You are way over analyzing this. Speedo's are not accurate. 2-3 mph slower than you're speedometer is perfect working.
set it to 771, and your odometer stays oem, your speedometer stays oem.

 

Yea, I'm going to have to agree with you Manny. I'm probably just looking into this way to much. It sucks now that I got CR to update to 800rev/mile. I know the man is busy, I don't want him to have to do it again.

 

You should be able to do it your self. When it ask to change optional parameters, that is one of the options. That is , if Chris left it open for you

BTW, are you certain that you have a 3:55 rearend? Cause maybe ( and I didn't do the math here) you have a 3:31 and that is throwing you off.

 

Naw, I've got the guy in the service dept to fax me the list of options that came on my car. You give them your vin, they give you 2 sheets of all the options in your car. Yea I can change it manually the way I have been changing it.

 

Just go to the tire manufacturer's web site, I'm sure they have all the info you'll need.

 

I'm throwing in the towel then

I hope you get this worked out. GL

 

Thanks for everyones help! I think I'm just goint to have to live with being a few mph off and keep my odometer correct.

 

I have the exact same problem and that's basically the point I got to as well. I'm going to explore using the data logging feature of my SCT tuner to see what the vehicle speed sensor says at speed. I just need to figure out how to do that first.

 

There are certain parameters in the sct that you can select to data log. It is really sweet when you hook the tuner up to your laptop and drive around. I love playing around with the different graphs. I like the load on the engine graph. Right now the way I have the car setup at 67-68 ish i'm doing right at 65. I just would rather know how many miles are on the car than the speed. I mean i guess in the end it would really only be like 6k miles difference if I drove it for a 100k which isn't enough to worry about.

 

So does the graph from the datalogging match what the speedometer needle says?

My other idea is to find a long stretch of abandoned asphalt, measure out 1 mile using a measuring wheel, then put your back tire square on the starting line, reset the trip odometer, and see if it actually rolls over 1.0 miles when you cross the finish line. I'd like to try it but I dont' have the empty pavement nor the measuring wheel.

 

I like the idea but I haven't tried data logging in a long time. I just wanted to do an inital checkout on it.

 

I usually use my GPS which displays the speed to verify my speedo ...

 

I'm really surprised this hasn't been brought up here. It's pretty common practice in the auto industry to set the speedometers to read slightly optimistic. I have not changed tires, or gears in my car and have not changed anything in the tune. The speedometer reads 2-3mph optimistic at all speeds.

Hell my 335xi BMW that I had previously read 3mph optimistic at 30mph and 6mph optimistic at 75.

However, that's what the speedometer is showing, but not what the car is calculating for mileage. THe mileage was always correct, the speedometer is just set optimistic.

Your speedometer was likely this way before your tire size change, but you never noticed it because you assumed it was correct to begin with (which it was). Set the rev's mile back to 771. It's more important for your mileage to be accurate rather than your speedometer.

 

I just (finally) got my Vista laptop to play nice with my SCT tuner and you're absolutely right. the VEHICLE_SPEED matched what the GPS said at every speed I tried (up to 82mph), but the needle was consistently 3 mph or so fast. I know I'm sticking with tire manufacturers spec from now on. It always seemed to be off by more than that but I had always forgotten to take parallax into account (the needle looks like it's in a different position if you look at it from an angle (where you usually see it from) vs from directly ahead)
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On a side note Jason, the data logging was ridiculously cool. I'm going to mess with some layouts and get some graphs going. I just need someone to hold and read off the laptop while I'm driving. Now I see the appeal of a car computer

 

Yea I did the exact same thing. I had a friend of mine ride with me on the way to Ocean City MD for New Years to break in 09'. He almost messed himself at how cool it was to see all of the parameters that you could change. It was nice having someone hold the laptop up where I could see it too.

 

Wow,

The mystery is finally solved, I'm glad it worked out for you! Nice job using the SCT to see what the computer said you were going!

 

Who makes your tires Jodade and what is the width of your rims? The manufacturer might have the tire data online. Also going with a wider rim than that used to measure by the manufacturer will effect over all diameter somewhat (say the measured dia was on a 10 inch rim by the tire manufacturer and you used a 12 innch wide rim for your app, the tires will actually measure a little shorter in overall dia.)

 

All of the info is in my signature. I've got 20x10 with toyo proxes 4 295/30's on the rear. I got the info off their site and have it as close as possible now.