Look at the diff. Both 6MT with the same FD but off by 200 rpms. 200 rpms is typically the diff between 3.3 and 3.5 FD

 

my car is bone stock as far as the FD .... so..?

 

its not uncommon for the tach to be off by that much though... I dont know about being off to another car - but off from actual rmps (as noted on several dyno pulls)...

 

My tach is off by about 50 RPM if I'm cruising. But if I floor it, the tach does 'lag' behind the real RPM. IDK by how much. I'd have to datalog to see.

 

ya, that can happen. Sometimes the tachs are not very accurate. Plus, you have to get the rpm while you are on flat ground.

I'm sure you have the 3.53 Final Drive differential it just shows you how they can be off.

 

Won't it depend on the tire dia also? Even with the same tire size, the actual dia various amount different makers.

 

^^^ speed yes, rpm no.

 

^That post kinda sounds like you're saying dire diameter would affect your RPM or make your tach read wrong. I know what you mean though. Changing tire diameter would change the ratio between RPM and actual speed. It shouldn't change the ratio on the dash though since the speedo works off the tire RPM (independant of speed/tire size) but if your were compairing RPMs to speed on a GPS then it would.

 

tire size will affect the RPM slightly, but should be no more that +/- 50 rpm. Smaller diameter tires will change the true speed of the car - thereby affecting the RPM at that given speed.

^^ same thing you were saying i think haha, repost.

 

^Add to this the fact that you could have a +/- 100 RPM inaccuracy in the tach combined with a +/- 5 MPH inaccuracy in the speedometer... and if they're inaccurate in opposite directions... well, you get the idea.

Really the best way to get an accurate reading is with datalogging software showing your true RPM and a GPS to show your true speed.

 

This is why cruise control is disabled with a different FD (it has to do with the TCU)

 

Toll- I'm assuming your CC is still working below 79mph?

 

does yours work Dave?

 

Haven't installed it yet. Might be another week or two.

 

Yes, my CC works, and it works all the way up to 86 MPH.