speedo error: gears, tires, mpg?

[compleckz]

so i went from 3.27s to 4.56 and 29" to 33"..

from 29-33 on 3.27s, the speedo read slower than actual, i forget how much..


i now regeared to 4.56.. and according to my gps, speedo at ~60 is actually ~50mph.. so it shows me going faster than actual, meaning my odometer is over-rated too correct? i see tire calculators on the net and gear calculators, but no combination of the two..

also it seems my gas mileage is worse than before somehow.. i filled the tank, reset the trip, and with it over-rated it says ~150mi and i'm down to 1/2 tank already (20.5 gal tank).. that would be ~15 mpg if the odometer was correct, i was getting 13mpg on 3.27s and 33s (yes, with the error factored in)..

someone wanna school me?

 

[ZSK]

It's not ONLY your speedometer that is off. Your Odometer is also off. It's a ratio for the gear located in the transfer case.

What was your stock size tire? What's the tooth count on your speedometer gear? Is it stock also? I'm assuming 3.27 was the stock ratio.

 

[MAKG]

You don't care what the stock anything is.

Measure a PRECISE speed or distance, and compare it to the speedometer or odometer. Pull the drivegear from the transfer case and count the teeth. Change the teeth in proportion to the displayed speed so it's correct. It takes maybe 60 seconds to yank a drivegear. You need an 8mm (if I remember right) box end wrench or ratchet and a flathead screwdriver.

Note that most drivegears have a little less than 20 teeth, so you need a better than 5% distance or speed measurement to be able to determine it. I prefer to use highway survey markers on a fairly straight road to get a precise distance (but if you use mileage markers, you'll need several tens of miles due to the precision issue). You can use a GPS as well, but you'll have to hold a steady speed and it can be hard to get a number good to a few MPH that way (so you have to go FAST).

If you start making assumptions about stock and replacement tire size, you'll quickly discover that there is quite a lot of variation. You want to make the correction to your actual condition, not what some lying scumbag marketer said it is. Make sure your vehicle is normally loaded, and that your tire pressure is adjusted and nominal, when you make the measurement.

 

[rickcdewitt]

ok so here's a hillbilly take on it.what axle is geared 10mph slower than your truck stock?(3.73?)if you take the gear from a 3.73 truck it will put it in the ball park-about 2mph off.then you could go up or down one tooth to get it right on.here's some info on teeth #'s and color coding-http://www.explorerforum.com/forums/showthread.php?t=158739&highlight=speedo+gear
the first formula new teeth=(current teeth x speedo reading)/actual mph might be best as you can just calculate and buy (or scrounge) a new gear. so if your old gear was 17 teeth you'd want a 20 or 21

pretty much what makg said,get an accurate actual mpg compared to the reading on the dash and then use that ratio to change driven gears.

 

[compleckz]

thanks for all the replies, i will be getting the right gear asap, then we'll see if my gas mileage estimates are accurate, if so there will be another thread asking wtf my mileage is still crap.