I have a 2004 Ram 2500 (1500 in the US) with 20s, I've read that those wheels diminish the acceleration of my truck how much of that is true?

 

It diminishes the accelration in 3 ways:
1) It changes the gearing of the truck
2) Larger wheels are usually heavier (except for certain light weight wheels) so it adds overall weight to the truck
3) Large wheels also create more rotating mass, meaning the engine has to work harder to get them turning

 

main thing is you need to measure the overall tire height of yours and the new ones. The tires on the 20's are a lower profile so I am sure that the overall isn't going to be that much different in the end.

 

Ok, so
1) So I need more reving for the same speed?? what does the 3.92 axle does?
2) I have the polished alluminium wheels (stock 20s), how much more heavy are those than the average 17" (also stock)
3) So, it really can make a difference or it I couldnt tell the difference if I get the 17s.?

 

1) If one truck had a lower profile wheel/tire combo, but they both had the same gears, the the truck with the lower profile combo would require more RPM's to maintain the same speed as the other truck. The 3.92 gears means that for each revolution of your driveshat, your wheels are turning 3.92 times that.
2) I'm not sure on the weight of either wheel, as I have the 20's with the chrome wheel covers.
3) There will be a slight difference, but overall it won't matter too much.

 

Also, about the gearing, it will depend on the overall height of both sets of wheel/tires. If you know the tire size used ont he 17's you can use this calulator to find the difference on overall height.

http://www.miata.net/garage/tirecalc.html

 

You have it backwards. The driveshaft has to make 3.92 revolutions to make the tire spin once.

 

If anyone has a bare stock 20" rim they can weigh, I have a stock 17" from an 04 Ram. From the 04 Ram shop manual;
265/70/17 661 revs /mi.
275/70/17 649 revs
275/55/20 655 revs
275/60/20 636 revs

So it looks like the 17's have a smaller diameter, in general, than any of the 20's. That combined with the lower weigh of the overall tire and wheels combo.....

 

Thanks for the correction, I was afraid I would mess something up. I've been up for the past 40 hours. [&:]