After 5 spd conversion, speedo reads 10mph lower, why?

[dubois]

The only thing that has changed is the tranny, 5sp from E21. I even pulled out the speedo gear on the tranny and compared it to the 4 spd one, and it looks the same. New speedo cable. I checked connection on the tranny and it is solid. The speedo does not skip or jump on the dash.

Usually on 4th gear at 4K rpm I was doing mid 70 mph, whereas after installation of 5 spd the speedo is only registering mid 60 at those rpm's. The last thing I will check is the connection of the speedo cable to the instrument panel. Does anybody know if I should be looking elsewhere?

[_z_]

strange.

4th should be 1:1 on both right?

Do the numbers produced from this little program match up with your observations?

http://www.jeffersonfreestate.org/gears/

like maybe the old readings were high and now they are more accurate -- just a thought

[dubois]

strange.

4th should be 1:1 on both right?

Do the numbers produced from this little program match up with your observations?

http://www.jeffersonfreestate.org/gears/

like maybe the old readings were high and now they are more accurate -- just a thought

Yes, Yes and No. I remember checking the speedo "accuracy" to the numbers on the spreadsheet when I had the 4 spd. and I was in the ball park.

[SHEN]

how do you know what diff is in your car?

[InkaSteve]

The "donor" tranny may have come from a car with a 4:11 diff. and speedo geared accordingly. Is that different than yours?? That would effect the final speedo reading. a 4.11 would read 66mph at 4K, a 3.64 would read 74mph at 4K. The tranny doesn't know what diff you have. it just sends the output revolutions to the speedo according to its original diff.

[dubois]

The "donor" tranny may have come from a car with a 4:11 diff. and speedo geared accordingly. Is that different than yours?? That would effect the final speedo reading. a 4.11 would read 66mph at 4K, a 3.64 would read 74mph at 4K. The tranny doesn't know what diff you have. it just sends the output revolutions to the speedo according to its original diff.

yes, that would be correct - but I thought all trannies were the same regardless of final drive and the correction was done in the speedometer itself. A factory E21 with a 4.11 would be rare too. Maybe someone can shed more light on this...???

[MichaelP]

What Steve says makes sense. It sounds like your trans might have come from a 320is, which had a 4.11 diff. Look at the speedodrive again. It must be geared to match the diff. Try your old speedo gear maybe? You say it looks the same, butcha nevah know...

Stupid question, but I'll ask: it's not a cr trans, right?

[ingraham]

Are you saying that 4th gear in such a tranny is not 1:1?

If the tranny is geared to match the diff, there would be no point in having different diff ratios.

I'm not dissing your suggestion; I'm intrigued. It's got me thinking so much, my head hurts.

Nice to meet you on the Shamrock Rally.

[ingraham]

with a 1:1 5th gear?

[_z_]

well if its a CR its got the dogleg pattern right? On yours fifth is up and to the right ya?

[Slavs]

The tranny should have nothing to do wth the reading. It is the diff that makes the difference. Your speedo is geared to the final drive ratio.

If you didn't change the diff ratio, you should be OK !

If using a speedo geared for a 3.64, but your final drive is 4.10, you will indicate too high. The other way around, you would indicate too low.

The diffrence is about 12 percent ( pretty notable )

Slavs

[Slavs]

The tranny should have nothing to do wth the reading. It is the diff that makes the difference. Your speedo is geared to the final drive ratio.

If you didn't change the diff ratio, you should be OK !

If using a speedo geared for a 3.64, but your final drive is 4.10, you will indicate too high. The other way around, you would indicate too low.

The diffrence is about 12 percent ( pretty notable )

Slavs

[InkaSteve]

Are you saying that 4th gear in such a tranny is not 1:1?

If the tranny is geared to match the diff, there would be no point in having different diff ratios.

I'm not dissing your suggestion; I'm intrigued. It's got me thinking so much, my head hurts.

Nice to meet you on the Shamrock Rally.

I am not saying that the actual Tranny Ratios change, just that the speedo system has to have variations to accomodate the change. I really don't know where it happens, in the speedo or in the cable drive from the tranny. I am assuming that the change must be in the tranny, or Michael wouldn't be having any problems. Unless, he is witholding some important info from us. But, there, obviously, is some difference. It makes my head hurt too.

I enjoyed meeting you guys, but honestly, without FAQ nametags, I didn't know it was you until I got home and had time to read all the handouts. Had a great time converting Km's to Miles and back.

[Paul7002]

(nt)

[Paul7002]

(nt)