Odometer correction

[tmcalavy]

Okay, my 93 Ranger XLT came stock with 3.0L V-6, stretch cab, auto tranny and 7.5" 3.45 gear ratio rear end. I had to drop in a 3.73 rear end and I know the speedo typically reads 3 to 5 mph fast so I correct for that easily. Question is...is my trip odometer showing more or fewer miles than reality? There's a .28 difference between 3.73 and 3.45. How do I work that into a percentage miles over/under so I can correct the miles driven and figure actual mpg when I'm at the gas pump. It's one of those math days when nothing naturally computes in my gray matter...any input is welcome.

 

[Full Throttle]

I speedo is off cause i put 31s on my 92 by 4 mph by gps on phone and 5 mph by my wife. So i add 5 miles to every 60 miles on trip meter. So i figure you mph hour off. But still dont now how accurte that is but it works for me. Least according to trip your probally getting better mpg! Lol

 

[Mark_88]

I put 15" rims on mine and the GPS consistently shows about 10 km/hr higher than my speedometer (don't like using speedo because it reminds me of a slinky bathing suit)...

Best way for me is to fill up, drive a specific number of kms and then fill up again...figure out how many kms and how many liters and bingo...usually wrong every time!

 

[ab_slack]

Bigger tires will make odo read lower miles for a given distance.

Changing ratio from 3.45 to 3.73, assuming odo reads off the trans, will make odo read higher miles for a given distance.

Big tires and diff change could cancel out depending on exactly what is changed.

I once had a free sent of tires, smaller diameter and a trip that I knew was almost exactly 600 miles from previous trips suddenly read 660 miles.

You can't judge odo accuracy based upon speedo accuracy. Requirements on speedo allow for zero tolerance for reading low (with stock tires and rear end) and a positive tolerance. In essence the speedo is required, if not accurate, to read higher than actual.

Since the odo works independently and is mechanically linked it has entirely different tolerance compared to speedo which is magnetically coupled to the rotating shaft (assuming not a electronically derived speedo/odo).

Generally speaking tho, if you see a percentage change in the speedo it should translate to odo. That would be a before/after comparison, but if you have no baseline info, you can't necessarily conclude odo accuracy is same percentage off as speedo.

 

[tmcalavy]

Not dragging tires, etc., mechanical or electrical speedo, into this at all. Tires are stock size...215 or 220X70RX14...always replace them stock size. But I gather from the input that my odometer reads more miles driven with the 3.73 gear ratio than with the old 3.45 ratio. There is a difference factor of .28, 3.73 minus 3.45 = .28, so I need to reduce the mileage on the odometer by multiplying by .28 and subtracting that from the total to get actual miles driven, then divide by gallons used. Or is my brain still math-impaired?

 

[ab_slack]

The correction factor is 3.45/3.73 = 0.925. So multiply odo miles by 0.925 to get the corrected miles. Thus 100 miles on odo = 92.5 miles. From there divide by gallons used so if you used 5 gallons in that 100 indicated miles, you would then divide the corrected miles (92.5) by 5 to get miles per gallon which would = 18.5 mpg

 

[tmcalavy]

Thanks. That's what I needed...what a brain fart day, thick headed and sick with a cold. Anyway, the Ranger's made 281 miles on three quarters of a tank so that puts it at 17-18 mpg in mixed driving...mostly town driving here lately. It's probably good for 22 mpg at speed on the interstate.

 

[ab_slack]

I never rely on the fuel gauge, but if you started with a full tank, fill her up and then you know exactly what the gallons are.

22 is what I got on my other B2s. My Current B2 has done as much as 21.5, but has slipped down to 16. Mostly because I been doing way too much idling (cleaning snow) and then in 4WD in the snow.

 

[pjtoledo]

Thanks. That's what I needed...what a brain fart day, thick headed and sick with a cold. Anyway, the Ranger's made 281 miles on three quarters of a tank so that puts it at 17-18 mpg in mixed driving...mostly town driving here lately. It's probably good for 22 mpg at speed on the interstate.

any mile marker posts on that interstate?
then drive with your speedo reading 60 for exactly 1 mile, each second you're off is approx 1 MPH off

dosen't every cell or whatever have a navigation APP? they are extremely accurate.