94 dodge cummins tops out at 65mph !

khwrx

Active Member
Joined
Dec 10, 2006
Location
lincolnton nc
this has been bugging me since i bought this truck. its a 94 dodge 3500 cummins 2wd with a 5 speed manual trans. it has low ,first, sencond , third and fourth/ overdrive. well at 65 mph im reving out at 2,300rpms. i can do 70 to 75 down hill. is this normal for this truck. i dont know if a previous owner has regear it or what.
 
Must have 4.10s and stock tires.

Mine(97 4wd) topped out at about 78 or so untill I went to a taller tire...now I can hit a whopping 85.

GSK kit too will help that. will let it rev to 3k or 4k depending on kit you buy..4k kit needs exhaust valve springs also.
 
4.10s and 235s (31.75") would have you topping out right at 70mph at 2300. I believe the stock tire size was actually 245/75R16 (or 215/85R16 if dually) which would be 30.5" Which would have you right at the 65mph at 2300 in O/D. Remember, your 235s are actually taller than the stock size, so you are actually running about 5mph faster than the speedo shows at that speed.

Throw some taller tires on there and it will make a difference keeping the RPMs down.
 
A little more info - a '94 cummins is a 12 valve (mechanical) engine. It uses governor springs to control how high it will rev. The stock springs will start to cut the fuel back around 2200 rpm and will not allow it to rev more than about 2500-2600 even at wide open throttle. This is what you are experiencing. You can buy an aftermarket 3k or 4k rpm governor spring kit that will allow it to rev higher, or else you can put taller tires on or regear the axles to 3.55 and it will go faster. You definitely seem to have 4.10 gears right now.
 
maddog411 said:
A little more info - a '94 cummins is a 12 valve (mechanical) engine. It uses governor springs to control how high it will rev. The stock springs will start to cut the fuel back around 2200 rpm and will not allow it to rev more than about 2500-2600 even at wide open throttle. This is what you are experiencing. You can buy an aftermarket 3k or 4k rpm governor spring kit that will allow it to rev higher, or else you can put taller tires on or regear the axles to 3.55 and it will go faster. You definitely seem to have 4.10 gears right now.
Click to expand...
72rockcruiser mentioned the GSK above.
 
FWIW you can buy some washers (ID/OD I've long since forgotten), mic them to get 2 very similar ones and install for 3k rpm. Mine were 0.103" thick, and I did initial+3 clicks to get it set. didnt touch idle adjustment either.
 
we were really slow at work today and i pulled the dodge in and pulled my afc and did a regrind of my fuel plate i found online. and turned the star wheel. man that woke that truck up. but still no higher rpms. i did read online bout the washer mod your talking about , but i didnt see anywhere that had any good pics of how to do it.
 
The short bed truck I built a while back had the same problem. It had 4.10s with 245s. Drove it back from PA with my foot to the floor the whole way home doing about 70. I put a 3.55 geared rear end, 33" tires, and a 3k GSK and it would do well over 100.
 
dodge cummins 4.10 gear

put 235/85/16 from 225/75/16. Rpms were 2500 now there 2300. But the motor sounds so much better at that rpm. around 55 its under 2000 rpm.
 
Just my opinion on what to do:

The cheapest fix without rednecking it is to buy a 3k GSK ($100)

The best way to fix it is to get 3.55 gears. It might be cheaper to find a whole axle with those gears to slide under the truck.
 
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