Differential swap?

Hey guys. I am new here. I have a 92 b2200 with carb. I wanted to ask for opinions about swapping diffs. There is probably a thread already started about this subject but I couldn't find it. My problem is that when I'm going down the highway at 65 mph my tach is reading about 3000 rpms. That seems high to me and I can watch the fuel level drop driving to school. Would swapping the differential to one from an rx7 make a difference? Could there be something wrong with the 5th gear in the tranny? I notice there is an arrow pointing to the 55 on the speedo. I just kind of figured that was the optimal speed for these trucks when they were manufactured. Any advice would be appreciated.

what size tires you using? a short tire will take more revs to spin it, putting taller tires on the rear will be 14,000 times easier than swaping the diff, it will bring down the revs - better fuel mileage

example: the stock tire size is 205/75/14, that is 26.1 inches tall. if you run a 205/50/15, that is only 23.1 inches tall. your speedo will run about 13% higher( bout 68mph when only going 60 really), plus your using more fuel

i have owned mine since it was new in 87, it has never got over 20mpg, these are not fuel misers.

the maximum speed in the US was 55, thats why its marked

Damn bob, cortnye's 87 b2k gets 24mpg on the highway at 70mph... We're running 205/70r14s on hers...

I've often wondered about the rx7 diff myself, thinking since it was designed for a car with smaller tires that the final drive might be better for my truck since I'm running 205/40r16s...

I think the wheels and tires are the stock size. Has any one done the swap before?

Correction: they are 195/75/14

That's what my truck and cortnye's were when we got our trucks

im running 180/65/14 and i get shit for MPG on the highway

205/40 is the correct size I believe, when I got my truck it had 3 different size tires on it yep 3 I think people just throw whatever cheap tire they can find if its close to the stock size b/c 14s are hard to find these days.

I get 20mpg around town and 22-24 all highway thats the best you'll do

any 84 85 gsl se with REAR DISC ONLY has limited slip and 4.10, not much for better fuel economy but is direct bolt on with the exception of swapping the drive plate for the shaft on the front of the pinion. The 86 to 90ish has the same LSD with 4.10 i think but has a long snout, ideal for using a one piece shaft in a cab plus truck.