Speedo Calibration for 4.08 gearing w/J30 speedo gear

[jamesmost]

So i have had my 408 for many years now. approx 3mph on the high side thanks to the J30 speedo gear.I had some work done in the trans recently which included swapping the housing.Now when speedo shows:20mph nav shows 16.525mph nav shows 20mph40mph nav shows 33.5mph60mph nav shows 47mph80mph nav shows 67mph

So was i missing something all along or did the transmission shop install my origional Q45 speedo gear

[Infinitiguy19]

No doubt Tech will come in here with an equation quite possibly proving that.

But why not use a 4.08 speedo or speedo adjuster which I think is made in Australia?

[Q45tech]

The pulses per drive shaft rev are set by the speed sensor gear, no equation necessary just common sense.

Always important to select and swap speed sensor when swaping transmissions.

The system has worked this way back in the 1950 except it was mechanical not electronic.

[Rex]

Those numbers look like my uncorrected speeds back when I had Da Beast.

My guess is you have a Q45 speedo gear again.

[elwesso]

What i would recommend is getting one of these bad babies..

http://www.dakotadigital.com/i...6.htm

Real easy install and you just keep adjusting the speedo until it matches your GPS...

Way easier than screwing around with those pinion gears AND allows you to easily correct for changing your tire size.

[Trumpkin]

I too have the 408 and the j30 gear. My numbers really never were right. It may be my 18's or from memory the j30 gear is not spot on. So with tires off and the gear not quite right, that may give the wrong readings. Wes's idea is a nice solution, but imo-why bother. You have your gps. Maybe the j30 gear works closer with 411's?

[elwesso]

I too have the 408 and the j30 gear. My numbers really never were right. It may be my 18's or from memory the j30 gear is not spot on. So with tires off and the gear not quite right, that may give the wrong readings. Wes's idea is a nice solution, but imo-why bother. You have your gps. Maybe the j30 gear works closer with 411's?

J30 is a 3.96 so it should be close...

Main reason I wanted it to be accurate is for the odometer... plus I like all my stuff to be "factory like"

[jamesmost]

i kinda remember mu J30 gear only being off bu a few MPH.Must have the Q gear back in.Is the speedo gear mounted on the housing, yes i believe so.my 91 trans was swapped into my 95 housing and i bet they ddnt even think to swap the gear....

[urbanskiben]

i am wih

J30 is a 3.96 so it should be close...

Main reason I wanted it to be accurate is for the odometer... plus I like all my stuff to be "factory like"

i am with you i like everything to look factory i hate those who jus kinda throw it together

[AZhitman]

I recall buying the replacement gear back when I had my 4.08 in the Q.

Didn't work. You need the entire housing, as the different gears have a different O.D. and won't physically fit in the hole.

[elwesso]

youre best off getting the dakota digital SGI-5. Easy install and works perfect!!

[jamesmost]

I recall buying the replacement gear back when I had my 4.08 in the Q.

Didn't work. You need the entire housing, as the different gears have a different O.D. and won't physically fit in the hole.

AZ - i swapped the J30 gear back in, perfect fit and speedo off about <5%

so 60 mph reads 62.7mph on nav

[AZhitman]

Strange.

All I know is I spent $$$ on a gear that wouldn't fit.

[jamesmost]

Strange.

All I know is I spent $$$ on a gear that wouldn't fit.

$ 30