SteveMongr

1996 XJ Automatic, 3.55 gears, 31" tires.
Been running 31" tires for 3 years and never changed the speedometer gear to match.
Does the jeeps systems use pulses from the speedometer in it's computations? In other words, do I need a correct speedo gear for efficiency or proper operation of vehicle?

jmm0122_

I never bothered to swap out mine since I've never figured out what size tire I want to stop at. My speedo is pretty accurate up to about 55 but after that its off by about 5-8mph. I've always though the lower registered miles will help the resale value lol.

SteveMongr

^ I believe there is more to it than stated mileage. Something to do with engine management and transmission shifts. Trying to gather more detailed info.

jmm0122_

Maybe so. When I drive my Subaru to work its exactly 60 miles round trip (all highway). When I drive the Jeep its 52. I can see something with engine management but idk about transmission shifts since some xj's are manuals.

Ianf406

I am almost positive it has nothing to do with efficiency, shifting, or anything of that nature. It is simply to read your speed.

That being said, your odometer will also be off, making measuring fuel economy almost impossible and making you a liar if/when you sell it

They are cheap and easy to do. Morris 4x4 has a chart with theirs that makes it really easy to pick the right one.

JeremyXJ

With 31's, I'm about 8-10 MPH off on the freeway. I use a two-dollar iPhone app to gauge my speed. It uses GPS and has an adjustable display (gauge types and colors). With the phone mounted on the dash, I have a constant, accurate, digital speed readout next to the cluster.

SeriousOffroad

Yes.

SteveMongr

Thanks fellas, I was after technical details of how/if computer uses speedometer data in its calculations.
According to GPS my speedometer is 4 mph slow at 55mph.

Here is what member CCKen say regarding the matter. Essentially what Ianf406 also said.

" I don't think that an incorrect speedometer gear would affect performance or economy. The PCM uses the vehicle speed signals for the speedometer and odometer readouts.


The PCM does look at the vehicle speed and TPS blade angle when the vehicle is stopped or decelerating, and adjusts the IAC motor to maintain an acceptable RPM. I think it also uses the VSS inputs (among other inputs) when going up a hill, or otherwise putting a load on the engine, to adjust the fuel injectors pulse width (rich/lean).


There's not enough information out there to tell us what else the PCM uses the VSS inputs for."

Thank You to CCKen for the above data.

HappyTrails

I am running 31's (actually 30" measured when mounted) and 3.55 ratio and stock speedo gear. Multiple GPS runs indicate at 65 MPH I am actually going 69 MPH.

To get actual traveled miles from the indicated miles, I divide indicated miles by .938 which is the error correction factor. Example, if the trip odo says 235, the I divide that by .938. That gives 250.5 actual miles traveled. Divide that by how much it took to fill the tank (assuming the trip odo was zeroed out at last fill up) to get MPG. Example: 235 /.938/9 gallons will give 27.8 MPG.

(PS The math is right. I'm just joking about the 9 gallons. It actually takes about twice that to fill the tank after 250 miles of city driving)

Fred/N0AZZ

You would see an increase in mpg and power with a gear change........in the differentials plus not have the up/down shifts all the time.

SteveMongr

Been itching for 4.10 gears for two years. I'm a stay-at-home dad and can only work two days a week.
Hoping to find a pair of stock 4.10 axles from a 2.5 liter XJ.

denverd1

should be negligible for 31s. I've only found my 33s to be off a few mph at 55

Fred/N0AZZ

Steve I envy you being able to stay at home with your children, I missed most of those years working all the time. But have been able to spend time with my grandchildren while they growing up and my grandson who lives just a few miles away.


Hope you come across a set of those stock gears for yourself, the 4.10's are hard to find but would be a big help to you.