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'65 GMC

'65 GMC

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'65 GMC

New User | Posts: 1 | Joined: 08/06
Posted: 08/03/06
01:57 PM

I have a few questions regarding my 65 GMC half ton truck. It has a 283ci (4.6 L) engine with a 3-speed auto trans (don't know ratio). Problem is, my truck revs quite high while going down the highway. To be more specific, at 110km/h, I'm revving at around 4,000rpm. Not only is this effecting my top speed, but it is making my gas milage bad. I use about 35 litres to every 100 km. My friend says thats about 7.5 miles to the gallon. So my question is would a late-model GMC transmisson bolt right onto my truck, assuming its the same engine size? Would switching a 3-speed to a 4-speed be an easy task?

My other problems is my gas gauge and speed-o-meter (and possibly tachometer) don't actually work properly, they are out of calibration to be specific. How do I fix these? How do I calibrate my speed-o-meter? Do I need a new fuel sensor? How do I know if my tachometer is working in perfect calibration? I've been able to figure out my actual highway speeds and when to fuel up, (using my crappy gauges) but it would be nice to have working gauges.

My truck is frickin' sweet, but these are some problems that need fixing.

Hope you can help,

mcsjr454 mcsjr454
New User | Posts: 12 | Joined: 10/06
Posted: 10/11/06
09:29 PM

Ok lets tackle the guages. The tach is the easiest to check, buy or borrow another tach and hook it up and see where your truck idles at, and compare that to your tach. If the numbers don't match then try to source a REMAN tach for that year V-8 truck. Fuel guage is another easy fix, First make sure your tank is grounded good, second fill up a 5 gallon gas can and set it in the back of the truck, now drive until out of gas and taking note of when you hit empty, now put gas in from can into tank and go to filling station and fill it till the pump shuts off automatically and your guage should read right dead on the full mark. If the readings are off on one end or the other you can try adjusting the float inside the tank. Now for the speedo, I would check it against radar, a local friendly policeman would be helpful. If it is off there are companies that make adapters to fix the problem.

Now for the crappy gas mileage, try going a higher rearend gear or swap to and OD tranny. Gearvendors has an Overdrive kit that requires very little Modification that can add an OD to your truck. This will lower the RPMS also at highway speeds. Generally in a truck I like a set of 3.40:1 gears in the rear, and a TH700R4 or TH2004R as the tranny, with 15 inch rims and a tire that sets around 5 to 7 inches off the rim depending on the type of tire.

any more questions post here and I'll try to help as much as possible