1998 silverado rearend ratio

My dad and I have been talking about re-gearing has 98 silverado it has a mild 350 with around 300-315hp and a firmed up 4l60e it currently has 3.73 ratio but we want more of the line acceleration thinking more like a 4.10 or 4.56 and a eaton posi what do you guys think.  

I would do a 4.56 as 700R4/4L60e has a pretty tall OD and even 4.56 is pretty tame cruising. You will like the way it launches with a 4.56 too. Upgrading to just a 4.10 is not really worth the effort here. BTW, "firming up" 4L60's with shift kits is not very wise because sprags in them are not strong nor is splines on sun gear drum. Shifts are modulated for a reason and I would not change them without first beefing up those parts. Shift kits are more hype that real world benefit.  

It dosent have a shift kit but I think the trans has been reprogrammed (i think that is the word I would use) to make it last a little longer with the diffrent engine.  

Reprogramming is not going to make it last any longer and misleading. (it might shorten its life if you change shift thresholds) Deeper gears will extend tranny life in hard usage because it reduces the average torque load on tranny because less engine torque is required for some effort on ground with deeper axle gears.  

ya i think 4.56 will really liven his truck up as long as it can get traction it already whips his friens nissan titan.  

How would you get the speedo to read the right speed with the diffrent gears. In a 98 truck is it a cable or is it electronic speedo.  

Reprogram ECM. There are a few after market tuners that do this nicely. HyperTech for one. (ECM supports up to at least a 4.56 on that body style so it is not a problem)  

I have been told that diffrent gears will mess up the abs system or something like that is that true.  

Not if you reprogram ECM properly. What kind mess with it is tire size more than axle ratio as ECM only officially supports up to 34 inch or so but 4.56 is fully supported. Beyond that it can be a bit iffy with ABS.  

Quick question - I have a 95 k2500 with 285/75/16's & auto trans, don't know the axle ratio's, but have noticed the loss of take-off power. Should I look into changing gears, at least to preserve the trans, or will it be ok?  

Deeper gear are never bad with a OD tranny. Your power loss is because your tires have raised your effective gear ratio and made it taller so performance has been reduced. Gears are the great equalizer.  

If I was to change the gears, what ratio would be best without affecting the mpg too bad?  

People fret over this MPG thing too much and think they have to cruise at 1800 RPM for best MPG. WIth your tire size a 4.56 would play well because of tall OD and city MPG would likely increase because it would upshift sooner and spend less time on MPG robbing converter stall. Highway should stay same or slightly increase depending on where you are at now.  

Thanks for your help, SnoMan