Gear help.

[68_GMC]

Just a guess gang... What gears would make the speedometer read 70 and my GPS say 60? 1 to 1 drive gear with no overdrive. 235/60/15 tires.

I am guessing 388s round about. It is geared for pulling at 55. Has the granny low 4 speed in her. She has a lot of power for pulling but really starts to scream around 60 mph.

[Steeveedee]

The truck would have come with a .78 or so aspect ratio, so the .6 aspect ratio tires you have are smaller diameter. I think that that may be the cause for the difference.

[Mike C]

The gear ratio in the rear is irrelevant to some extent. It relationship of the speedometer drive and driven gear and the total effective gear relative to that. Tire diameter as mentioned above plus the gear ratio give you the total effective gear ratio that the combination of speedo drive/driven gears need to show right speed. But the speedo is off because the tire size is off. Go up in tire height and that should fix it.

Those 235 60 15's are just 26" tall while the 235 75 15 that the truck would have come with are just over 28" tall. Fix it with some 235 75 15's or some 255 70 15's or on my truck I have 275 55 17 which is pretty close as well. !

And looks much cooler. FTW!

[68_GMC]

Good information to know. I just did a 100 mile round trip in her and that motor was screaming to much at 70. I will look into purchasing the tire size it came out with to make it run a little better on top end. Thanks for the help.

[weim55]

Steeveedee is correct, those super short tires are reducing your MPH per the same engine RPM W/ stock size tire. 2 solutions: Replace tires W/ stock size (235-75-15 is close) this will drop the highway engine RPM significantly and correct the speedo issue. #2: pull the speedometer driven gear out of the transmission and increase the tooth count 1 or 2 teeth til you zero in on the correct speedo reading. These old mechanical speedos were rarely dead nut accurate so variations are common.

Steve weim55 Colorado