Speedo question HELP

[CST10]

My speedometer is off? It's not off like 5 mph all the time but progressively shows higher the fast you drive.

Actual mph = shown mph on speedo
25mph = 25mph
40mph = 47mph
55mph = 65mph

Progressively shows higher than actual speed.

Any ideas what it could be, I have change speedometer out already. Someone told me it may be the cable?

[TBONE1964]

Has it always been this way? Have you changed your tire size lately? Have you changed the rear end (axle) or the transmission lately?

The cable will not cause this. Is the speedometer needle bouncing when you drive it or is it smooth?

If you have a local transmission shop, they can calculate your gear ratio and tire size and put the matching speedometer gears in your trans. That will make it read correctly. Any time you change gear ratio or tire size, it will make the speedometer read fast or slow depending on what change you made.

[CST10]

It's been like that since I bought the truck a year ago barely running. It's the original open carrier 3.08 rear end. Tires are the stock height. Transmission was changed before I bought it but it's the turbo 350. Same transmission type that came from the factory.

I talked with a tranny shop. They said that if it was the little plastic gears it would be off an even number , like -+ 10mph all the time. Not progressively getting higher.

Thoughts?

[CST10]

It's been like that since I bought the truck a year ago barely running. It's the original open carrier 3.08 rear end. Tires are the stock height. Transmission was changed before I bought it but it's the turbo 350. Same transmission type that came from the factory.

I talked with a tranny shop. They said that if it was the little plastic gears it would be off an even number , like -+ 10mph all the time. Not progressively getting higher.

Thoughts?

[TBONE1964]

OK, so it is the same 350 turbo trans as the one that was removed. If the truck the new trans came out of had a 3:73 (or what ever it may be 3:42??) rear gear ratio, then the speedometer gears in the trans will be different. They are color coded and have different teeth count than one with 3:08 gears.

I understand what the trans shop said but i think that is where your problem is. Make sure you have the right gears in the trans for the gear ratio in the rear end and the tire size and go from there.

If you have changed the speedometer and got the same result, then there is really nothing else it can be.

[geezer#99]

[QUOte- Someone told me it may be the cable?[/QUOTE]

It is the cable. Pull it, clean it and relube it with oil but not grease (which is what's likely on it now).

[BADAZ chevy guy]

Go here. http://www.tciauto.com/tc/speedometer-gear-calculator
Here also. Good pics here. http://www.transmissioncenter.org/GM...eter_Gears.htm
And NO... it's NOT the cable. A sticking cable will cause the needle to jump and makes grinding noise. A progressive error is caused by mismatched speedo gears.

[Already Gone]

Would mismatched speedo gears make a factory tachometer read wrong as well? My speedo, verified with a GPS, reads 95mph and the GPS shows 93mph. My tach with a turbo 400 and 3:08 rear end and 255/70 15 tires shows 3000rpm at 65mph where it should be 24/2500rpms with that setup.

[TBONE1964]

Originally Posted by Already Gone

Would mismatched speedo gears make a factory tachometer read wrong as well? My speedo, verified with a GPS, reads 95mph and the GPS shows 93mph. My tach with a turbo 400 and 3:08 rear end and 255/70 15 tires shows 3000rpm at 65mph where it should be 24/2500rpms with that setup.

The factory tachs are wired to the ignition coil and read engine RPM only. The trans, gear ratio and/or tire size will have no effect on the actual tach reading.

However, if you for example run 65 mph and your tach shows 3000 RPM and you changed your tire from a 255/70 15 to a 235/70 R15 then your tach would read a different RPM at the same 65 mph.

It is possible that you tach is out of calibration. Have you checked it against a timing light?

[BADAZ chevy guy]

Also, check to see if there's a selector switch on the back of the tach. Some Chevy tachs were used on 6 as well as 8 cylinder engines. It may have been bumped to the wrong position.

[jocko]

what Tbone said - speedo being incorrect won't cause tach to be incorrect - it reads directly from the engine based on firing pulse.

If stock tire size and rear gear is the same, then the TH350 you swapped in would be from a vehicle that did not have a 3.08 rear originally. In other words, the speedo gear on the output shaft of your TH350 is incorrect for your setup. You can live with it (I had the SAME scenario in my 71) or you can have a trans shop change out the trans tailshaft speedo gear in order to correct the situation. If you have the old trans - IT will have the correct gear on the tailshaft, grab it! You can do this yourself also. Not exactly a fun job and you need to have some auto trans experience. Personally, I'd live with it til it's time for a rebuild. Just knowing what the issue is kinda makes it easier to live with...

EDIT: Also, there are speedo/trans mis-match compensators out there - old Stewart Warner units. I had one, tried it on my 71, still wasn't the correct ratio, so just left it off.
I was also going to post what BADAZ posted - the 2nd link is particularly useful.

[truckster]

Sorry, your trans shop was just wrong. A mismatched speedo gear in the trans will cause a progressive error in the speedometer reading. Whether the problem was caused by a gear change, change in tire height, or trans swap, the solution is the same - get the right speedo gears in the transmission.

[luvmytruck]

Truckster has it right. Had same issue many times. Last time was v8 vega went from 4:11 gears to 3:36 just a simple gear change (no need to pull tail shaft) fixed all

[Already Gone]

Originally Posted by TBONE1964

The factory tachs are wired to the ignition coil and read engine RPM only. The trans, gear ratio and/or tire size will have no effect on the actual tach reading.

However, if you for example run 65 mph and your tach shows 3000 RPM and you changed your tire from a 255/70 15 to a 235/70 R15 then your tach would read a different RPM at the same 65 mph.

It is possible that you tach is out of calibration. Have you checked it against a timing light?

I hear what you are saying and its funny you picked those two tire sizes because I used to have 235/70 15's on the truck and now have 255/70's and the tach is out the same amount with both tire sizes. Could it be the tach is pooched or because the factory tach is hooked to a HEI which it was when I bought the truck.

[CST10]

Thanks guys. I'm call a different tranny shop today. The link is very helpful.

[TBONE1964]

Originally Posted by Already Gone

I hear what you are saying and its funny you picked those two tire sizes because I used to have 235/70 15's on the truck and now have 255/70's and the tach is out the same amount with both tire sizes. Could it be the tach is pooched or because the factory tach is hooked to a HEI which it was when I bought the truck.

I have never heard of anyone having a problem like that running HEI. You should get the same pulse from an HEI coil as you would a point style coil.

If you can find someone with a timing light that reads RPM's, it will be a good reference against your tach. It is possible your tach needs to be calibrated.

[Already Gone]

Originally Posted by TBONE1964

I have never heard of anyone having a problem like that running HEI. You should get the same pulse from an HEI coil as you would a point style coil.

If you can find someone with a timing light that reads RPM's, it will be a good reference against your tach. It is possible your tach needs to be calibrated.

Thanks for the info I'll check into it.

[kwmech]

Simple math

40 divided by 47 = .85
55 divided by 65 = .846

I'd say your speedo is off by 15%

So what was said about the gears in the rear end the trans came out of was probably right on 3.08/3.42= .88 or 12 % pretty close

[cdowns]

if your tranny guy is tellin you that the speedo should b off a set amt accross the board get another tranny guy who knows how the speedo gears work

[CST10]

Very helpful guys. Much appreciated.