speedo tuning

[camaro1]

hey, i just got a gps/nav unit and thought id try to tune my speedo a little closer than it is, here is what i came up with my current settings:

94 z28 auto 3.50 rear gear, p275/40zr17 tires(stock for camaro ss)

speedometer scalar=29
speedometer scaalar, fraction=1.057

35-45 mph speedo is 1 mph fast
50 mph speedo is correct
55-75 mph speedo is 2 mph slow

it is close and i can live with it but id like to have it right on, and if you enter my data into tunercat speed calibrator it comes up with scalar=28, fraction=.985 and the speedo is over 6mph off

is there a more exact formula than what tc uses for its calibrator

thanks,

[playtoy_18]

Just a tip,but never go by the tire ratio.The height's of the separate brand's differ between the same ratio,alway's use a tape measure and physically measure the tire height.
I'm thinking it might never be dead on alway's,tire size fluctuate's at different speed's and temp's.

[Ed Wright]

Well, you can't make the speedo read faster at one point, and slower at another. Your changes will be the same percent everywhere. Don't bank too much on your GPS, as what you are seeing about differing errors is not unusual with those. Ditto highway mileage markers. Guys with stop watches using highway mile markers to check thier odometers drive me nuts. Can't change the odometer without changing the speedo. One clown insisted his odometer acruacy varied with ambient temp. Jeesh!
Yep, he was an engineer.

[Stealth97]

Odometer varying based on ambient temperature? And the guy is an engineer? Hopefully not automotive or for any product I'll ever utilize.