Gear ratio/odometer question

[tmcalavy]

I'm suffering from old fart tired brain syndrome today and need help from younger sharper minds. I replaced the 3.45 differential in my 93 XLT 2WD super cab with a 3.73 rear end a few months ago cause the old diff was eat up. I'm running stock wheels/tires, 215X70X14 (I think), and the speedo calculator says I'm running 60 mph when my speedo shows 65 mph at cruising speed (above 55 mph). I've adjusted to that, but how do I adjust my odometer calculation so I can figure mpg by dividing miles traveled by actual gallons consumed when I'm pumping gas? Am I driving more or less miles with the 3.73 rear end, and what percentage more or less? Thanks in advance.

 

[kunar]

you're actually driving about 92.5% of whatever your speedometer/odometer says.

 

[tmcalavy]

That's what I was thinkin...but wanted to be sure. Otherwise my mpg doesn't make sense.

 

[Surrey]

should be a pretty easy calculation, just divide one gear ratio by the other to get how much change you get... so 3.45/3.73= 0.9249. multiply that by 100% and you get 92.5%, just as Kunar stated.

 

[tmcalavy]

Thanks guys...after a night's sleep and a coupla cups of Joe it makes perfect sense. My mpg around town is down slightly, but not enough to care about.

 

[GentleRF]

So, a question relating to this topic would be: if I'm currently running 26-inch tires and a 3.43 gear ratio, what ratio would I need in order to run 31-inchers to maintain engine speed at the same road speed?

Rick in East Bremerton

 

[tmcalavy]

Approximately 2.88 according to this calculator in the tech forum:
http://www.therangerstation.com/tech_library/Gear_Tire_RatioChange.html

 

[kunar]

Approximately 2.88 according to this calculator in the tech forum:
http://www.therangerstation.com/tech_library/Gear_Tire_RatioChange.html

you did your math backwards... taller tires need deeper gears

 

[GentleRF]

Thanks to that handy dandy calculator, I need 4.10/4.11 gears. Now what I need to do is find some for a 7.5 rear and go from there.

Rick in East Bremerton