I drive a 97 dodge 1500 with a 3.9 liter v6 and a five speed manual transmission with 173,000 miles and I'm having problems keeping speed in 5th gear I know that 5th isn't for gaining speed but I can run 100 mph in 4th and when I put it in 5th it goes back down to 70-75 I've changed spark plugs, wires, tried fuel additives, changed o2 sensors, air filters. And the truck runs like a bat out of hell until the end of 3rd but becomes doggy afterwards. I'm running tires slightly larger than stock on the rear end so I'm not sure if it's a gearING thing, or a bad sensor, distributor, but it fires right up right away. Check engine light is on for random misfire on all 6 cylinders but dose not happen in low end only in high gear. I need help I have daily thoughts of burning it to ground and claiming bad wireing.

 

I would say that part of your problem is the random misfire on all 6 cylinders, that by it self would cause the lose in power. So, you need to fix that problem first before anything else. Also, the larger tires on the rear are effecting the gear ratio.

 

Drop in a 5.9.

Check and see if you have oil pooling in the plenum.

How did the old plugs look when you pulled them?

Still running original O2 sensors?

 

It's a V6 full size truck with larger than stock tires, sounds normal. Needs lower gears.

 

I have changed o2 sensors

 

What would be appropriate gears for 235/85 r16?

 

I have also been told that it may be vacuum advance / electronic advance was wondering if that sounds right