hankthetank

You've been told a total of nine times to change your speedo gear first...just do it man. You need a baseline mpg number from an ACCURATE odometer. Then, methodically, you go from there. Trust me...I just went to 32's, and my odometer only shows like 180 km on the top half of the tank. It used to be 220+...

stev-o

How would I go upon changing my speedo gear

F1Addict

For the record, I checked my mileage using a Magellan GPS unit that calculates to the hundredth of a mile. Even though I have stock-ish tires, my speedo is off by between 5 and 10 mph depending on speed and my odo is off by about +4 miles per 100 (so when my odometer indicates I've traveled 100 miles, I've actually traveled 104 miles).

The only way to get an accurate mileage calculation is to start with a reliable gauge. The speedo gear calculates revolutions and it assumes a certain distance of travel for each revolution based on the tires that came with your Jeep. If you increase the size of the tires without swapping out your speedo gear it will continue to assume that you are running small, OEM tires and your mileage will not be accurate. Here's some math to bore the crap out of you...

Most Cherokees came with 205/75 R15 tires, these tires have a diameter of 27.106 inches. To determine the circumference of those tires just multiply by Pi (π) which yields an overall tire circumference of 85.156 inches. That means that the speedo gear assumes that for every revolution of your stock tires, your Jeep has been moved forward by ~85.2 inches and it moves your odometer accordingly.

Now, put larger tires on there, we'll use 32" tires since you've already told us that's what you have. 32 x Pi = 100.53 inches of circumference so for every revolution of your tire, you're now traveling 15 inches further than you would have on the stock tires your speedo gear assumes you have so when your odometer says you've traveled one mile, you've actually traveled one mile, plus 948 feet due to the larger tires. This will add one full mile every (approximately) 5.5 miles actual miles traveled. Assuming you once got 250 miles out of a tank of gas, the 32"s would now yield (based purely on the math and not accounting for the additional mass, rolling resistance, wind resistance, etc. of the larger tires) 242 miles per tank. Add to that the parasitic losses mentioned earlier which will probably account for another 20 miles per tank and suddenly you're only getting an indicated 220 miles per tank.

Changing your speedo gear will gain you back at least 1/3 of the apparent mileage lost due to your larger tires.

Here's a pretty good source for speedo gears and a calculator to determine exactly what you need...

http://www.jeep4x4center.com/jeep-sp...FQJ9hwodcQRy1Q

XJ.89.Limited

Are you by chance a physics major? I learned most of that in my class in high school. haha. You sound very intelligent.

stev-o

I definitely am not getting 220 miles a tank

JHumes

I have a 1989 renix with 4.5 of lift on 31" tires with 4.56 gears. I get 15 MPG in the hills and 17 MPG on the flats.
It was bad. I did a full tune up, changed the cps and fixed all the vac leaks. This helped but repairing and using the damn cruise control really added to it.

DFlintstone

Here I use the county mile markers, those electronic road signs (which are often off a bit), or sometimes my second hand on my watch a stretch where I can hold it on 60 along with the county markers.

Seems to read correctly with 215-15's, but with 225-15 (only about 1/2 inch larger dia), it's 4% off. 4%= 4/100 so to go 50 mph, it reads 48. It will under read because of the larger diameter tires, and that's just 1/2 inch, (over ?27 or so?) so sure, you go to 32 inches it's gonna be way off.

The odometer and speedometer and on the same cable so when it said I went 50 miles I, really went 52. Then trying to use the gauge is all but worthless. It drops slowly on the top 1/2, and faster below 1/2, I'm pretty sure.

When I'm watching mileage (almost always lately), I park at the same spot, in front of the same Shell pump, then with the lever full on so it's blasting let it click off three times. Then write down the odometer reading and how much it took. (I don't think I'm over topping it to where I'm gonna have issues with fuel going back into the Evap line, like I would it I went slow and really topped it up).

Unless you know what percentage to change them, or change the gear on your speedo cable, (unless it's already been done), those odometer numbers are worthless. (if your 10 ths of a mile are supper close to your odometer after 2 or 3 miles using the county signs, you might already have the correct speedo gear).

Nearly any sensor can cause mileage bummers. MAP, coolant temp, and the 02 are right at the top. Gotta have good vacuum as well. If the map reads low vacuum, it will dump more fuel thinking you have your foot in it.

Without first knowing your actual mileage, you wont know if something "got" you another couple mpg, is why you are getting mixed replies here, I'd guess! I was thinking about trying this here in this link...someday! >http://www.naxja.org/forum/showthrea...ht=sensor+test

stev-o

I was told by the gentlemen I bought my Jeep from that wen it says your doing 45 your actually doing about 49

XJdryva

My Jeep has 31X10.50X15s on it and the speedo is 1mph FAST. No regear and no speedo mods so figure the $hit out

austinjoe13

I bet that thing burns'em

I also have an 89 with 31s and similar lift and I average 16-18 back and forth from school and 20-30 on long flat trips.

DFlintstone

MMMM, that seems....odd. If the little difference in my 15's was 4%, how could 32's be only about twice that. (45-49 is+about 9%). Anyway an easy check. At 9%, if you hold it right on 60mph, you would cover a mile in about 55 seconds.

For me, since I'm not going to swap any gears, I use the county mile markers on a longer trip to figure out what % I'm off. (or not).

Or there is the math. C=2x(Pie)xR, so C, circumference (=dist. of one rev)=2x(3.14)xRadius, (1/2 dia, 32") so R=16). C=100.48 inches. Mine on my 90 is 86.35 (if the tire is 27-1/2"), so that comes out more like 16% (16.4).

That would mean when your odometer says you went 200 miles, you really went more like 233. 50 on your speedo would really be about 58.. Anyway there's a start.

I could be way off. With ADD and/or dyslexia it's a house of mirrors to me!

Metmknice

My MPG/accel sucked when I put 31's on my renix auto with stock 3.55s. When I got it stock it had 235/75s and after tune up, etc. Highway could get 22mpg easy. City was about 15-16. Even when I lifted 3" and had 235s mileage did not change.
Now, with 31's and 4.10 gears my city mileage is about the same. Highway, still needs to be measured more, but on the last couple trips I have taken hauling a bunch of crap averaged 19.5mpg hwy.
Currently, stock 4.0L; strokedjeep.com 60mm TB, Ford 5.0L 19lb injectors, Accel 8000series plug wires, champion plugs, NAPA Gold air filter/fuel filter, magnaflow 14" 4x9 oval muffler/turndown.

stev-o

I have No idea if the po swapped the gears out...
Maybe I should try to give him a call
I think he swapped out the rear axle to a dana 44 unless that came stock

austinjoe13

For an 89 it was an uncommon option

stev-o

I thought that it had been swapped I'm sure he must of regeared it then