Speedometer way off!..and third gear rpm question

Speedometer way off!..and third gear rpm question - 03/30/11 03:39 PM

My Chall has 255/60/15's on the rear,auto and sure grip .First off my spedo is 20mph off till about 50 then climbs to 30 miles off consistantly.At 75 on the GPS my speedo is on 120.Any ideas??Alos guy i bought the car from claims it has 3.91's but 4000rpm's at 70 mph seem right??.Thats flat azz screamin!Thanks for any help.

Posted By: Mr.Yuck

Re: Speedometer way off!..and third gear rpm question - 03/30/11 03:49 PM

Jack the car up and count the number of times the driveshaft turns per one rotation of the wheel. 3 1/2 turns = 3.55, almost 4 = 3.91's a tad more than 4 = 4.10's.... 4k at 70 seems high to me however I usually run 275/60/15's and they are about 1" taller 27" vs 27.99"

whoops forgot, you need the right speedo gear. find out what rear and check e-bay for the correct gear.

Posted By: Mopar_Country

Re: Speedometer way off!..and third gear rpm question - 03/30/11 03:55 PM

With a 27" tall tire at 70 MPH, 4000 RPM's sounds more like 4.56 gear. Did you check that 70MPH with the gps or the speedo? You would also need to switch the speedo gear in the trans to compensate. 44 tooth gear I believe. 70 with 3.91"s should put you in the 3400 rpm range.

There are differences in shaft length on the speedo gear, so make sure which one you have. Also when changing the gear you have to use the correct setting on the case.

Attached picture 6557621-TFspeedohousingandseals.jpg

Attached picture 6557621-TFspeedohousingandseals.jpg

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/30/11 04:06 PM

Quote:

With a 27" tall tire at 70 MPH, 4000 RPM's sounds more like 4.56 gear. Did you check that 70MPH with the gps or the speedo? You would also need to switch the speedo gear in the trans to compensate. 44 tooth gear I believe. 70 with 3.91"s should put you in the 3400 rpm range.

...with the gps.The 4g's was 120(on the speedo) or basically pegged but showing a consistant 70mph(gps) on a wide open highway with no hills or satillite obstruction.
I'll go out and turn the wheels and let you guys know what i found out.First gear only good for tire spinning.

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/30/11 04:08 PM

Quote:

With a 27" tall tire at 70 MPH, 4000 RPM's sounds more like 4.56 gear. Did you check that 70MPH with the gps or the speedo? You would also need to switch the speedo gear in the trans to compensate. 44 tooth gear I believe. 70 with 3.91"s should put you in the 3400 rpm range.

There are differences in shaft length on the speedo gear, so make sure which one you have. Also when changing the gear you have to use the correct setting on the case.

Hey i'll go out and do it now..be back in half hour thanks

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/30/11 06:46 PM

Half hour turns into three,uncle had heart attack,duaghter is sick had to go get her,well they are definetly 3.91,just shy of 4 revolutions..would the 3500 stall make any difference?I will check the setting on the trans this weekend thanks for all the help!!!

Posted By: Mopar_Country

Re: Speedometer way off!..and third gear rpm question - 03/30/11 07:00 PM

You may have to physically change the gear in the trans, simply setting the adapter will not fix the problem. 27" tall tire 3.91 should be 38 tooth gear. If it's not that is why your speedo is whacko.

Posted By: John_Kunkel

Re: Speedometer way off!..and third gear rpm question - 03/30/11 07:00 PM

With 255/60/15's and 3.91 gears you need a 38-tooth speedo pinion. (Color blue P/N 52068060)

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/31/11 12:36 AM

Ok,i'll order that one.What is the setting on the trans for,fine tuning?Still 4000 at 70 on the GPS..i'll get the wife to follow me out the highway some night.Dont know the accuracy of the gps.Thanks again.

Posted By: Pacnorthcuda

Re: Speedometer way off!..and third gear rpm question - 03/31/11 12:51 AM

Quote:

Ok,i'll order that one.What is the setting on the trans for,fine tuning?

