Speed for 5.38 gears

[HuskerFan]

Hi guys,

So i've tried to look online and understand what happens mechanically in this scenario, but i'm still confused. I have 5.38 gears on a 14 bolt with 44" swampers. I swapped out 33's on the truck to fit it on a trailer as i was in a bind, but drove it to town to pick up an extra ratchet strap as one of the 4 i had with me broke of course, and in driving down the highway with the 33's on, only at about 50-60 mph, but accidentally got to 70 when i wasn't lookin at the speedo on a downhill portion, felt a little bit of a knocking/humming noise. My question is, what happens if you were to go, say 80-90 mph with 5.38 gears?

Thanks in advance!

 

[HotRodPC]

Loaded question. Overdrive Transmission or not? What engine? For what period of time? Stock engine?

You have to take all these things into consideration.

I can tell you, 80-90 with 5.38s on 33's for a long period of time without OD in a stock big block is not a good thing. A stock small block would handle it better due to less rotating mass, but a properly built small block for higher rpm would be better.

 

[77 K20]

It also depends on your speedometer. When you said you were doing 70 were you really?
For instance if your speedometer has been reset/recalibrated to be accurate when you run 44" tires and you then switch to a 33" tall tire you speedometer will be off by 17.5 mph. Meaning the truck will say 70 mph... but you are doing 52.5 mph.

http://www.onlineconversion.com/bigger_tires.htm

If 70 was your actual speed there are calculators to look up engine RPM based on gear ratio, tire size, and a 1:1 trans ratio (automatic with lockup non OD, or manual trans non-OD)

In theory your truck doing 80 MPH if you have a 1:1 trans would be running at 4,382 RPM. If you have a different trans ratio you can enter that in. (for OD)

http://www.crawlpedia.com/rpm_gear_calculator.htm

 

[HotRodPC]

And then there's that too !!!

 

[WFO]

Sounds like too many RPM floated the valves.