math problem

see if any of you math geeks can figure this one out....

in my caprice... the odometer reads 38 miles when i get to my moms house. but i know for a fact that my mom lives 55 miles away from my house.

so when i need to get gas my odometer reads 260 miles to a tank (takes 17 gal) so that puts me at 15 mpg. that seems a bit thirsty even for a tbi car. given the discrepancy i cant help but wander what the real mpg is?

Hi Ian:

Assuming your odo is working but totally inaccurate (typical problem with TBI cars), 38 is 69.09 percent of 55. That means if your odo is reading 260 miles than that equates into 69.09 percent of the real mileage or 375 miles. Again assuming you put in the same 17 gallons your getting 22 MPG which is just about right.

If you haven't replaced your rear end gears, or replaced the tranny and forgot to do the speedo gears, I would suspect either the odo (probably) or the VSS buffer (unlikely). Have you compared the speedometer to a GPS? If not you may be reading 40% low. Not good for speeding tickets. Reading 60 and your really doing 84

Bill

Good point If you register 60mph , but seem to be passing everyone, it could be trouble with points on your license.

Another way to check is to find out if there is a "Measured Mile" in your area. Local police will use it for calibration as well. You can ask local Highway Dept's if they know of one nearby. When you line up on the mark, reset your trip odometer, drive the mile to the next mark and see what you have, It should be dead center on "1"

The gear sets required for alteration or ratio are available through GM parts counters, or local transmission shops could help. As pointed out, you'd want something that you give you a change of about 30%. But, you'll need to know which gear set is already in there. They are color coded. Each ratio is it's own unique color.

Dave

You can calculate the gear ratio you want for the VSS. If you changed rears, you need the original rear ratio. Having it, the FSM will have a chart with the original VSS gears. By knowing the drive gear, you can then determine which driven gear you need.

If the transmission has been changed, you may not be able to determine the drive gear size without removing the VSS. There are 2 different VSS units. Make sure you have the correct one for the driven gear you are using.

Your tire size will also figure into the formula.

A couple of the larger driven gears are no longer available, but they are not the likley candidates unless you have an extremely high numerical rear ratio.

Here is a site that you can get the gear ratios and gears from. http://www.transmissioncenter.net/. You have to go down the page to the speedometer section, and go to that site. They have charts and formulas to get the most correct gear ratios for your car. You can call thm to fine tune the ratios if you have any questions.

Once you have determined the drive gear size, it is a matter of using the formula to determine the driven gear size.

As a last option, you can get a pulse modifier to put inbetween the VSS and the VSS module under the dash. It can be adjusted to correct any error electronically. I do not have a source for one of these. It may be the easiest of the possible routes, because it is a once and done fix. You can fine tune it as well. If you are going to have someone else do the mechanical work, it may even be cost effective.