my bro just bought a 96 z28 6speed, and we were going from town speedo read 45mph and a cop said he clocked us at 75mph. my bro had just got a ticket earlier that day for doing 79 in a 50(he was steady at 50). what would make it do this? also what mph can you cruise in 6th, cuz the car seems to bog down some in 6th. usually my bro cruises in 5th instead without the rpm going past 1500-2000 rpm at high speeds. any help is greatly apreciated, thanks guys

 

In my car 1500 in 5th is around 50 but 1500 in 6th is around 75.

 

Did the pcm get re-rpogrammed for the gears?

 

Don't know why it's so far off - but have him verify what gears are in the car, then go to a gear calc site like www.f-body.org/gears/ and make a "cheat sheet" so he can use his tach for actual cruising MPH so he won't get any more tickets!

 

LOL he couldnt tell the difference between 75 and 45?

 

my bro just bought the car about a month ago, and he doesnt use it too much. we both noticed that the car seemed to be moving faster than what the speedo read, but never payed any attention to it. i noticed that the rear end isnt the original, and whoever had it disconnected the abs sensor on top of the rear end. it is missing the rear swaybar also, i know it was swapped out. how can i check what gears it has, and what are the gear ratios in the tranny to use the calculator?

 

by bad, disregard the last question. just saw all the options in the menu.

 

Only way to be absolutely sure is to pull the cover and count the teeth. For example stock M6 41/12 = 3.416666 (or 3.42). Hard to be accurate this way, but you can also try jacking the car from the rear, mark the driveshaft and tire locations with chalk, then rotate the tire one revolution and see how many times the shaft turns - just under 3.5 turns would be 3.42, just under 3.0 would be 2.73, just over/near 4.0 would be 4.10, etc. Think my math is right.

 

if the car has 2.73-3.23 that might be the cause of the speedo at 45 when its really 75. cuz my bro had a 94z w/ 3.73's but the car had 2.73's and it would read 120 when it only trapped 100mph in the 1/4. is this theory correct?

 

45-75 is a huge difference for the same RPM (that calculator assumes stock T-56 and tire size unless you change it), I'm not an expert but think something else is going on. Just played with that calc:

3.42, 5th, 1500 RPM = 45
2.73, 5th, 1500 RPM = 57

 

Reading 45mph at a true 75mph is hard to explain away with gears. To get that kind of a difference (75/45 = 1.67) you would have to be driving a car that has the PCM programmed for 4.56 gears, while the actual gears installed in the car are 2.73's (4.56/2.73 = 1.67).

Is it possible the tranny was swapped from an A4 to an M6? The difference in pulse/mile from the VSS plus a gear swap might possibly account for the huge error.

 

hmm, A4 to M6 swap would account for it, the A4 used 40 teeth on the reluctor ring, and M6 used 17..............

 

Yup, it would have to be the tranny swap + a gear swap, since the tranny is going to make it read low by a factor of 40/17 = 2.35

For example, if you swapped in a T56, then went from 2.73's to 3.23's:

(40/17) x (2.73/3.73) = 1.72

I know when I swapped in the TH400 with the VSS/40 tooth reluctor gear out of a 90's GM pickup truck, my 155mph speedo was pegged when I was driving 67mph.....

 

it probably got the trans and rear end from the same car, all he needs is a speedo correcting flash, it would be best if he knew the rear gear ratio for sure