Big tires & accurate mileage.

[Green Gidget]

Bare with me as this may seem like a girl question: trying to recount mileage since gas gage is out of sorts - since the Jeep speed is off by 15mph on gage due to size of tires, would the mile counter be off too? Trip(s) mileage way off. Not enough miles on the counter.

 

[mtnwhlr]

Yes

 

[Belizean]

Yes

 

[Green Gidget]

Oh this is funny! Doing the math tells me for every 100 miles the counter I'm actually traveling 172. *college prof was right. Math is every where. Thanks again for the help.

 

[RevDrCueBall]

Oh this is funny! Doing the math tells me for every 100 miles the counter I'm actually traveling 172. *college prof was right. Math is every where. Thanks again for the help.

Better check your math.... your prof might fail you

That's a 72% increase in tire circumference without an axle gear change. I somehow doubt that.

Edit: Also, speedometers don't get off by an exact mph, they get off by a percentage. At what speed are you 15 mph off?

 

[Green Gidget]

Gears were changed. Not sure what they are. Redid the math after checking actual distance. Gage off by 48 miles not 72. There's very little original on the old girl so I'm having to figure it out as I go! Thanks for the reply. Speedo also needs gps checked. When gage says 15, I'm actually doing 30. So I just made the assumption that is was 15 off across the boards. What a learning curve this project is.

 

[RevDrCueBall]

Gears were changed. Not sure what they are. Redid the math after checking actual distance. Gage off by 48 miles not 72. There's very little original on the old girl so I'm having to figure it out as I go! Thanks for the reply. Speedo also needs gps checked. When gage says 15, I'm actually doing 30. So I just made the assumption that is was 15 off across the boards. What a learning curve this project is.

Still doesn't seem right. What were your original gears, and what are they now? Also, what is your tire size now?

 

[Green Gidget]

Rev-
Only thing I can tell you is tire size. They are 38.5. It was a birthday present years ago that others worked on as needed. Have no idea what was original or what it has now. I do know that they were changed at least twice. The only reason for all this is to figure mileage because gas gage is wonky. Easiest fix in my girl brain is to fix the gas gage! None of this is any real issue. Just an education!
*and the tires - they are old and not so much worn as they are shrunkin. Meaning at full air pressure they only measure 34. Yes, that is correct.

 

[hole]

Tires are rarely the size that are claimed on the sidewall, but 4 1/2" is a lot

 

[Green Gidget]

That was an honest measurement. Taken when a Jeep with new 38.5s was parked beside it on concrete. They're so old, over the years several have wondered if they weren't mismarked in some way. No an issue. It's fun to say it has 38s but my old arse only has to climb in past 34.

 

[Lumpy Grits]

Does your phone have GPS?
Use it, and it will show your error on the Jeep's speedo and help you to determine what gear to put in the t'case to correct this.
IIRC: I went at least 2 or 3 less 'teeth' on the speedo drive gear.
You are traveling faster than the speedo is showing. The smaller tooth count will cause the speedo's cable to turn faster, and get you back in the ball park on accuracy.

LG

 

[Belizean]

I think you can do it with snap chat also. My daughter did it while riding with me.

 

[yellow85cj]

And there is always changing the speedo gear in the t case.
https://www.quadratec.com/c/reference/jeep-speedometer-gear-tooth-chart

 

[BusaDave9]

From your original post it sounds like your concern is when will you run out of gas now that your gauge isn't working. First off carry extra gas. When you fill up you can see how close to running out you were. Watch you odometer. It doesn't matter how far off it is. It's still consistent. So if your odometer says you went 300 miles on a tank then go by that. If it says you only go 50 miles on a tank go by that. You don't need to do the math to find out your actual mileage. That just complicates the issue.

 

[gr8dain]

^^^^ What BusaDave said. It is all relative. Run what you got fill up and see haw many gallons it took. Based on the tank size you have, you can gauge how far to go between fill ups. Keep track. Done.

I have had a little notebook in my console to write the miles down each time I get gas. This started when my fuel gauge quit on me. Even though I have a working sending unit and aftermarket gauge, I still write it down because you never know when the gauge will quit again.

Last year I regeared and my mph got back to accurate. Then I put larger tires on this year and I am off again. No biggie. I usually have my gps on when on the highway so I can see an accurate mph

 

[Belizean]

X3. I'm about 225 on a tank with a little still in the tank. I fill up before I go 200 miles ea time. Works really well. Got the little yellow sticky pad taped to the dash with mileage and date