Highways around here were fine in the 70s (speed limit was 55mph) when these trucks were built. Now they raised the speed limit to 65mph some years ago and an old truck with no OD doesn't really like 65-70mph.

 

Speed limit was 65 when I was first driving, which was fine with 3.70 gears, then it was raised to 75 and that was pushing it.

Josh

 

Depends on state. At 70 I'm still under 3k rpm but getting close to it. What rpm are you turning at 70 with and whats your setup currently?

 

As of today I have the 275-60x15 rear tires which are actually 27.25" tall. I have 3.50 rear gears. At 70 (GPS) and my loose convertor I am around 3200 RPM and around 3400 for 75 MPH.

Either Wednesday or Thursday I am swapping out my 3.50 gears for my 3.00 gears. I no longer can drive my company truck home so I will have to start driving my truck and motorcycle to work, so I figure I will experiment a touch. At 70 MPH I should be turning around 2600-2700 RPM with the 27" tires.

When I throw on the BFG 31" (30" actual) tires (possibly around Fall) that will drop the RPM to 2744 for 70 MPH with 3.50 gears or 2352 for 3.00 gears.

I have in the past driven with both sets of gears, but don't remember the gas mileage difference.

I also have a set of 2.75 gears, but I think that will be way too tall of a gear for my engine power band and my torque convertor.

I'll probably drive my truck and motorcycle until Fall when I plan to get some sort of beater that gets better gas mileage for this winter. Pretty much all the roads to work are 55, 65 and 75 MPH highways with hardly any stop and go.

As a disclaimer I haven't really driven my truck daily since 2000 so this is sorta all new to me again.

According to my calculations with the 3.89 gears and the 28" cheater clicks I was turning about 3300 RPM to go 70 MPH which seems low because I remember the tach going over 4 grand to stay up with traffic. Maybe I was going 80? No GPS at that time!

Josh

 

Ok so you don't have a lockup converter in your auto. I have a 4spd in mine so the engine and trans input are turning 1:1 with eachother. I'm not shure of what gears I have in mine as of now or if I have a LS.(Why I asked) I have hardly been driving mine also since 04', got a new truck so... I have to GPS mine but I have to get the carb buttoned up first. I would really like a tremec 5 spd in mine but then I'd want to freshen up the 302, add gears rebuild stuff and etc and can't justify it for a truck I don't drive much. It would kill me also if I got in a crash with my truck.