If you mean the clocking of the pinion housing....no. Its because pinions with more teeth are larger than those with less. The 'center' hole in the housing is not centered, for that reason. Correct clocking will ensure correct tooth engagement with the worm gear portion of the output shaft of the tranny.
It has no 'fine-tuning' effect whatsoever.

Posted By: Mopar_Country

Re: Speedometer way off!..and third gear rpm question - 03/31/11 12:55 AM

With the 38 tooth set it just like the chart I posted above. If you look at the speedo cable threaded portion you can see it's not centered on the adapter.

You should be able to find one on ebag, I just picked up one for 20 bucks, in great condition. If you can't find one there Brewers Performance should have one.

Posted By: Golden-Arm

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:25 AM

Quote:

i'll get the wife to follow me out the highway some night.Dont know the accuracy of the gps.Thanks again.

just take her car and the gps, and see for yourself. you'll know in a matter of seconds, if it's accurate. if it knows you're going 23 mph it should know your going 70. if the convertor is slipping, your rpms will be excessive. are you positive it's shifting into third? run through the gears manually, and see what is what.

Posted By: kenworth_goose

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:49 AM

You've got a set of 4.56's.

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:51 AM

I just never messed with that part of the housing other than taking the speedo cable off..thanks for all the help i have a manual valvebody and i am definetly getting into third.

just take her car and the gps, and see for yourself. you'll know in a matter of seconds, if it's accurate.Yeah it works great but didnt know if a gps can be affected by weather.Dont know much about them.Her Edge and Garmin are dead on together.

Appreciate the help..been into Mopars since '91 striaght through and definetly dont know everything,even some simple stuff like this which was foriegn to me.I enjoy learning..so much appreciated men.

Posted By: kenworth_goose

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:54 AM

If you really have 3.91's your convertor is operating at about 80-82%. And that's not good. 4000 at 70 and that's what it comes out to.

Posted By: Mopar_Country

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:55 AM

Quote:

You've got a set of 4.56's.

That's what I'm thinkin'.

Let us know what you find out, inquiring minds want to know.

Posted By: Mr.Yuck

Re: Speedometer way off!..and third gear rpm question - 03/31/11 01:34 PM

Quote:

Quote:

You've got a set of 4.56's.

That's what I'm thinkin'.

Let us know what you find out, inquiring minds want to know.

I think he counted the rotation of the wheel vs. the driveshaft. He said it went almost 4 complete turns.

Posted By: patrick

Re: Speedometer way off!..and third gear rpm question - 03/31/11 02:25 PM

without converter slip, 70mph with those tires and gears is 3400, I'd expect actual cruise RPM to be 3600-3700...the loose converter might be slipping more, getting up to 3800-3900....

275/60R15's drop that 100 RPM.

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/31/11 02:49 PM

Yeah i put her on stands and marked the driveshaft on the bottom and the inner pass side tire so i could spin drivers and read the other side..just barely 4 so we know the ratio..so the converter is possible not locking up all the way?..maybe?Wow i think the best thing for me to do is a make a video and post in car of Garmin and my tack readings and the fact you cant hold a conversation with full exhaust at 70mph

.
I'll try to put this together today and get back on here..week of vacation has been fun..couple honeydoo's to do but after that i'll try to get video put together...stay tuned...

Posted By: Mopar_Country

Re: Speedometer way off!..and third gear rpm question - 03/31/11 03:29 PM

Posted By: 451 D*O*D*G*E

Re: Speedometer way off!..and third gear rpm question - 03/31/11 07:57 PM

Sh** dude you're gonna need another bag.Its raining pretty good now.Camera is charged and honeydoos are done so hope the rain stops.I gots no heater,wipers..nothin

The car was a QUART low on trans fluid i know pints but it took a whole quart in nuetral to get it up to fulll line.Foamy as well.Be back soon as weather will allow.