I was wondering if anybody has a math formula to figure out gas mileage accurately after installing larger tires.

My trucks a 99 QC swb 4x4 5spd. It had 265's on it when I got it, now I am running 285/75r16's,, the door sticker says it came with 245's so I'm assuming that's the size tire that the odometer/speedometer are calibrated to.

Also, how do I find out what my rear axle gear ratio is? There's a tag in the glove box but it just lists a code for rear axle, I forget what it is at the moment but it was just one or two letters
Thanks

Matt

 

http://www.dakota-truck.net/TIRECALC/tirecalc.html

Pretty easy to use, just put the tire sizes in and click the right button. And just because the sticker on the door lists 245s doesn't mean your truck came with them. My sticker is exactly the same, but I bought the truck new and it definitely had 265s on it. How you find out for certain, I'm not sure, but I've heard you can have a dealer print a list of equipment based on your truck's VIN, so that might tell you what size tires it came with.

 

Use that calculator to tell you the outer circumference and the revolutions per mile of the tires you have, then find a Dodge dealer in your area that will take 5 minutes to hook up their computer to your truck and change the setting. Be sure to give them those numbers and make it known that they won't have to do any calculations. Some places might try to charge you--don't agree to it. You can find a place that'll do it for free if you shop around and don't get taken advantage of.

 

do I need to do this too? I have a 95. what will it change?

 

Having the computer changed to reflect the proper tire size will affect the speedometer, overhead computer (if you have one), and the odometer (!). Very important, if you ask me.

 

All you should have to do on the 95's to adjust the speedo is change the gear out in the tailshaft of the tranny. Someone with that year truck will have to help ya on that but I know that's how ya do it on the older mustangs.

 

It should be about 8% off