speedometer puzzle

[brian]

Hi

on my little yelow muleI(89 mj 4x4) I replaced my busted 3:55 rea rend with a used 3:07 and then replaced my busted 5speed tranny with a used one same tranny -different year.

the speedo was not operating so I disconnected the cable from the tranny and turned it with a drill to see if the problem was the cable or the gauge itself. turned out the cable had disconnected at the back of the gauge and I simply plugged it back in.

 

So my kid is driving down the road last night in the jeep when a state trooper pulls him over because he is doing 67 in a 55. The kid swears he was doing 55 and procedes to tell the cop we just messed with the speedo cable and suggests that maybe the mix-up lies there. The cop doesn't buy it but lets him off with a warning. (WHEW!)

 

So ..... what is the trouble? is it;

(A) the different tranny

(B) the different rear end gears

©the speedo got screwed up when I spun the cable backwards with my drill

(D) my kid was going 67 but claimed he was doing 55 ?

 

thanks in advance for your expertise

Brian

[Geonovast]

When you change axle gearing, or tire size, you MUST change your speedometer gear... especially when you go to a higher gear ratio... which I'm a little puzzled by. 3.07 sucks.

 

The speedometer gets its reading from the end of the trans or transfercase, so it's essentially reading how fast the driveshaft is turning.

 

Let's assume you're in 4th (0.79 in the 5 speed), with 215/75/R15 tires

 

You're running 2000 rpms.

 

With a 3.55 axle, the wheels are spinning at 713 rpm, 58.76 MPH

With a 3.07 axle, the wheels are spinning at 824 rpm, 67.95 MPH. significant difference.

 

Essentially, the truck will be going faster than you think it is, if you go to a higher gear axle, or bigger tire.

[Geonovast]

How long were you running a 3.55 axle with the 5 speed?

Did you put the 3.55 axle in, or did you buy it that way?

If you did put the 3.55 in, did you change the speedo gear then?

 

If you did put the axle in, and the 5 speed trans has its original speedo gear, then you'll be able to simply swap the gears and get your speedo accurate again.

 

A better fix, however, would be to get another 3.55 axle and get rid of the crap 3.07.

[brian]

Thanks geo-

The truck had a 355 rear when I bought it used. Tuned out to have a 307 front end so the rear busted as well as the tranny. I have no idea what was original and what had been replaced.

I replaced the rear with a 307 to match the front. I found another 5spd to replace the busted tranny.

Can the speedo gears be swapped easily?

 

brian

[ComancheKid45]

Yes swapping a speedometer gear is a 5 min job

[Eagle]

Let's see: 55 / 3.07 x 3.55 = 63.6

 

I'd say your son probably wasn't really doing exactly 55, but it was the rear axle ratio change that did him in. That's assuming, of course, that the speedo was accurate before you swapped the axle. If the tires were larger than the speedo gear "thought" they were, even with the old axle the actual speed would have been faster than indicated.

[fiatslug87]

Check out this link, step by step how to change the speedo gear. http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm

[brian]

As always ...good info from you guys. the link provided is just what the Dr. ordered.

I'll see if I can find one to swap out.

thanks

brian