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Speedo and Odometer Question

C

cj11

NAXJA Forum User
Location
Easton Pa.
I did a search and could not find an answer to my question. I have used the calculator and found that when i am running at 60mph it is more like 71mph. So how do I figure out how off my odometer is would like to figure out my actual miles per gallon. I seem to get about 230 miles per full tank but if the odo is off then what the hell would it be,

97 Xj with 32x11.5x15

And I did read GOJEEPS write up on the speedo gear and will do this soon looks like I need a 31 tooth gear. where is the cheapest place to get this from and how difficult is this to do looks easy but did not understand the last part about adjusting the gear in the t case
 
I haven't changed a gear before, but it looks pretty straight forward. As for MPG, it should be percentage based. If you are doing 71 when it reads 60, then your actual travels are +18.3% of what the odo says. 230 miles equals 272 real miles.

However, I read something just yesterday on another thread where a guy said the odo and speed readings are separately controlled, so the difference would not be equal. I do not understand that at all, and at this point doubt it is true, but if I am wrong, then I have no idea how to calculate :)


cj11 said:
I did a search and could not find an answer to my question. I have used the calculator and found that when i am running at 60mph it is more like 71mph. So how do I figure out how off my odometer is would like to figure out my actual miles per gallon. I seem to get about 230 miles per full tank but if the odo is off then what the hell would it be,

97 Xj with 32x11.5x15

And I did read GOJEEPS write up on the speedo gear and will do this soon looks like I need a 31 tooth gear. where is the cheapest place to get this from and how difficult is this to do looks easy but did not understand the last part about adjusting the gear in the t case
Click to expand...
 
Speedo and odometer are both run by the same VSS output. Chenging the gear will effect both.
To work out your MPG you must add 18.3%, as mentioned above, to the mileage before doing the final calculation.
 
As far as adjusting the gear, I believe it talks about turning the sensor as a whole in the transfer case hole. If you look at the gear/sensor, the gear is slightly off center. This is so you can rotate the sensor to fit a larger or smaller gear on the sensor, but still have it mech with the gear inside the transfer case. There are markings on the side of the sensor which indicate how far to rotate it depending on how many teeth there are. If you pull it out, switch gears and put it back in, it will not go back in. I lost a lot of oil trying to figure why it wasnt going in.
 
Best way to figure out how far off your odometer reads is when driving down the highway watch when you pass a mile marker and when you pass the next one. Then you know that was a mile and you see how far your odometer said you traveled. When I had my 31's on it was off by 10% so 100 ODO miles would really be 110.
 
Obviously, if you went for 10 miles of markers, you would be more accurate. For speedometer test, hold it at 60MPH and you should pass a marker every minute, or 10 in 10 minutes. If odometer shows you went 9 miles, you are 10% off. And it would likely take 9 minutes to travel the 10 miles. (If speedo and odo are off the same as likely.)
 
interesting thread....thanks, never really though about it like that
 
I found out how much I was off by going next to markers, pushing the mileage button and ended up getting exactly 1.2 miles for every mile. In addition to that, I figured out that when my speedo said 48 I was actually going about 40, and as my speed increased, the difference in accuracy increased.

Based on that I divided my miles per tank by filling up everytime, dividing however many gallons of gas I added to the amount of miles I got on the last fill, then multiplying by .8 to get my MPG. Not sure if this helps, but figured I'd throw it out there anyways.

~Scott
 
Well I checked with a gps and it seems that i am off by 5mph at 70mph. If you go off the calculators on diff. sites it says i should be 10mph
 
Depends on how much air you have in the tires, remember that your rolling radius is what's important not the actual circumference of the tires. i.e. if you have 33s but you're low on air and they actually have a radius of 14 inches instead of 16.5, you'll have the same effective mileage/speedo differences as a 28 inch tire.

EDIT: I also saw the post by the guy saying the speedo and odo are controlled separately. I scratched my head and wondered how exactly he could think that, and how the ECU/TCU would determine the actual diff gearing and tire size it was dealing with...
 
GPS is the way to go for calibrating a speedometer. Mine reads in 1/10 of a mile per hour. Being that I like to cruise at 9MPH over the limit and run through state police radar all the time without tickets (except when they catch me at 20 over), I know where each of my vehicle is actually at 79MPH. Every one varies.
 
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