High rpm in 4th but bogs down in 5th

i’m not sure what it could be but i just bought a 1992 chevy k1500 on 35s and on the road in 4th gear it is in the high rpms and burning gas. shift to 5th and it’s got no power and starts to bog out. any ideas on what this could be? i’m thinking maybe needs a new ratio in the rear.

Or the right size wheels and tires.


k1500 is 4wd? a front/rear diff? stock ratio is 3.50? or 3.73? 700r4? or 5spd manual? they did not put a 4l60e in a 92?

Was it regeared? If you have 35’s, then assuming you have the TBI 350 and 3.73 gears stock, you should have something along the order of 4.56 gears to compensate for the 35’s. Many people lift their trucks stuff larger tires on it, but half-ass it, and don’t do the supporting mods.

i just bought the truck and don’t know the ratios yet i’m assuming they
just lifted it and half assed it like you said.

does truck have tachometer?

no tach but from sound my guess at like 55-60mph in 4th it’s sitting at
like 4400-4900 rpm

The TBI 305/350’s redline at around 4500 RPM. and if you had 3.73 gears and 35 inch tires and 4th is a 1:1 ratio. you’d be going about 125 MPH at 4500 rpm in 4th gear. So I kinda doubt you’re turning 4400-4900 RPM.

then maybe like 3200 then i know it’s screaming in 4th at 60mph

Really none of my business, but was it like this during test drive and if so why did you buy it?

actually I was wrong, I forgot that the effective ratio would be less that stock. The effective ratio is about 3.19 with 35 inch tires. meaning, in 4th gear at 67 MPH, you should be turning about 2000 RPM. When you shift into 5th gear, which in your case is a 0.73 ratio. You’re turning around 1300-1400 RPM. Which would likely both things down.

Keep in mind this is assuming you have the optional 3.73 gear. 3.42 gears were standard with the manual K1500 4WD with the 350 TBI. If you have the 3.42, RPMs will be a bit lower. Either way, the engine shouldn’t be screaming even at highway speeds in 4th gear. You might want to have someone pull the rear diff cover off and see what ratio you have. Usually the numbers are stamped into the ring gear.

35" tires is 576 revs at 60mph
so, 3.73 gears puts you at 2200 rpm for motor
with a 1:1 trans output such as 3rd gear on an auto OD trans
a 0.73 OD would put you at 1500rpm
"screaming" does not sound correct in this situation
3.42 gears is 2k rpm and/or 1350rpm with OD

You’re making the same mistake I did. The 3.73 or 3.42 ratio is assuming the stock sized tires (about 29.5 inches tall). The 35 inch tires change the effective axle ratio. In practice, the 3.73 ratio becomes a 3.19 (or thereabouts) with the 35 inch tires. Also 35’s are rarely 35 inches tall in the really world, typically they are closer to 34.3- 34.6 inches tall.