Speedometer correction after gear swap?

[GAmes]

We called the area around Dothan (Ft. Rucker) Lower Alabama

 

[Joe Mc]

Wondering if my 99 is the same fix as above? Running 305/70/16s speedo is consistently 5mph lower than GPS. Or is fix a program thing?

 

[GAmes]

No, starting in '98 the speed signal comes from the ABS sensor in the rear differential. I don't know if the PCM can be programmed or not. Because the G56 doesn't have a sender on the transmission I get my signal from the ABS sensor using a Dakota Digital SGI-5. It's easy to install (4 wires) and adjustable.

 

[Bamtech1]

Mileage report: 14 city, 17 highway.

On the way down to Florida, got 16.8 at 72 mph. Coming back did 17.3 at 70 mph (2000 rpm)

Those figures are strictly the interstate/ cruise control miles. Fueled up getting on I75 and again getting off, just to segregate those miles. Towing about 2000 lbs.

 

[GAmes]

It isn't the weight of what you tow as much as the aerodynamics. I get better mileage towing my boat with a cover on it than I do with the cover off.

 

[Bamtech1]

How do you feel about a camper shell? Seems like it would be slipperier than without one, but I have no data. Camper is blocky but pretty small and round.

 

[Bamtech1]

I’d say the ratio swap was worth it just for engine wear. Conservative math says 300,000 less revolutions just on this trip.

 

[GAmes]

View attachment 113963 How do you feel about a camper shell? Seems like it would be slipperier than without one, but I have no data. Camper is blocky but pretty small and round.

I'd say that combo is about as good as it gets. The shell smooths the flow to the camper. You might want to check on those tires though. If they are STs the norm is 65 mph speed rating.

 

[Bamtech1]

You’re right, they are ST tires, brand new for this trip. I had thought about going to LT radials just for that reason (speed rating) but didn’t have my research poop in a group when I spotted irregular wear 2 days before the trip. Just called and got the same as the last two sets. To be honest, I’ve runn all of them too fast for too many miles. Don’t make it right, though.

 

[Bamtech1]

Another update-
Trip from Atlanta to Murphy,NC laden as shown, 13.3mpg. Pretty much all in the mountains, with a good bit of 25mph grinding switchbacks. I ain’t mad at it. My buddy on the same trip got 9 in his suburban towing a car.
In better news, my last all-commuting tank netted 16.1mpg. Getting the AFC Live setup conservatively seems to help. Less gratuitous smoke (money) out the pipe.

 

[Matt42]

View attachment 113963 How do you feel about a camper shell? Seems like it would be slipperier than without one, but I have no data. Camper is blocky but pretty small and round.

Bamtech: Your front bumper is exactly what I have been looking for my 1996. Where did you find it?

 

[Bamtech1]

Bamtech: Your front bumper is exactly what I have been looking for my 1996. Where did you find it?

It was on the truck when I bought it and is a one-off, built in place I suspect. It’s welded to the truck (very nicely). I added the spare tire mount from a boat trailer.

 

[Bamtech1]

 

[Bamtech1]

Just a study in how mileage related to speed. Working up from the bottom- first tank is all interstate with cruise set at 70. Second tank is all 2-lane flat Florida, 55 mph. Third is mixed with some 55 and half the trip home at 65. Top line is from south Ga. home to Atlanta cruise at 65.