Speedo Gear Impacting Mileage

I installed the speedo correcting gear in mine after re-gearing, and got 13.5 MPG, not good. I checked it against a GPS mileage wise and the jeep was lagging behind the GPS. I used the GPS mileage and am really getting 15MPG. So, even with the correcting gear installed my mileage was still off quite a bit.
 
MutantNinjaTJ said:
I installed the speedo correcting gear in mine after re-gearing, and got 13.5 MPG, not good. I checked it against a GPS mileage wise and the jeep was lagging behind the GPS. I used the GPS mileage and am really getting 15MPG. So, even with the correcting gear installed my mileage was still off quite a bit.
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Yes, that's the beauty of the speedo-healer, you can dial it right in.

From the factory, there is (what I would call) a wide range of accuracy in them. Typical is 3% but I have seen 6% error in new vehicles.

Sometimes, if you really want to play with it, you can adjust up or down the speedo gears to dial it in.

My personal opinion is that (and I do not wear a tinfoil hat) the manufacturers tend to error on the speedo reading higher than actual. (I have never seen a new vehicle read lower than actual) This does two things: 1) uses up your mileage warranty a tad quicker and, 2) keeps people going slightly slower than what they think- for safety reasons of course.
 
MutantNinjaTJ said:
I installed the speedo correcting gear in mine after re-gearing, and got 13.5 MPG, not good. I checked it against a GPS mileage wise and the jeep was lagging behind the GPS. I used the GPS mileage and am really getting 15MPG. So, even with the correcting gear installed my mileage was still off quite a bit.
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I swapped from a 32 to 39 tooth gear using Chris' chart in the How To post, and it brought my speedo to within a 1mph of my GPS. That's works for me. I gave up trying to increase MPGs a long time ago. I just drive the damn thing and empty my wallet around every corner... lol
 
jazngab said:
So the odometer reading would be reading lower miles than it actually is, correct?
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Absolutely correct, the mileage with bigger tires will read lower than actual.

So if you want to keep your mileage down, put 40’s on it.
 
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If you want to calculate your mileage use this formula:
Actual Seed / Indicated Speed * Recorded MPG = Actual MPG
For example if you speedometer reads 59 when you are actually going 65 and you record mileage of 15 MPG you would actually be getting 16.5 MPG
65/59*15=16.5

If the speedometer reads low you are logging fewer miles on the odometer
If the speedometer reads high you are logging more miles on the odometer
 
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dbbd1 said:
From the factory, there is (what I would call) a wide range of accuracy in them. Typical is 3% but I have seen 6% error in new vehicles.
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Chrysler speedometers are pretty good. If you want to see the error just do an instrument cluster self test and you will see exactly how far off the needle is. Everything else is digital or related to tire size and gearing.
 
Pcleaners2 said:
So if my speedo is reading I’m going 2mph faster than what I actually am going then I wonder what my mileage really is if I’m getting 13mpg.
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You need to know the percentage your speedo is reading fast. If it’s 10%, your odometer is reading 10% more miles and you would be getting 14.3 MPG. The speedo won’t read 2 MPH fast through all speeds.
 
Pcleaners2 said:
So if my speedo is reading I’m going 2mph faster than what I actually am going then I wonder what my mileage really is if I’m getting 13mpg.
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2 mph makes no difference. Every Jeep I've checked the odometer and speedometer were off by a few percent. Although the speedometer takes the same information it's an analog needle and simply isn't tied exactly to the odometer. If you adjust based on 2 mph you are just as likely to go the wrong direction, it isn't an exact science. The only way to be really precise is to calibrate the speedometer signal internally using obd2 and gps, otherwise 2 mph doesn't matter. Honestly your fill-up errors tank to tank are probably huge compared to a few percent on the odometer or speedometer.
 
pagrey said:
2 mph makes no difference. Every Jeep I've checked the odometer and speedometer were off by a few percent. Although the speedometer takes the same information it's an analog needle and simply isn't tied exactly to the odometer. If you adjust based on 2 mph you are just as likely to go the wrong direction, it isn't an exact science. The only way to be really precise is to calibrate the speedometer signal internally using obd2 and gps, otherwise 2 mph doesn't matter. Honestly your fill-up errors tank to tank are probably huge compared to a few percent on the odometer or speedometer.
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Ok well does 13mph sound ok? 33s 4.88. I would say 5th is still not great uphill. Although the new cats did help some
 
I've heard worse, if you don't have a check engine light and you drive like everybody else in CA on 33s it sounds normal to me. How many tanks in a row have you checked?
 
Pcleaners2 said:
How do I figure out percentage Then
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Tire manufactures often list revolutions per mile as part of their tires specs. I checked stock tire vs my oversize tires and did the math. Came out to 13% fewer revolutions per mile with the larger tires compared to stock. So, my speedometer and odometer are 13% slower/lower than actual. I confirmed it with a GPS. It is aways going to be a percentage difference rather than a straight MPH difference.
 
wildbill said:
I swapped from a 32 to 39 tooth gear using Chris' chart in the How To post, and it brought my speedo to within a 1mph of my GPS. That's works for me. I gave up trying to increase MPGs a long time ago. I just drive the damn thing and empty my wallet around every corner... lol
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GPS can have errors too because it filters signals through some program a guy wrote.

I tired to fix mine by regearing but the new gear had something wrong with it, so I sent it back. I'm not too far off anyway. Before you changed your gear, you could have taken your trip meter mileage and multiplied it by 39 (the gear you should have in there) then divided it by 32 (the gear you do have in there) to get your actual trip mileage. Then figure out your mpg.

Trip * 39/32 = actual miles. It gets you closer.

I stopped caring because I stopped driving to work. I've been getting 5 months to the tank lately with this stay at home stuff. Now that I'm driving to work again, I'll get a speedo healer to dial it in this time. I get 14-16 mpg in town depending on my driving style.
 
Pcleaners2 said:
1 tank so far only
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I'd average a few, I log all my fills and it's really common to go 15, 19, 14, 18. Just depends on how full you really get the tank each time. If it shuts off early you'll get really good mileage one tank but the next one will be terrible because you get more gas in. I average almost 17 but get 11 from time to time because of this, it's not a big deal. You probably wont get that mileage because of the tires and maybe how you drive but that's the general idea.
 
InOmaha said:
GPS can have errors too because it filters signals through some program a guy wrote.
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The speed output from a GPS receiver has basically zero error as far as vehicle speeds are concerned. The speed isn't calculated from the change in position rather the location is calculated based on the exact speed and the Doppler shift of the signals. If the GPS receiver doesn't know it's own speed within a fraction of a MPH it would be off by miles in location. Some modern devices have a bit of lag reporting what the GPS chip is telling them but there isn't any real error in the data. Even when selective availability reduced location accuracy the speed data was not meaningfully reduced for this reason. I know it's allot of jargon but basically you can really trust GPS speed data.
 
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