock with 14" tires? Tire height will impact speedo accuracy too. The challenge is that all the data out there is for T-5's and AOD's with 16" tires (for the most part). So every piece of yours is different. You may ultimately have to follow jrichker's advice to get it to work together.
You didn't answer some questions - is it already hooked up and inaccurate, or is it not hooked up at all? Do you know if the Stang's speedo cable/gear is compatible with the C4? Those tires are about 24.4" tall (if you're correct about the size) - compared with about 25.7" tall for most of the HO Stangs out there. That makes your 3.73 gears about like a 3.89 gear in a Stang with stock 16" tires. But here's the challenge - the C4, I believe, is a 3 speed tranny with a 1:1 final drive (boy is that thing gonna be a revver on the highway!!!). The AOD's and T-5's have a top gear ratio of about .68 or .63:1. So your top gear is about the same ratio as if you drove along in an AOD in 3rd gear, or a T-5 in 4th gear. Consequently, finding the right gears to make an accurate speedo is a tough task. It will likely require changing the drive gear inside the C4 - which is usually a major pain (on the T-5 it is) -- that's why I said following jrichker's advice about getting a speedo box to correct the error may be the best way to go.
Do you want the speedo to work, or do you just need to know how fast you're going? If it's the latter - you provide me with the size tires on the rear of the car, the final drive ratio, the final drive ratio of top gear in the tranny - and I can run some calcs for you that will tell you how fast you're going at certain rpm.
Even a 23 tooth will be WAY off because of the lack of OD on that tranny. It seems to me that you don't understand the speedo implications of what you've got in the combo. If you had an AOD, a 23 tooth would get you close, but because of the difference in top gear ratio (1:1 on your tranny; .68:1 on an AOD) - you'd still be over 30% off. Are you contemplating another rear end/gear change? I'd get that and whatever wheels and tires you're ultimately gonna have on the car before I tried solving the speedo. I'd also get underneath and see if the Mustang speedo cable will even hook up to the C4 without some sort of adapter. You've got lots more homework to do before you can begin to figure out what gears are needed to make things accurate. And even then, I suspect jrichker's solution is what will be needed when all is said and done.
They put the C4 in 79-82/3 mustangs correct? Dont you think i could get a speedo gear for like, 3.55's and be close, or will it still be way off due to wheels and what not.