Re: [A/S] More on towing

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

I can't speak for GMC, but my Ford's computer uses a speed sensor that
is driven by a conventional speedometer gear set at the tail of the
transmission. Then an electric cable connects that speed sensor to the
computer. My early '98 requires changing the gears when changing axle or
tires. Later Fords are fully adjustable in the truck's computer. I'd
think a '93 GMC would use a similar sensor (if not the same one) and
conventional speedometer gears. The only way to know for sure is to ask
a good GMC parts department for the gears to go with the 4.10 axle (or
whatever ratio you select). Ford has a separate parts book on the topic.
I chose to go the after market gear route because Ford's route wanted to
change the driver and the driven gear which meant pulling the drive
shaft and the back off the transmission.

In my case the driver was 8 teeth and the driven 18 teeth. A 24 tooth
drive would have been almost perfect, but Ford stops at 22 teeth. The
Ford book change was to go to a 6 tooth driver and 19 tooth driven
(which introduces some error over a 6 tooth driver and 18 tooth driven
with would have a bad wear pattern). Saleen and other Ford after market
places sell a 23 tooth driven. Its not quite as perfect as the original
gears but its within Ford tolerance. I indicate 57 mph to actually go
down the road at 55 mph (according to my stop watch and the 1/4 mile
speed checking marks on Iowa interstates) and when I've traveled a true
ten miles the odometer says 10.2... When I received the 23 tooth driven
I could see why there is not 24 tooth driven. There just isn't room
within that diameter gear for 24 tooth spaces to pass the driving gear
teeth and still have more than paper thin teeth. I have a spare 23 tooth
for the day when these thin teeth break off. Changing the speedometer
gear was no problem at all. 5 minutes with a ratchet and it was done
once I had the gear. Saleen took 6 or 8 weeks to deliver.

With the higher ratio axle gears you will be running the engine faster
and will be passing more exhaust (and intake) volume to get horsepower,
and free flowing intake and exhaust can be of the best benefit at high
engine speeds. Wait until you have to replace the exhaust anyway. Its
sure that the original will die of rust and rot. Might as well get
maximum use of the old parts because the change isn't going to be a
drastic improvement.

The Ford 8.8" axle is particularly difficult to adjust. I've done one
other Ford axle where the works came out making it all bench work. This
Ford axle doesn't come apart that way and the case has to be stretched a
bit to preload the bearings. A pain to work on. A bigger pain without
proper tools. My gears cost about $229 new, most of the $480 charge was
for labor and I was happy to pay it after I studied the shop manual a
while.

Gerald J.