Transmissions and DrivetrainNeed help with your trans? Problems with your axle?
Welcome to ThirdGen.org!
Welcome to ThirdGen.org.
You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, at no cost, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, join the ThirdGen.org community today!
I could just tell you what speedometer gears for what axle ratio, but this only works if everybody runs the same size rear tires. So, here's the only correct way to know exactly what speedometer gears you need. First, you need to gather the following information: (1) your rear tire size, such as 245/50R16. (2) your transmission, such as 700R-4, or TH350, or T5 or whatever. This is to determine what tooth counts of driven gears are available to work this formula. For most of you using a GM trans that isn't an overdrive automatic, you ca net drive gears with tooth counts of 6,7,8,9 and 10. For the 200-4R there's 10, 11, 12 and 13. For the 700R-4 there's 15, 17, 18 and 19. (3) your exact axle ratio, such as 2.7333333:1, 2.769230769:1, 2.92856142857:1, 3.07142857:1, 3.0769230769:1, 3.230769230769:1, 3.27272727:1, 3.4166667:1, 3.45454545:1, 3.70000:1, 3.72727:1, 3.90909:1, or 4.100000:1. So then, you need to know your rear tire size's diameter when new and not carrying the weight of the car. This is easiest found at TireRack.com, or it can be calculated. For 245/50R16s, most are 25.7" diameter. Now, the formula followed by an example: 20168.0672268(the constant that makes this work) divided by tire diameter, then that times the number of teeth on the speedometer drive gear, then that times the axle ratio, then divide that by 1000. Then repeat with every other drive gear for your trans, until you find the closest match. Now, the example: get your calculator and follow along: 20168.067 divided by 25.7(for 245/50R16s) is 784.74968 times 15(for a 700R-4) is 11771.245 times 3.7272727(for 3.73:1 axle) is 43874.64 divided by 1000 is 43.87464. So, you need a 44 tooth driven gear. This does exist, so you're good. But what if your closest result for your combo is something like 38.5? Then, to avoid a speeding ticket, round it down to 38.
This ad is not displayed to registered members. Register your free account today and become a member on ThirdGen!
Sponsored Links
Registered users do not see this ad. Click here to register for free!
This thing is perty cool. I always thought I was going faster than what my spedo said so tonight me and my dad clocked it and my car said i was doin 55 but he said I was doing around 67. Dose anybody know the rear gear for a stock 2.8 84 camaro, if not how do I find this out. I need to get this thing straightened out before I get a ticket. thanks
There seems to be some demand for the more popular combos to be pre-figured, so I'll post the best. It seems the 2 most popular tire sizes are 245/50R16, and 275/40R17, which has the convenience to me of both being 25.7" diameter. That, with a 700R-4, works like this: 3.08:1 use 17:41. 3.23:1 use 15:38 or 17:43. 3.42:1 use 15:40. 3.73:1 use 15:44. This also works for 225/60R15 and 245/45R17 and 245/40R18 and 315/35R17.
I prefer a 275/40R18 because it better fills the fender openings, and with a 3.23:1 use 18:44. 3.42:1 needs 17:44. 3.08:1 takes 18:42. This also applies for 245/60R15 and 295/50R15 and 225/60R16 and 245/45R18.
Atilla, just wanted to give a quick thanks for this useful post. After doing the math and figuring out what I needed for my 4.10, speedo is now dead on.
For anyone interested, with a 4.10 ratio, I needed a 15 tooth drive and a 45 tooth driven gear.
__________________ The best way to judge someone's character is to watch how they treat a person that can do absolutely nothing for them.
While that is the closest possible combo for any tire shorter than 27.55" dia., it will be optimistic, so if you have a 15:45 with a 4.10:1 and tires shorter than 27.55", then you'll be going a wee bit slower than what your speedometer says. I'm glad to hear that someone took the time to try, and found it useful. If he or she can , and I can, then you can, too!
I've had to do this so often that I just made a calculator in Excel, with handy part number and color references. All you do is enter your tire size, axle ratio, and the number of teeth on the drive gear. For a 700R4, there are only four options for drive gear, and they are listed. I would generally assume you already have one, so start with it. If not, keep trying until one works.
