Speedo 5-7mph off

http://home.neo.rr.com/chambersracing/speedo.htm that should tell ya what u need to know. I'd change the drive gear to a 6 or 7 tooth, as the driven gear will wear out quickly with the 8.

hey bro, could you tell me what RPM's your turning at certain speeds? Im trying to figure out what gears i have in my rear.

im turning 2,000 RPM at 55 in 5th gear

if you know what RPM's your turning at different speeds in 5th and could post that to help me out it would be awesome

stang2004 said:

hey bro, could you tell me what RPM's your turning at certain speeds? Im trying to figure out what gears i have in my rear.

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Put the rear up on jackstands, put a chalk mark on the tire, put another one on the driveshaft.
Turn the tire exactly one turn and count the turns the driveshaft makes. You may need a helper to watch the driveshaft while you turn the tire. Turn the wheel once and the drive shaft turns 3 1/2 times = 3.55 gears. Turn the tire once and the driveshat turns almost 3 3/4 times = 3.73 gears. You get the picture...

yup...that's exactly what I was gonna say to do.

wish there was an easier way to do that, its a real B1tch to get the car off the ground

isn't there somethin like whatever rpms ur car is turning at XXmph in 4th is what ur gear is? maybe it's 70mph? check it out...

5.0guy said:

isn't there somethin like whatever rpms ur car is turning at XXmph in 4th is what ur gear is? maybe it's 70mph? check it out...

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thats what i heard,, the one guy told me thing Mustang5l5, that at 70 mph in 4rth he turns 3,700 rpm and he has 3.73's

so if i have 4.10s at 70 i should be turning 4,100

and if i have 3.55's ill be turning 3,550


if thats correct

yeah, that sounds right. go test it out!

Ive got a tremec 3550 tranny 3.73s 235/50/16s and when my speedo says Im going 60 Im going 50. When I go 75 Im acctualy doing 60. The way I figured this out was I had my dad drive next to me while he did 50 then 60. He has all stock **** on his truck. His could be off by maybe 5 but still mine is way off. Any ideas on howto fix???!?!?!?!

it's supposed to be off. u need to get a different speedo gear. like a recommended, get a 6 or 7 tooth drive gear and then most like u'll need a 23 tooth driven gear, but use the calculator to find out.

All the speed/rpm comparisons only work if 1) you have the same diameter rear tires, 2) you're certain the tach and speedo are accurate - most stock tachs aren't; many speedos aren't. It's simply not a reliable way to check rear gear ratio. As others said - jack the car up.

jlhall_10 said:

yea im doing 55 in 5th gear at 2000 rpm.

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Wow, I have a tall 5th. I'm doing near 80 at 2000rpm's in 5th gear.

Im kinda having the same problem , when I go 55 my speedo says 70 . I have 4:10 gears also and have the right gear in the tranny. I think my odometer is go faster than it should. This may be a stupid question but how many gears are there supposed in the tranny for the speedo ?

There is a gear on the output shaft of the tranny (6, 7 or 8 teeth) that meshes/drives the gear on the end of the speedo cable (anywhere from 15 to 23 teeth are available for the T5 although not all those came stock in the Ford model). As the output shaft of the tranny turns spinning the driveshaft, the speedo drive gear turns the driven gear on the end of the cable. To make adjustments is a simple matter of ratios. If you know how far off your speedo is, and if you can multiply and divide, you can figure out what gears you need to make your speedo accurate.

So, turboman - let's use your case as an example. Speedo says 70; car's going 55. 70/55=1.27 -- converting to a percentage, your speedo is 27% fast. So you need to slow it down by that amount. To slow it down, you need either fewer teeth on the drive gear, more teeth on the driven gear or both. How do you figure it? The ratio of driven to drive gear (numbers of teeth) needs to change by the same percentage that you're trying to slow the speedo down by - in your case, 27%. So, turboman - crawl underneath and find out how many teeth you have on your drive and driven gears, and come back and post them, and we'll work through the math to tell you what combination you need to make the speedo accurate.

There are websites out there with helpful info (http://www.geocities.com/mgnedell/notchback/tech/speedo.html) -- but it's all based on stock diameter tires and assumptions that the factory speedo was accurate to start with. Many times those assumptions are simply not valid. A couple of tips - use mile markers on the interstate to figure out how far off your speedo is now. Going off of someone else's car isn't a good idea as their speedo may be off too. Use the color of the drive gear (Black = 6 tooth, Yellow = 7 tooth, Green = 8 tooth) to determine the number of teeth. Due to the way the gear is designed/produced (modified worm gear) you CANNOT look through the hole in the tranny and count the number of teeth. You CAN count the number of teeth on the driven gear. And, if you're up in the 4.10 rear gear range, you'll probably have to change the drive gear to return accuracy to your speedo. THat will require pulling the tailshaft off the tranny - a fairly major undertaking (tranny has to come out of my car to do it).