spdrcr29

joewill

don't know specifically what the tremec has or does, and you did not say at what speed you are off by 26 MPH in order to calculate a percentage error?

but in general if your speedo is showing slow then you need to speed up your cable by dropping the number of teeth in your driven gear. looks like your online calculator agrees

in standard trannys, the driven gear is pretty common to buy and several are available in different tooth counts. if that is true on the tremec then just buy you 2 or three with various lower tooth counts and experiment.
they shouldn't be too expensive.

if all else fails then you can get you one of those gear reduction boxes that you plug into the end of your cable. most tranny shops will know where to get one and I see them on ebay all the time.
but double and triple check your math before you order one with a specific ratio.

most speedometer / cables will spin at 1000 rpm and the speedo should then read 60 MPH

spdrcr29

derekderek

speedo low or high? is the odometer off the same amount? if 26 mph high at 50, you should be doing 1.5 miles odometer for every real mile traveled. if going 70 and speedo reads 45, if odometer is accurate or close, it is the magnet in speedo itself wearing out. that is most likely scenario. Tremec has a prety good idea common rear ratios.

wer2xu

Does the speedometer make any noise? Is the needle steady, or jumping around a bit? Does the trip odometer read actual mileage, or mileage based on inaccurate speed?

v2racing

Usually with a low gear ratio (higher numerically) like a 4.11, the speedo will run high if not setup for it, not low like yours is doing. I would check the odometer by GPS or mile markers. If it is close, then you speedometer has a problem.

Mike

spdrcr29

Speedo reads low. For instance, when my GPS says I'm doing 40mph, the speedo reads 14mph. Have not done any highway driving yet so not sure about how it reads over 60-70mph

The speedo does not make any noise and the needle seems steady and does not jump around at all. If I remember correctly, I found it weird that the trip odometer was reading less mileage than what the actual mileage was. Maybe because of the inaccurate speed reading?

The next time I get the vette out, I will check the odometer by GPS as you suggested and see what it reads. At this point, I'm down to the issue being the incorrect speedo gear or something wrong with the speedometer itself. Already swapped the cable for the heck of it and know its good. Ordered a couple more speedo gears (20 & 21 tooth) to replace the 23 tooth gear that's in the trans now. Will see how that changes things after swapping gears. If not much of a change or no change, suspecting speedometer head issue...

MelWff

did you buy the TREMEC TKO-600 new and tell the vendor what rear gears and tire size you are running or you bought this used?

Bikespace

Can the speedometer head unit get "clocked" inside the housing? A gearing mismatch would cause a ratio change. It seems like it has turned internally, however many degrees corresponds to 26 mph. If there weren't a lower limit on the needle, you would read -26 mph when sitting still. It may be a simple fix to rotate the speedometer within the housing, relative to the dial face.

I don't have an early dash to look at, so I can only speculate.

spdrcr29

Thanks for all the info guys. I have an update on this...I did change out the speedo gear in my Tremec from a 23 tooth gear to a 21 tooth gear like it should have had in the first place. However, the speedo reading was still way off. At 60mph, the speedo read 26mpg and at 40mph, the speedo read 14mph. Basically like no change from when it had the 23 tooth gear in it. So pretty much suspected an issue with the speedo head itself. To test this theory, it dawned on me that a friend of mine is building a 72 and has the interior completely gutted. I asked to borrow his speedometer unit so I could test things out. Simply disconnected the cable from my speedo and pulled the cable down from behind the dash and was able to insert it into the 72 speedo and kind of hold it while I was driving. Low and behold it read correctly (well almost). It was anywhere from 2-4mph off, but that could just be the difference in the GPS reading on my phone. None the less, the 72 speedo reads pretty darn close to what it should while driving. I have since located a shop down in FL that does speedo/tach restorations & calibrations. I called them today and the guy seems to think there may be an issue with the "magnet being weak" and therefore causing the reading to be way off and not reading correctly. His cost for calibration is $50 plus whatever other odds and ends the speedo may need. He says about a 1-2 week turn-around time too. So guess that's what's in store next...take the darn dash apart again to get the speedo out so I can ship it off to him. Yay. Not a job I look forward to! It will probably be a couple weeks till I get around to doing this, but will keep you all posted once I get it back from him and get it reinstalled...