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Gear question

Ray Reynolds

Ray Reynolds

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what gears where typical in the Jimmy's and stock K5's with 350/700r4. My rear doesn't have the tags and was curious. I'm gonna pull the diff cover eventually but not anytime soon
 
My 90 has 3.73s, but it also came with an engine oil cooler and a hitch, might be a tow package of some kind, though I've never pulled the RPOs.
 
Well here's my issue. I think, and it's kinda messing with me, that they've been changed. I'm running 35's and that should make the Speedo off by about 10-15 mph. So if it's reading 65 I'm going damn near 80, but it's actually doing the opposite. At 65 on the Speedo I feel like I'm only going 55. Going to have a buddy go 55 tonight as I follow him just to get a definite read
 
Larger tires make the speedometer read slower...smaller tires make it read faster..more revolutions per mile = higher readings..

This tire comparison chart shows the difference in the amount of revolutions between two different tire sizes ..type in both tire sizes and you'll see..
Tire Size Comparison
 
Ya I get that so why is it doing the opposite. I'm guessing someone has either changed the gear that drives the Speedo or changed the axle ratios
 
If you have an Android phone you can use GPS Status to show your current speed, it's free. Not sure if available on iPhone or not.
 
You didn't state the year of the truck but I'm guessing it's the one in your build thread (1987). The gear ratio depends on the current standard equipment for that model year plus the options ordered on that truck. That said, my bet is 3.07 which is by far the most common.
 
3.73's are extremely rare in a 1/2 ton, not saying you're wrong just that they aren't common. My '91 had 3.42's.
 
The '85 K10 Suburban I have has 3:73's factory...I think the trucks with 700R4's had other ratios besides the 3:08's more often than those with three speed automatics..
 
it was tagged as 3.73 and when I pulled the gear set to change out the pinion bearings, the ring/pinion were 3.73. Maybe a special order?
 
Best I can tell with the trailering option you could/would get 3.73s. There are three listed ratios for the '87 K5 in the dealer brochure (3.08, 3.42 and 3.73).

In any case, 3.73's were spec'd with a 12,000 combined vehicle weight rating (truck and trailer) in a K5. No idea how they got three different combined weight ratings in the K5, but they did.

Can't recall if the K5's could be spec'd with different brakes, I want to say yes, only because I've seen different brake RPO's listed on parts websites, not necessarily because that is the truth lol. Perhaps it was along the lines of trailering package or not, then trailering package with or without better brakes.
 
Jack up one of the rear tires, transfercase in neutral and count the driveshaft revolutions to spin the tire once. 3 times 3:08, about 3 1/2 times 3.42, and almost 4 is 3.73. If you live on a hill make sure you block a tire that is still on the ground.
 
AZK5 said:
Jack up one of the rear tires, transfercase in neutral and count the driveshaft revolutions to spin the tire once. 3 times 3:08, about 3 1/2 times 3.42, and almost 4 is 3.73. If you live on a hill make sure you block a tire that is still on the ground.
Click to expand...
ya I thought about this. I'm gonna do it tonight. Thanks man
 
If the diff lube is old,and could stand to be changed,you could pull the cover off and read the ratio stamped in the ring gear,it'll have two numbers like 41:11(which = 4:11 ratio) ,you divide the larger number by the smaller one to get the ratio..that is the best way to be 100% sure what ratio it is...the spin the tires method can be confusing and not accurate because of the spider gears..
 
I never was much good at math without a calculator !.. :blush:..:doah:..
I was going by memory--I looked in one of my Chilton's manuals and they stated 41-11 is 4:11 ratio too,so they printed the wrong info !..
 
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