i have 235/60/15...360 fe , c-6...and 3.25 rear ....i think its revving pretty high on free way and dont know what cruising speed rpm is good . do you? im equipped w/ headers duels, edel intake and carb. all msd ...thanks

 

Personally, I'd try to keep it under 3000rpm steady cruise. I've heard of people running 4000 all day long, but I wouldn't try it.

 

my 75 runs about 2500 @ 70 but Id like to get it lower with OD for hwy cruisin

 

Engine rpm is dependent on rear axle ratio ratio and tire size. Without those numbers no one can tell you anthing about your setup, only what their setup does.

 

Yep, what Bear 45/70 said.

Mine had a 3.25 in it too and with 235/85-16's it for sure wasn't gettin over reved! (360 with T18 here)

I like my new ratio!
Went to 3.50 when the pilot bearing surface on the pinion went bad.

My odometer is still off but the speedometer just happens to be right on the money now. I adjusted the speedometer years ago and never was quite enough but was all I felt like I should... it would just barily return to zero.

The odometer could be corrected now with a nylon gear change, but ain't gonna do it.

Alvin in AZ (30-30 bear, mt.lion, jackrabbits, lizards, mud puddles and whatever else was around

 

Keisha9, if your running 60 series 15s that is part of the problem, you've got shorter tires than stock, effectively lowering your final gearing. Try going back to 75 series tires or switching to 16" rims. That should get your rpm back to where its supossed to be.

 

i actually have 275s on it right now but i was going to go to 60s. i'll probably go to 16's and work from there. i wonder what the final gear is on a t-18. that would be sweet

 

Unless it is acutually an overdrive tranny, final output is 1:1; same as any other tranny.

http://victorylibrary.com/mopar/car-tire.htm

If the link works, there is a pretty nice chart that gives you an idea of installed height for different section width and aspect ratio tires. The math is already done, the chart stays put, it is very easy to use.

 

yea, that T-18, is like just about any other old truck 4-sp....1:1 4th gear!

 

t-18 with overdrive the way to go ?? lower rpms on streetrod??

 

It's expensive, but it would work.

It might be money well spent to get a tach and find out just how fast the engine is turning. Might be that your auto tranny is sliping, contributing to a higher RPM. Trying to push a truck through an O/D might get you into a tranny rebuild even sooner.

I took the info you already gave, used the tire chart and a gear calculator to see what RPMs you should be turning at "free way" speeds. Here's the calculator: https://www.ford-trucks.com/calculat...mph_rpm/a-rpm/

3.25 R&P, 26" nominal tires:
55 mph/2310 RPM
65 mph/2730 RPM
75 mph/3150 RPM

Yeah, you could drop some RPMs and be ok. For the price of an O/D unit, you could rebuild the tranny and put new tires-or whatever on the truck.

Just some more food for thought, Mike