Since the car came with the tuner (Hypertech power programmer 3). I decided to retune the car or to tune the car depending on what the previous owner had.

Well now my speed is off about 6-8mph

Tires are 275-40-17=rear 245 in the front. I "dont" think the gears were swapped out.

So with the tuner i know i need to adjust or give it some info to apply.

My question is where do i start, will the tires be the issue or maybe it does have gears installed. Can you guys judge by how far the MPH is off. It takes about 30mins to flash and i dont wanna keep flash guessing.

Thanks a lot

 

My speedometer on stock gears shows about 5mph over what my GPS says at highway speed. Some say that the manufacturers make the speedo say a faster than actual speed to keep us from seeing blue lights all the time.

I don't think your tires would cause that big of a fluctuation in mph as the stock tire size is 245/45/17 or 245/40/17. Trucks tend to have the huge mph variations when they run massively oversize tires.

I'm sure more people will weigh in with ideas.

 

before you retuned it, what rpm did it turn at 60 mph? that would tell you if it's had gears installed. On the tires, 275 40 17 is the same height as 245 45 17 so unless the stock aspect ratio is 40 the tires aren't the cause.

 

ODB says im going 40mph and GPS will say im going 34mph. Never noticed it this bad before the tune, i wanna believe it was correct before....

RPMs are around 2100-2300 if i remember correctly ill edit once i head in to work and see.

Its not really a game changer for me i can do the math

Thanks

 

In what gear are you turning 2100 to 2300 rpm at 40/34 mph?

With stock rear-end gears and a 5-speed it would have to be 3rd gear, with a slushbox it doesn't work at all unless the converter was not locked.

While traveling in 4th gear with a 5-speed (or 3rd gear with an auto) record some solid mph readings from the speedometer and GPS, and the engine speed at those vehicle speeds, and figuring out what gears you have will be easy...

 

If your spinning at 2100 at 60mph ina 5 speed you have 373s no doubt....