Overhead console mpg accuracy?

Ok,
So I've put on bigger tires. I now assume my speedo is running slower, as is my odometer. Here's the question: Where does the overhead console computer get it's info from to calculate the MPG??? and can it be adjusted to make it accurate with oversized tires???
Thanks!

I'm almost convinced everything recalibrates itself when you go to larger tires. I've been running 31's for a very long time ... and recently I've noticed when I approach one of those new portable speed traps that flashes your speed at you ... my speedo it's right on the money every time.

... and I believe those portable guns are calibrated on a regular basis.

It would be nice to know if the computer re-adjusts the guages automatically.

I don't know if it automatically recalibrates its self, but I wouldn't think so....I'm probably wrong though.....but it should work off the VSS which is in the tcase output houseing, so I couldn't imagine how it would learn that you've installed larger tires downstream of the sensor, but I'm new to xjs, so maybe I'm wrong????

Justin

It takes it reading from the transfercase and the speedo drive that is there. You must just change out the drive for one with less teeth to have it read correctly again. It also effects you shift points on the auto like when the torque converter locks up.
Go to my site and you will see how to this very easy and cheap mod and which speedo gear to buy.
http://go.jeep-xj.info

Gojeep said:

It takes it reading from the transfercase and the speedo drive that is there. You must just change out the drive for one with less teeth to have it read correctly again. It also effects you shift points on the auto like when the torque converter locks up.
Go to my site and you will see how to this very easy and cheap mod and which speedo gear to buy.
http://go.jeep-xj.info

Click to expand...

That is about what I was thinking. It's gotta take the reading from the tcase. So with bigger tires that reading will be off.
Thanks for the info!

bchulett said:

I'm almost convinced everything recalibrates itself when you go to larger tires. I've been running 31's for a very long time ... and recently I've noticed when I approach one of those new portable speed traps that flashes your speed at you ... my speedo it's right on the money every time.

Click to expand...

It's more likely that you were off with the stock tires. Unless the Jeep has an independent method of determining location and speed (like a GPS-equipped nav computer) it wouldn't have a clue whether you've got lo-pro tires, stockers, or 48" tractor tires...last time I looked that was never an option in the Jeep. In your case it doesn't sound like you need to change the output gear in the t-case but that's typically what is needed.

There are charts galore all over the web showing what tooth count you need on the output gear, based on the tire size and the gear ratio in the rear axle. For example, when I moved to 32s and regeared to 4.56 I needed a 39-tooth gear to correct my speedometer and odometer. It's a 5-minute job to replace

bchulett said:

I'm almost convinced everything recalibrates itself when you go to larger tires. I've been running 31's for a very long time ... and recently I've noticed when I approach one of those new portable speed traps that flashes your speed at you ... my speedo it's right on the money every time.

... and I believe those portable guns are calibrated on a regular basis.

It would be nice to know if the computer re-adjusts the guages automatically.

Click to expand...

i have found that mine is correct after the 31's also i have checked with one of the aformentioned portable radars and on a trip of 400 mi many times and it was so close that i couldnt tell if it was off or not

I found that my odometer was accurate with 30 9.50s. When I switched to 31s I am about a tenth off with the odometer. The computer doesnt care what diameter tires you are running but the actual mpg will be lower than indicated due to the fact that the jeep "thinks" it has gone less distance than you have really traveled. Of course, you typically will lose economy with the larger tires. For example, when the avg economy shows 16 mpg over 100 miles, I am really averaging 17.6 miles to the gallon (16 on the odometer is actually 17.6 miles true distance or 16 + 1.6 or 1 mile equals 1.10 actual)

regardless, I always try and fuel as soon as the fuel light goes on.

Joel

So whatdoyouthink ... is it possible the sensors read the driveline speed through the TC and sends signals to the CPS, TPS, GPS ... or whatever, and recalibrates ?

I'm struggling here with the idea that Chrysler shipped thousands of XJ's with an inaccurate gear ... or maybe they did because the standard gear is within a tolerable range given the different wheel/tire combinations for the XJ ?

Or maybe they designed the XJ's with enough vision knowing that we were eventually going to lift our XJ's and mount 31's ?

Pick one ... I'm grasping at straws here.


IMO: If they had vision they would have never discontinued the XJ.

From what I've heard from several sources most new vehicles are some 5% inaccurate. That is, when they read 55mph they could be actually going 50 or so. The automakers apparently figure it's better to err on the side of safety and have everyone drive slower than to err on the high side and open themselves up for lawsuits and safety problems. When we install slightly larger tires on our Jeeps they made up the difference so the speedometer is actually closer to accurate.

The VSS (Vehicle Speed Sensor) reads the rate of revolution of the t-case output and that is interpreted as the expected vehicle speed, assuming the factory tires and gears. When you change the gears or tires, the speedometer doesn't have a clue that there's a difference downstream from the sensor, and it still shows the incorrect speed based on the only thing it knows...rotation of the t-case output.

The overhead console calculates MPG by monitoring fuel usage and the indicated miles traveled.

Thanks for the detailed explanation ... that makes perfect sense.

Avoiding lawsuits ... I should've known.

It does not recalibrate.

Just got back from a cross country road trip in the 2000 Classic. I already knew the speedo was "optimistic" with stock tires. I thought it was probably off by about 2 MPH at 60 MPH. Went through a lot of construction zones that had those flashing speed warning radars, and it turns out it's off by about 3 MPH at an indicated 60 MPH (actual speed approximately 57 MPH). That's a 5% error, which would make it just about dead-on accurate running 30x9.50s. The difference between 30s and 31s isn't that much, so I'd probably not bother to re-gear the speedo even with 31s (except that I also would not run 31s with stock axle gears, and changing to a different gear ratio would require changing the speedo gear to recalibrate.

Eagle said:

It does not recalibrate.

Just got back from a cross country road trip in the 2000 Classic. I already knew the speedo was "optimistic" with stock tires. I thought it was probably off by about 2 MPH at 60 MPH. Went through a lot of construction zones that had those flashing speed warning radars, and it turns out it's off by about 3 MPH at an indicated 60 MPH (actual speed approximately 57 MPH). That's a 5% error, which would make it just about dead-on accurate running 30x9.50s.

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That sounds about right. When we drove it home from TN, I was doing 75mph on the interstate posted 70, just so my wife wouldn't complain. She was driving the '01 Limited. She complained I was doing 80. Turns out, at 75, ours was reading almost exactly 5mph off. So I was thinking that I might be closer to accurate with the 30's on it. Now the question is- Is the current mpg calculations MORE accurate too? :doh: hmmmmm
Thanks for all the input!

ChuckstrPT said:

That sounds about right. When we drove it home from TN, I was doing 75mph on the interstate posted 70, just so my wife wouldn't complain. She was driving the '01 Limited. She complained I was doing 80. Turns out, at 75, ours was reading almost exactly 5mph off. So I was thinking that I might be closer to accurate with the 30's on it. Now the question is- Is the current mpg calculations MORE accurate too? :doh: hmmmmm
Thanks for all the input!

Click to expand...

So what do you think? Is my mpg more accurate now? Do you think the 'good' mileage I was getting was just opomistic because the speedo we registering fast?
Thanks for the input,