I recently switched from 3.90 gears to 3.73 gears and I think that my speedo may be off.

I adjusted the speedo using an XCalibrator (using XtremeTune) but I'm not 100% sure it worked coz it "feels" like the speedometer is reading faster than I "think" i'm going. As we all know, SOTP impressions are fallible but I just can't shake that feeling that it's not correct. I just don't have any proof. Any way to test this?

I guess one way to test it is get somebody to pace me side-by-side and both agree to go to the same speed (say 50mph) and see what happens.

 

or you can set a stop watch to your odometer.... set your cruise for 60, and see how far you get in 60 seconds. this isn't going to get you exact, because reaction time to hit buttons, etc, but you'll be in the ballpark.... you can see it change when you're watchin' the clock.

 

May be I can get a kind cop to radar me?

 

Where did you get 3.90:1 gearing for a 2005?
To determine your true speed, set the cruise control for a constant speed and measure the time it takes you to travel one mile:
Vehicle Speed (MPH) = 3,600/seconds (to travel one mile)

 

I got the 3.90 gears from Motive and my current 3.73 came from Precision.

The only problem with using distance to guage the speedometer is that you gotta know how far you've travelled. That in itself is not so easy.

 

problem is if the speedo is off... odds are the odometer is off the exact same amount

 

Thanks for the update on the 3.90:1
Washington State freeways and highways all have local mileage markers every mile, with this exception:
If the mileage marker is near an on-ramp or off-ramp, you should skip that one and continue your timing to the next marker. This is because it may not be placed exactly on the mile mark. If so, just multiply 3,600 by the number of miles traveled then divide by total seconds to travel the multiple mile segments.

 

I live in Los Angeles, CA - not sure if there are mile markers - i never paid attention.

I guess, if you know the exact distance you're traveling, all you gotta do is look at the odometer difference. And if it's not exact then you're off - right?

 

One final thought on this.
If you are still calibrated for 3.90:1, when your speedometer indicates 60 MPH, your true speed would be:
60 MPH Indicated * 3.90/3.73 = 63 MPH Actual Speed.
If you somehow flashed your computer for the stock gear ratio of 3.55:1, then you would get:
60 MPH Indicated * 3.55/3.73 = 57 MPH Actual Speed.

 

All Interstates have mile markers. Not sure about freeways. Get out on I-5, I-10, or I-15 (depending on what part of L.A. you're in) somewhere outside of town and set the cruise. Don't just do one mile. Take the average over about 5 miles.

 

Easiest thing is to follow someone and have them do a steady 50 or 60 mph.

 

Actually, CA doesn't seem to have them. I see them in AZ and NV all the time, but not here in SoCal. I wish they did. But we do have call boxes about every 1/2 mile of so!!

 

That's weird. I never noticed that. I thought it was federal law to have them since the interstates are federal. I can see how the freeways may not have them, but the interstates??

 

Just out of curiosity, is the odometer tied to the speedometer. I mean, if your speedometer is off, would you odometer reading be inaccurate too or does that reading come from an unrelated sensor - may be the wheel rotations vs the differential or shaft rotation.

I'm actually thinking that perhaps the flash worked coz if it was still at 3.90 settings then my actual speed would be faster than the speedometer reading. My fallible SOTP sensor was suggesting the reverse - lol.

 

Just out of curiosity, why did you change from 3.90's to 3.73's? The Motive 3.90's are on my list of planned mods......

 

I changed out my motive 3.90 gears becasue it had a whine at around 50 mph when coasting.

The installer reccomended Precision 3.73 gears so that's what I put in but it too makes a lot of noise. He said it requires a 300 mile break in so we'll see ...

 

I think you may have to get out of town a bit before you actually see them. I seem to recall seeing them on my commute from National City to Escondido on I-15 back in the day. :scratch:

 

I travel I-15 quite a bit... from Mission Valley, all the way to Temecula. Don't ever remember seeing any. I'll be headed to Las Vegas on Labor Day weekend, so I'll make sure to pay attention on that drive.

 

How about a hand held GPS? If you don't have one, ask around. Someone you know is bound to have one. Heck, you may want to buy one. They're getting pretty cheap.......$100 will get you a bare bones model.

 

What's the acceleration difference like between 3.90's and 3.73's? A lot of people seem to think that you wouldn't be able to tell the difference between 3.73's and stock. What do you think?