PROMETHEUS PRIME

Im on your same boat

cerino2000

Funny you bring this up. I just bought my 06 a few days ago and when doing 80 MPH it just doesn't seem like it is actually doing 80. I timed it using the mile marker posts on the hwy and 80 MPH is actually 78 MPH. 70 MPH reading is actually 67 MPH. Wonder if they do that on purpose.I have a GPS and it's not uncommong for that to be off just a little.

I am going to get someone to pace me some time in something totally different and see what they show.

SlinkyBMX

would this change you odometer reading?

Stewart

I know BMW's do this stock. The difference is exaggerated as you go faster meaning there shouldn't be a difference more than +1 or 2 mph before 50 but after you get to 60, 65 it wil start to exaggerate more. I tested it with the in car GPS going around 90 according to the gps and the speedo read 96. I wouldn't worry about it unless there's a diference at low speeds.

howarmat

Mile markers are not that accurate...I wouldnt use them for a precise measurement.

wmtheflash

Yes, it does change your oddometer reading. I drove the vehicle around a little tonight to add to my data this what I've found so far. I'm using the trip meter on my stock 2005 Focus, assuming that's correct for comparison.

1) Distance from point A to point B

Focus trip meter = 12.5 miles (checked it twice, it's definitely 12.5)
Mustang with 3.73 and STOCK revs per mile = 13.1 miles
Mustang with 3.73 and 770 revs per mile = 12.7 miles
Mustang with 3.73 and 805 revs per mile = 12.1 miles

1) Distance from point B to C

Focus trip meter = 10.3 miles
Mustang with 3.73 and 780 revs per mile = 10.3 miles

GPS was reading low about 2 mph on the highway with 780 revs per mile. (BTW. Tire rack says my tires are 768 revs per mile.) At slower speeds, though, it seemed to track very well, strange.

STOCK above is defined as whatever SCT defines the Stock revs per mile to be...from what I can tell the STOCK value isn't right. I wonder if the tune was screwing up my speedometer before my gears? I guess that doesn't really matter now. :-)

I guess my best bet is to take my vehicle on the 12.5 mile route and see if it comes out the same at 780 revs per mile. There are a few possible conclusions:

1) The trip meter on my Focus is wrong and I should set the revs per mile to 770.

2) The trip meter on my Focus is correct. I should set the revs per mile to 780. Maybe the tuner just isn't adjusting the revs per mile correctly?

3) The tuner is adjusting the car properly. Perhaps new Mustangs generally read 2% or so high from the factory and the SCT tuner is simply restoring my speedometer to the original factory error?

Why would a car purposely read 2% higher from the factory? To make me think I'm getting 0.5 mpg more? Maybe they think I'll speed? Maybe they can make more money penalizing buyers with leases? Is there a penalty for driving it over a certain miles per year?

It's funny, but the local news did a story on this sort of thing tonight. The compared a Ford, Toyota, Chevy and a Nissan (Altima, I think). The Ford, Toyota, and Chevy were perfect the Nissan read about 2 or 3% high from the factory. The lawyer they interviewed for the story said he was about to file a class action lawsuit against Nissan, because the vehicles were gaining miles faster than they should.

Here's an article:

http://www.wzzm13.com/news/news_arti...?storyid=71002

Honda and Nissan have the same problem.

"The Society of Automotive Engineers' voluntary standard is plus or minus 4%, or no more than 4 miles high or low in every 100 miles. The National Highway Traffic Safety Administration says it doesn't regulate odometer accuracy."

Orion_240

Maybe you need to put more air in the tires???
I've noticed that whenever I come across one of the machines that read your speed alongside the road that show you how fast you're going that I seem to be clocked at about 2 mph slower than my speedo shows.

Ford_Dude

ya
I'd go make sure of exactly correct rear tire pressure
and compare 4mile @ low speed to 4mile at higher speed
also the focus is fwd right so it measures from the point of contact of the front tires and the stand does from the rear
on the focus ensure exactly correct front tire pressure
its possible that at higher speeds the tire is not exactly round (get someone with a video camera and a steady hand to record your tires at 80mph)
also if you can get on a dyno to free spin and get it to 80 without moving then compare how fast the dyno says to the speedo
and any tire deformation(its not really bad just how some tires may act when spinning at high speed (remember their rubber and rubber flexes)
also if you do the dyno idea compare 20 to 80 to 147

Jurnymn

Rather be going a little slower than a little too fast when I meet a cop.

wmtheflash

Tire pressure is factory (32 PSI) for both vehicles. However, we are talking two different tires and two different vehicles.

I assume that tire pressure recommendations from the factory are set more for vehicle handling/safety and tire life than speedometer accuracy. I probably can't assume that my Focus is 100% accurate, either.

I'm going to load the manufacturer's revs per mile for my tires at 770 and leave it at that. If I come accross one of those highway signs that list your speed, then I'll take a glance and see how well the two corrolate. The truth is it won't make much of a difference over the life of the car.

The speedometer reading on my GPS also doesn't appear to be exactly accurate. If I get ambitious I may try a different one.

The only questions left unanswered are:

What in the heck is the "STOCK" revs per mile found in my SCT tuner?

Why is the "STOCK" number so far of from 770 revs per mile?

Does anyone have another source that lists the revs per mile of the stock tires? I couldn't find it on the manufacture's site.