Need some help with a bit of a Speedo/odometer mystery...

[pw700z]

This is long, sorry...

I purchased a 92 cherokee laredo about 1 month ago. So far, so good, but the MPG has been terrible -- 8.5MPG the last time I calculated with a bunch of stop and go, short distance trips.

Well, today I had the GPS in the car for a first family longish distance outing in "Mr. Green", and I noticed that while driving 55, I seemed to be passing a disproportionate number of people. Then I pulled up the GPS info, and I was actually doing 71.4mph. Uh... ok. I changed the speed sensor when i first got it because the speedo was all over the place. Did I get the wrong part?

Then I was tracking the odometer vs. the mile markers, and every 1 mile marker equaled about .7 miles on the odometer. Where does the odometer feed off of?

Currently, the jeep has 215/75R15 tires on stock jeep rims, tirerack.com says the optional oem tire was 205/75R15, so it would seem the tires are a bit bigger than stock, but certainly not anything near enough to account for a 30-40% difference.

So, while this is good news from a MPG perspective, what does this really mean? Is there a different transmission or transfer case in the car? Are the gear ratios besides the speedo gear "wrong"? And what should I do about it? Should I be concerned?

My ultimate goal for the jeep is to tow a small camper, and take the family to various campgrounds around the northeast on the weekend, maybe do some lightweight "showroom stock appropriate" offroading. The jeep replaces a LR Discovery which got less than 12mpg highway.

So, considering the towing I want to do, how should I approach this:
1. Get the right speedo gear (i just changed the speed sensor, could I have the wrong speed sensor? (there was no gear involved with changing the sensor))
2. Also consider lower ratio diffs, to compensate for a differently geared transmission?
3. Is tirerack correct, or could this jeep have really come with 215s?

Thank you for your consideration!!

 

[mud4feet]

First off, welcome to the "Z"!!!!! Glad to see another struck with the Jeep bug!!!

The 215 - 205 tire size would not make near the difference you're speaking of...........I'm runnin' 31's on my '03 Wrangler (Stock 205's), have never changed the speedo gear, and it's about an 8% (roughly) difference. Not knowing a thing about XJ's, I would suspect the speed sensor that you purchaced. Did you get it from the stealership??? Seems to me they would point you in the right direction and/or make good on their mistake .:shock:

Sorry I could not be of more help, but welcome aboard anyway!!!

 

[Bounty__Hunter]

Sounds like you didn't reuse your original speedometer gear on the new speed sensor. The speedometer gears are available with a lot of different tooth counts to accomodate many different tire size and axle gear ratio combinations.

Google "Jeep Speedometer Gear" and you'll find many useful articles on what I'm talking about. Swap out speedo gears and your speedo will be accurate, you'll be able to more accurately determine your fuel mileage, and your fuel mileage will likely improve as the jeep computer uses the speed sensor as a parameter for engine operation.

 

[pw700z]

When I replaced the speed sensor, i did not remove or otherwise deal with any sort of gear. I will get the proper gear.

What do you suppose it means that my speedometer is so far off (baiscally) stock tires? The previous owner (of the trans?) had different diffs?

Also, where does the odometer feed off of?

 

[jeepless20]

the odo. should feed off the same deal the spedo reads from. all from the spedo gear coming from the rear of the T-Case.

 

[TwistedCU]

If you have the original speedo gear and have stock size tires (or very close to them) then maybe the new speed sensor is either faulty or the wrong application.

 

[pw700z]

So, the odometer is electronic, and feeds off of the same speed sensor? For some reason I assumed the odometer was mechanical.

 

[jeepless20]

far as i know it is. i thought it ran off the same gear that the speedo reads from. ive been wrong B4

 

[Bounty__Hunter]

Speedo and odo are same feed from the speed sensor. The reason your speedo and odo are off, they speed sensor you installed had the wrong tooth count speedo gear. If you no longer have your stock speedo gear, check out www.tacomaspeedometer.com for a new one.

 

[pw700z]

I feel like i'm beating a dead horse, forgive me.

Should the replacement speed sensor I got from pep boys have come with a gear? It didn't come in any sort of gear. It was an oddly shaped device just had a little shaft sticking out of it, which I swapped in place of the original unit. Perhaps pepboys gave me the wrong unit.