1990 Burb Gear Ratio Change Killed Gas Mileage

[1990 Suburban]

HELP,
Ever since the ratio change in my 2WD 1990 Suburban, the mileage has been worse. I started out with a 3.73 gearset and was averaging a solid 16-17 MPG. Since having the rear diff replaced with a 3.42 gearset it's been a steady 12-13 MPG.
I have a VSS, not a speedometer cable. I'm wondering if the reluctor gear on the output shaft of the transmission(700R4) needs to be changed.....I was thinking it might have something to do with the Vehicle Speed Sensor reading it wrong.
Anyone with advice please respond......

[maxwedge]

Well now the miles traveled per rpm is more so your mileage calculations are off.

[2000CAYukon]

On the 700R4, you need to change the speedo gears to correct for the gear change. See http://www.700r4.com/speedoCalc/speedocalc.shtml

//2000CAYukon

[1990 Suburban]

Please understand that my gear ratio change was a 10% difference, so now when I travel 1 mile, my odometer registers 0.9 of a mile. Given that equation, my 37 gallon tank that used to give me 600 miles, should now give me 660 miles. The problem I'm having now is I can only go 500 miles on a full tank if that.

[777stickman]

Actually your calculations should be just the opposite. You geared up which means you're odo is turning less miles than before. So your 10% difference would really equate to 600-60=540. Less miles on the odo=lower mpg. Just my ....Steve

[maxwedge]

Correct, simple math!

[1990 Suburban]

So my mileage hasn't changed at all!!!???