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I have an 85 chevy truck that I don't know much about. When I push on gas pedal it seems to take a lot just to get it to move. It is a 4x4 and I think it has a 700R auto. It has 38.5 in tires on it and I can't here the trans slipping. Is this normal for a truck with big tires? Would a trans with a big stall do this. Has a 350 engine that seems very snappy until you put it in gear.
Thanks Tom
Tom, I'd wonder more what your differential ratios are on the truck. Often guys put big meats on without considering the impact to overall driveline ratio. Can you tell me what rpm you're turning at a specific speed in Drive (not OD), like maybe 60mph or 50mph? Really any speed will do, just get it steady state and mark the rpm. Then drop it into OD and note the rpm change. With those we can quickly figure out the ratio and see if that's your problem.
I don't drive in the winter months, because it is still rust free. I do know that it has a 3:08 gear.
Tom, that's your problem. Assuming that you indeed have a 700R4 the tranny ratios are 1st 3.06, 2nd 1.62, 3rd 1.0 and 4th 0.70 OD. Running 38.5's and 3.08 gearing you've got to get up to almost 20mph before you're into your torque range. It runs out that you're only doing about 1300rpm at 70mph! You need to get a much lower gear in the truck - you're really straining your tranny as it is. I'd probably go at least 4.88's, but 5.13's or even 5.38's are not too much, depending on how you use the truck. Points of reference, consider 70mph cruise, assuming a 700r4 OD, 4.56's turn 1950rpm, 4.88's turn 2087, 5.13's turn 2194, and 5.38's only crank 2300rpm at 70mph. Drop in a pair of new ring & pinions and get that gearing where it needs to be and it'll be a whole new truck. You're gonna love it!!Quote:
Originally Posted by tominiowa
I do that as soon as it warms up a little, But the truck is so tall I can't even get in it.
Yeah, but a truck on 38's has got to be lookin' super cool! Get a fold down step!Quote:
Originally Posted by tominiowa
I may have the clue others have been missing-personal experience.
Does your converter have lockup control?
Get the B&M lockup control. You'll appreciate the improvement.
Firebird, lockup only happens after you get up to speed and back off of the throttle. Tom's problem is that he's lost almost all mechanical advantage to get rolling by keeping the 3.08 highway gears, and swapping on tires that are ten to twelve inches taller than stock. The lockup is a good idea for the highway, though.Quote:
Originally Posted by firebird77clone
If he could give some feedback with a tach, it could help. A hi stall isn't going to do much before its stall speed, but we may be both right. At any rate, he would sure feel the difference from a set of 4.11s
Tom, I'm not saying that the stall speed of the converter may not be high for your engine's torque band and rpm capabilities, but it's a moot point with the gears in the differentials as there is simply not sufficient mechanical advantage with the tall tires. IMO 4.11's are not nearly tall enough, but to each his own. Pick your desired cruise speed depending on the mix of driving and highway cruise rpm you want, and pick your gear ratio (front & back) accordingly. Now the numbers are based on the assumption of a 700r4, but....Quote:
Originally Posted by firebird77clone
Good luck with the truck, and if you've got a good 4X4 shop in the area drop in and chat with them about your truck and I'm pretty sure they'll echo what I've told you. I've found our 4X4 shop (the "good" one is an hour's drive away, in Topeka) to be an excellent place to have gears set up, because they're into differentials all the time with off-road rigs, and a good shop will know their stuff. Post some pictures of the bad boy if you get a chance!Quote:
Originally Posted by rspears
I tend to agree with Roger. Installing much higher rear gears would be the first thing to address.
Here is an excellent chart that will help to pick the best gears for the application. With the overdrive trans. and 38.5" diameter tires a 5.38 gear would be "just right" and make the truck much more enjoyable to drive. You'll think you tripled the horsepower!
Be sure to read the notes about overdrive transmissions that are discussed in the text above the chart.
Hope this helps... and post up some photo's of your truck Tom. No doubt it looks great with those big meats!
http://i645.photobucket.com/albums/u...ps0ccd1738.jpg
http://i645.photobucket.com/albums/u...ps0c03ad74.jpg
Great response John - the visual really helps to explain the issue. Big tires are a real disappointment if rear end ratios are not addressed. It's nice to have the additional clearance for off road adventures but if you can't get your horsepower to the ground it's no fun at all. I'd with the 5.xx gears with the 38.5" tires if you're off roading at all - 4.88 would be okay for a "street machine".
Great chart lakeroadster. Can you post a link?
This truck needs 5.29 ++ gears.
"Has a 350 engine that seems very snappy until you put it in gear."
Funny stuff Tom.......
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Yeah, a high stall is just a band aid with a 3:08 gear and those big tires. That thing can't be fun to drive. Get passed by a smart car fun. :LOL:
Here's the link to the chart I posted above: Jeep Parts, Jeep Accessories & Jeep Soft Tops From The Jeep Parts Experts - QuadratecQuote:
Originally Posted by techinspector1
The following link is to a chart that includes even larger tires and higher numerical gearing: Gear Ratio and Tire Size Calculator - 4x4 and Jeep Ring & Pinion
Well it may need a push to get started, but if you run the numbers out the theoretical top speed in OD at 6000 rpm is 319MPH! :rolleyes::LOL::LOL:Quote:
Originally Posted by 40FordDeluxe
Tom, please don't be offended by the goofing around. It's the cold winter slowdown time, and not much else to do.;)