Is my speedometer correct? 99 Blazer LT, 4x4, automatic, 4.3L.

With P235/70R15and3:73 gear at 2000 RPM, speedo shows 70 MPH.

 

Sounds about 10 mph too fast to me.

 

So you think I'm actually doing 60 whne the speedo says 70?

What do I need to do to get the speedo re-calibrated???

 

FWIW, this truck is bone stock. Tires are exactly OEM spec.

 

Pretty easy. Just need to program into the ECM your gear ratio and tire size. You can either go to the dealership or some other shop and they can do it for you, for a price, or purchase a programmer yourself.

 

Assuming you are quoting the RPM with the transmission in 4th gear and the torque converter locked up... The 700R4/4L60/4L60E has an 4th gear (OD) ratio of 0.7:1. With 3.73 gears, that results in a final drive ratio of 2.61:1. So for every 2.61 revolutions of the motor, the wheel will turn 1 time. A 235/70R15 tire is a ~28" diameter tire which goes through 722 revolutions per mile. This results in a speed of 63.68MPH at 2000RPM. At 70MPH, the engine would be turning ~2200RPM.

MI/HR = ( REV/MIN * 60 MIN/HR ) divided by ( Tire REV/MI * Final Drive Ratio )

Final Drive Ratio = Transmission Gear Ratio * Axle Ratio

Tire REV/MI = 63630 IN/MI divided by IN/REV [which is circumference = PI * DIAMETER]

 

As a comparison, my truck with 3.42 gears and the same size factory tires (which, at normal inflation, have a measured rolling circumferance of 27.25 inches), and reprogrammed for that with my Hypertech tuner, shows 1,800rpm at 60mph.

 

You mean rolling diameter, right?

 

D'oh!!!
And I can't even use New Year's hangover as an excuse.

 

Thanks guys!!

Now I have to find that thread about tuners!