Inaccurate mileage do to wrong gear ratio?

[MasonWS6]

So when I bought my car it seemed to fast for what the speedo was reading. Come to find out it was off by 15 mph. Someone for some reason swapped 2:73s in at some point. I swapped the 3:42s back in and followed my dad in his truck to check speeds and its reading right again. My question is does it effect the mileage of the car. If so does anyone have any general idea by how much. Car has 91k on it now

[Firebirdjones]

I'm not sure if you're asking if it affects the mileage mathmatically or if the gear change (cruise rpm change) affects the mileage.

Either way, yes to both of those. With the speedo off there is no way to accurately calculate mileage with the odometer. I prefer to just use the GPS, much more accurate.

As far as just MPG is concerned, yes, jumping from 2.73's to 3.42's will affect the overall mileage. How much is hard to say as it varies with everyones driving style and cruise speeds. With higher rpms to cruise at the same speeds, generally they sip a little more fuel.

[pajeff02]

Sounds like someone blew up the stock rear and just threw whatever they could find out back to get the car back on the road. From a ratio perspective, the 3.42's require just over 20% more rotation at the driveshaft than the 2.73's to make the rear wheels go around one complete revolution. This is a significant difference, however, the gain in performance would outweigh any mileage concerns in my book.

[Too Fast]

Maybe OP means, does the car really have 91k miles on it? Short answer is no, it has more. If it was driven 20k miles with the 2.73, and it's about 20% off on the odometer calculation, then it really has about 95k miles. About 2k miles more for every 10k driven. Roughly.

[oldmanZ28]

true odometer miles can only be calculated if it's known at what odometer miles existed when the 2.73s' were installed and then up to when the 2.73s' were removed.
As an examples:
if using 2.73s',the odometer doesn't show enough of true miles,
2.73s' were put in at 60,000,2.73s' were removed at 90,000. 30,000 with 2.73s'. 30,000 x 25% = 7500.
91,000 + 7500 = 98,500
if using 2.73s',the odometer shows too many true miles
30,000 x .8 = 24,000 6,000 less 91,000 - 6000 = 85,000

[Whamhammer]

Wow, could you imagine the engine speed at top gear with the 2.73's? For a T56, thats 1.365:1 final drive vs 1.71:1 with 3.42's in it! Wouldnt pull very hard in 6th, but stupid-crazy low rpms at cruise.

[oldmanZ28]

1100rpm @ 62mph in 6th. Some cars with a cam can't even idle that low.