28HopUp

So I don't drive our 2005 4.0L/auto vert all that much, but I recognized not too long ago that the car felt like it was traveling faster than the speedo indicated. I had the xcal2 with Evolution tunes set to stock rear-end ratio and stock tires (27" dia). My MPG's were also down to around 12 or so, indicating that something is off.

Getting ready for spring, our son Jack installed our summer tires, which in the back are 275/50R17 Nitto NT450's (28" dia). These are the same tires I used last year on my coupe. Using the xcal2, I changed the tire revs per mile to the Nitto-listed 745 revs per mile (from the stock 770 revs/mile), and loaded the 93 octane tune. Things seemed to feel a bit better regarding the speedo, and my mileage immediately jumped up to around 16.5 mpg.

So this afternoon I came upon a local PD's radar sign that tells you how fast you are going. I had the speedo just under 40mph, and the sign flashes 47 - WTFrick??? That's a 20% error, assuming that the radar unit is properly calibrated.

I wanted to get your thoughts as to the cause, and what would be the best method to correct the issue. It could be that the tune is off, since it was originally written for 4.10 gears (remember that I'm setting the gearing to "stock"). The handheld tuner doesn't have a gear option for 3.31's, but I could change the gear ratio to 3.08:1 with the tuner to get closer, and then adjust the tire revs to hone it in. But being an auto, I don't want to negatively affect the tune's command codes on the tranny. Conversely, the way it is working now certainly isn't right.

Thoughts?

Smooth_J

What else is done to the car besides a tune and tires?

28HopUp

C&L intake
Evolution Performance tunes
GT take-off muffler
stock 7.5" rear w/ 3.31:1 open gears
PH Aluminum DS
LCA's
Adj panhard bar
PP rear sway bar

I have a call into Evolution. The tunes were originally written for 4.10's, but I went back to stock gearing after blowing up the 7.5" LSD. Even though I used the handheld to change the optional parameters to "stock gearing", the tune may need to be adjusted.

siggy14

best way to tell if your speedo is off is by useing a GPS device, dont trust those police signes so much.

ink003

i would doubt the police sign before i'd assume a 20% miscalibration in my speedometer...

comando2929

When I get my new phone I was messing around with the GPS while driving around and it said my speedo was off by about 10%, said 55 when I was doing 50. Sure enough, the tune that came with my intake had set the gear ratio to 3.31, yet my car came stock with 3.55 gears and a LSD. After a quick change with the tuner the speedo was much better. Of course now its off again because of my new tires but that's a separate matter.

157dB

The new laser speed signs are dead on nowdays...

Red Beast

have you done any mods recentley? when i got my hood installed it threw off my spedometer for about 2 days. my exhaust threw off my spedometer for about a week, but after that the computer adjusted and got alot better. o maybe you just need ur computer to adjust

28HopUp

I ran up to Evolution Performance, and Fred reset my tunes at no charge to the 3.31:1 gearing. Reloaded my 93 tune. Speedo feels much better, and my highway mileage with the top down was over 27mpg.

Smooth_J

Nice

Its crazy how computers are everywhere these days, and I do IT for a living. Sometimes I wish I could go back to the old style type of cars without computers and without a bunch of other things...