themaniam1

I got the Hypertech programmer for performance and speedo correction. I have an auto tranny, 37" tires and regeared to 5.13s, I selected: Regular gas, 37" tires, 5.13 gears and NO to cooling modifications. The Jeep ran much better and the shift points changed, but instead of showing the speed 10mph slower (I use my GPS to check) now it shows 10mph faster then actual. So after changing the tire size up and down I have finally got it with in a few MPH. The tire size that worked ended up being 36.0". I know the actual tire messurements differ between brands so that does not bother me but running with the speedo showing faster then actual speed does because that puts on extra miles on the speedo. That said.

How important is getting the speedo correct besides the odometer not showing actual miles driven? Does it affect the way the Jeep runs that much?

BaltChief

Dealer told me it could effect the ESP as the computor senses speed and roll of the vehicle....He did say the effect would be minimal as you are just talking a few MPH.

Piginajeep

you need to measure the actual height of the tires, sidewall number dont work with the programmer


i think the guys crazy, my front tires are brand new and the rears are atleast half way gone. the ESP has no issues at all.

Red

The actual tire diameter and what is stamped on the side wall can, and often do, differ a bit. I can believe a 37" tire with an actual rolling diameter closer to 36." Measure it out to see what the tire actually is. Mark a spot on the pavement and on the tire, roll the tire along until it has one full rotation (or more and then take the average), measure out the distance, apply that whole circumference = 2 x pi x r thing you learned back in the 9th grade, and see if that comes out to something closer to 36 than 37. If so, that explains why programming for a 36" tire gets your speedo reading right. The Hypertech programmer is set for exactly a 37" tire, so, unless you have the exact tire size, it will be off by a bit, but within tolerances that any court should accept if someone ever really wanted to challenge how much mileage is on the truck or if you are contesting a speeding ticket and the issue comes up. Also, while the GPS is a great tool to use to see if your speedo reads correctly, don't forget, the GPS also has a tolerance for error, and it changes constantly based on a whole lot of factors. You should be able to check to see how accurate it reads and go from there. If you really want to be accurate about it, find a friendly police officer to help out with his radar gun, or find a flat stretch of road with accurately measured mile markers and a stopwatch... Bottom line is that programming for 36" might be better for your tires. Check with others that run the same tires and gears. Also, I am not trying to second guess you or make you paranoid, but are you 100% positive that the gears you have are 5.13? Perhaps the tires and the gears are on the extreme ends of the production tolerences? Maybe a shady mechanic put them in his truck and gave you something else? Who knows? As far as it hurting anything else on the truck, probably not. BaltChief's dealer may be right about the ESP, however, dealers say a lot of things. Ever wonder why dealer refers both to the guy that you bought your Jeep from, the guy on the street that wants to sell your kids drugs, and the guy that hands you a stack of playing cards? Okay, back on topic, yes it could have an effect on the ESP, though, the ESP measures a whole lot of parameters and it has a range of acceptable parameters and when it leaves these, then the ESP kicks in. I highly doubt that the difference from actual speed and measured speed is going to make the ESP kick in too late or too early. And, it definitely isn't going to break the ESP. With the odometer reading correctly, you hit the nail on the head with the problem of not adjusting your speedo after changing tires or gears, well, that and speeding tickets.

0311sgt

well put RED

themaniam1

The gears are difenitly 5.13s. I Checked when changed the diff covers but a good thing to know for sure, besides I really trust the folks who did the gears.

Thanks for the replies. I am sure that it is the tire actual tire size that is the issue and it seems technically that it really doesn't matter as long as when the programming is done the speedo is correct.