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hi guys. I have a 4 speed 41 gmc 1-1/2 ton . stock tire size and a 248 motor. anyone have a clue what to turn this thing at. speedo is dead on. 35-40 sounds like it's working really hard. thanks katrina

That is top speed on the early trucks.I got mine to go faster by useing 900-20's on the rear.The two speed rear and Watson three range takes care of the low end.

Not a GMC man but if it's all stock and the same as a Chevy of the same size I'd say your doin' pretty good to pull 40 mph out of it. These were not very powerful engines so gearing is the only way to they could get them to haul the loads they were required to do. If it's the look that your after and performance then your going to have to do a bunch of upgrading.
Denny Graham
Sandwich, IL

my 40 11/4 ton has a 228 and i can run 47 all day and can hit 50-53 if i need too. when i first got it 40 was about all i could get. the engine and tranny is noisey so it sounds like it is going faster than what it is. at 50 the 228 is turning 2432 rpm according to the rpm/mph chart and sounds like it is screaming. back it down to 45 its 2189 and in the 2/3rds operating range and you can actually hear yourself think. big difference in noise in only a 250 rpm increase.

if you have a two speed rear and its in low range it will be screaming and going nowhere. i see its a fire truck, depending on the fire body it could be a lead sled. the ashton had a 248 in it and weighs in at 10000 with nothing on it and could easily hit 50 but then the tranny is a sm420 and a lot quieter than the crash boxes.

If its a two speed rear it will have a ratio of 6.10 high or 8.something in low. A single speed would most likely have a 6.17 and even rarer a 5.43.

45-50 is about the fastest you'll ever want to go in a firetruck with the two speed rear axle. I've been to 55 but it seems to really too high of an rpm to maintain for very long.

I put a tach in for this very reason. Instead of speed I now just keep within a certain running range, which seems to be up[ to 3200rpms, which maybe 45.

(6.10/8.13)Not any speed ,but can pull a house clear across the country.

I'm guessing that's about what my '57 GMC 350 will do then. If they'll fit on the stock rims,I'm hoping to try a set of 11.00R-22.5's on it in place of the 9.00's it now has-hopefully it'll help the road speed a little. My truck has the 270 six,SM420 and a two speed rear.
Speed

Check this Tech Tip for info and for the Accuride Wheel Catalog linked in it
On page 39 is a tire and rim chart that gives the acceptable rim widths for different tire sizes. 11R22.5 tires need a 7.5" wide rim at a minimum. Occasionally this info contradicts what the tire manufacturer list, so double check the minimum rim width for the tires you actually plan to purchase too.

Grigg

All of the major brands of tires recommend 8.25 wheels for 11R22.5 tires. Newer wheels have maximum weight stamped on them. The weight limit of the wheels must meet or exceed the weight limit of the tire. 8.25 wheels are the standard of the industry now. If you find an old 7.50 wheel, it was probably built before radials became popular and was designed for 11x22.5 textile tires and not radials. This was a big issue about 30 years ago when carriers started switching to radials, but most of the old wheels have been scrapped since then.