Can the speedometer/odometer be adjusted based on tire size? - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 12-31-2013
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Can the speedometer/odometer be adjusted based on tire size?

My Ranger is a 2002 XL Regular Cab 4x2 with a 2.3L DOHC and 5-spd Manual tranny.

Everything is stock but the tire size. Stock tire size was 225/70R15 (~27.5" OD). New tire size I'm running is 235/75R15 (~29.0" OD).

Odometer reads behind an honest tenth of a mile for every mile I drive. So when my odometer says I've gone 10 miles, I've actually traveled a hair over 11 miles. This is based on mile markers and my Dads 2 year old Honda Civic.

According to my GPS the speedometer is about 1 MPH off at 25 MPH and 3 MPH off at 55 MPH, and progressively is further off the faster I go. (Speedometer says I'm going faster than I actually am.)

The Ford Dealership claims they can plug my truck into some diagnostic-scanner-thingy and correct it to some extent. BUT, they (Service Manager) also claim that the difference in tire size probably isn't enough to where they can simply change the "pinion factor" to correct things any closer than they are unless they just get lucky.

This leaves me with some questions that they didn't have answers to. I'm hoping the I can find those answers here. Is it possible the diagnostic adjusting doesn't factor in tire size with OBDII? Does OBDII have nothing to do with the operation of the speedometer, leaving a simple sensor in the rear differential to control speedometer reading?

Is it true that the "pinion factor" is the only parameter the diagnostic equipment can take into account to adjust the speedometer by?

I really want my speedometer to be reasonably perfectly accurate. Is that not necessarily possible when changing tire sizes? Am I the only one that thinks an odometer being off a hair over 11 miles for every 100 miles traveled is a lot? That can really add up.

Any detailed knowledge of the subject would be greatly appreciated!
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Old 01-01-2014
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On an 02 is should be on the tranny. When i swapped from auto to manual I adjusted mine with my tuner it takes some tweaking but you can get it spot on. In actuality the dealer sets them to read a few MPH faster than your going that way when you get a monster ticket you can't sue them for faulty calibration. And on that year its entirely done by the PCM theres not much mechanical left in these trucks. The sensor sends electric pulse to PCM PCM converts it to something the cluster can read i.e. a specific voltage the higher the voltage the higher the needle goes. No more gears and mechanical speedos the sensor is nothing more than a fancy magnet. Sorry thats kinda rambly but thats the basics of what you need to know. So to directly answer your question it may be more trouble/money to fix that little bit than its worth unless it just really bugs you because most new cars can be off by as much as 5% which is a bit more than losing a tenth of a mile.
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Old 01-01-2014
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Thanks for the insight. Maybe I'll just leave it alone for now and go by what my GPS shows for speed. I still don't like my GPS for miles driven because that's not spot on either. So for miles driven, just for my own worthless peace of mind, I'll just manually convert what my odometer says by the factor I know it to be off.

You had mentioned; "...theres not much mechanical left in these trucks." That said, it's even more surprising to me in this day of computer age that these sorts of electronics aren't only easily infinitely adjustable, but also easily done by the vehicle owner. I know, I know, it's about money. What would the dealerships do if they couldn't charge customers $100.00 just to hook any given vehicle up to they're coveted electronic diagnostic equipment. Such is life. Maybe soon there will be an app for that (not a free one of course).

If anyone else has anything to add, please do so. Thanks, and Happy New Year Everyone!!!
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Old 01-01-2014
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You can get this. Superchips FlashPaq Tuners - 670+ Superchips FlashPaq Reviews & Best Prices on Jeep, Dodge & Ford Gas Tuners

Personally I don't think it's worth the money but one the bright side, you can control many different things with it.
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Old 01-01-2014
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I think the super chips is worth it. Can change all kinds of settings, plus or can read obdII codes
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Old 01-01-2014
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Thanks for pointing out the 670+ Superchips Flashpaq Tuner. Looks like a good investment to me too. I don't have the $350.00 to spend on it right now, but in a couple months I may just go that route.

Anyone know what all the parameters are that the Superchips tuner allows speedometer calibration adjustment for? I guess I'm really asking, besides gear ratio, will it also allow adjustment by tire size? I just tried calling them, but of course no one is manning the phones on New Years Day.

TYVM guys! I may just go the route of that tuner in a couple months. Any other comments and suggestions are welcome.

Is Superchips pretty much the way to go with tuners? Are there more economical options out there?
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Old 01-01-2014
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no, there's kinda others. No More BAMA Tunes - Ford Explorer and Ranger Forums "Serious Explorations"

Basically superchips does their tuner, which is kinda a "good for all". But other people have tunes that are specific to ranger mileage or performance, etc.

Sorry, I don't know about any other economical options.
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Old 01-01-2014
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Good to know. Thanks for the guidance.
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Old 01-01-2014
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I have a super chips and SCT the SCT is hands down better but they don't have stock ranger tunes Fred at rogue wrote my tune and it allows way to many adjustments to list. if you want basic performance improvements with the ability to adjust for tire size and gear ratios go super chips if you want infinite adjustments then go SCT. Its all up to what you plan to get out of it and what future mods you plan.
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Old 01-02-2014
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Thanks for the heads-up. Will SCT cost like twice the money over the $350 Superchips offering?
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Old 01-02-2014
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No, the lowest model is same as superchips. Higher model costs 5something And it's better. Here's his website. Rogue Performance Custom Tuning. You can probably search the forum for more information
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Old 01-02-2014
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Thanks a lot guys, you've all been a huge help!
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