Re-geared, What a difference!

My speedo was more accurate with the 265/75/16 than with the 245.

 

Not your odometer though...

 

Actually on the manual transmissions (at least on mine) the odo and speedo come off a gear in the transmission. I tested it one time by unplugging it and driving around my block. Neither the speedo or odo worked. I would've double checked this before posting but it rained today and there's a giant puddle under my truck right now.

 

As far as I know, the ODO reads off a different sensor...

 

I thought speedo and odo were directly correlated. If your drive at A speed which is B difference from actuality, C miles will be D difference? That's how I thought it went.

 

I was under this impression as well.. I have been calculating my MPGs by minusing 18% off my odo reading as my speedo is 18% off..
I would love for someone to prove otherwise thou, as I would gain alot of MPGs back that I thought i lost

 

ODO is based off of revolutions per mile...speed is not.

 

Ok so your telling me this is wrong:

Speedo says 70, I am going 66 according to my GPS, difference of 1.06.

When I calculate my mileage per a certain trip and compare it to my GPS trip mileage, I am at a difference of 1.06 (trip contains more than 50 miles to remove some error).

So you are telling me there is no correlation between the 2? I find that very hard to believe.

In no way I am saying you are wrong, but with my observations and record keeping the numbers go with a different theory than you are mentioning.

 

hey oliver, any idea if the first gen front diff gears will fit the second gen? it would be sweet to find a set.

 

The truck is calculating your ODO by measuring the number of revolutions of the tire it takes to cover one mile. It thinks your tires are size XXX, but in fact you changed the tires to size YYY...it is still using size XXX to calculate your mileage. So in actuality, your ODO is reporting less miles than you have really traveled.

In order to correct your mileage calculations you need to account for the difference in circumference of your old tires vs. your new tires. It will probably something like 6-8% off. So you will have to add 6-8% to your ODO reading and then divide by the gallons you used.

 

This is interesting because as we speak I am driving and notice that the speed is not the same between my ultra gauge BUT the mileage is.....damn so I am actually getting freaking amazing mileage if in fact you are right.

398 x 1.075 correction factor then divided by about 17 gallons pits my mileage a little over 25. Hmm thanks for pointing this out I thought the speedily played a part in mileage correlation.

I must have no taken the exact same route that my gpa told me to take or the distance was not accurate.

 

Ok now that I am home.

Here is a picture of my info:


So without taking gearing into account, such as Andrew mentioned, my mileage shall equal:

Miles= 285.3
Difference in diameter per TW tire calculator= 8.5%
Actual Miles = 285.3 x 1.085 = 309.551

Fuel per this trip= 11.2

MPG per this trip = 309.551/11.2 equals......
27.64 MPG

How the hell can that be right lol

 

Are you taking into account the change in your gear ratio?

I know you're using an ultra guage but for reference the scanguage manual talks a little about how to adjust it to changes in gear ratios and tire sizes.

If you think you're speed is off then do both of the above calculations and then add the two numbers together to get your total deviation. You can always check your speed with a GPS.

 

Not to be rude but have you read the previous posts in this thread? I was taking into account the larger tire diameter and over gearing factor in my original posts and thoughts. The post above yours is in reference to what the Andrew person said that gearing plays no part.

It was more of a sarcastic post. There is no way in hell I am getting 27 mpg, which is why I posted that to maybe have him think a little about his approach.

In no way am I saying I am in the right, I am just presenting the facts and making ends meat of the numbers presented. My thoughts give me a real mpg that makes sense, the other way just doesnt seem real to me.

 

Yea you have me a little confused. I'm not positive if/how the re-gear affects your ODO readings. But if you merely changed tire sizes, then yes, the method I mentioned would be correct.

 

Yeah I've read the other posts. It seemed like there is some dispute over if what you're getting is correct. I agree with you that 27 mpg seems outlandish and not considering the gear change will lead to incorrect numbers. I posted the scanguage formulas so that you (and others) could see how they account for both gear and tire changes. The % that comes out of the formula is used on the SG to calculate correct speed AND mileage. I don't know how the ultraguage works or how it allows you to adjust speed and mileage.

I guess I missed your point in the previous post that you were NOT taking into account the gear ratio change. I probably should've explained what I meant a little better.

 

I'd also like to report that I confirmed this morning that my ODO and speedo are both tied to the sensor in the transmission. Unplugged it and drove about 2 miles with no change in speedometer, odometer, or the trip A or B counters.

There are also ABS sensors at each wheel but based on my findings above I do not think they affect the speedo or odo.

 

So would this be a good way to calculate mpg?

Take old gears divided by new gears: 4.10/4.56 = .899

Tire calculator tells me im 8.5% larger so that equals 1.085.

take the .899 add .085 and you get .984

.984 should be what I multiply my miles by then divide the gallons correct?

 

Not to add to the confusion,...
but I think you would multiply the 1.085 by .899 to get .975415 not add the ratios. Just a thought.

 

Is this for front and rear? I'm gonna be lifting my 4 banger soon and am trying to hone in on what my total cost is going to be for lift, tires, rims, and regear...

Sorry for reviving such an old thread...