Gear Ratio

[doby48]

I have a 1994 Chevy C1500. The gear ratio is unknown although I was told that the previous owner had changed it out for towing. I had the engine replace two years ago with a new crate motor and a new transmission also installed, the mileage is horrible and the towing power is horrible. According to varios charts they claim that the truch should run at approx 2800 RPM at 65Mph but current at that speed it is running at 2100 RPM, since I would expect the RPMs to be higher, not lower with the gear ratio installed by the previous owner, I'm really confused.

How can I find out what gear ratio I have in my truck? I had POSI installed when the new engine was installed as well so I am hoping that I don't need to replace the gears and am able to make the necessary adjustments with tire size, but this 9mpg is horrible expecially with a new engine. Spark plugs and wires are good and regularily changed. Airfilter as well. Fuel filter was replaced when the new engine was installed, oil and filter is changed regularily also. AC doesn't even work so decreased mileage is not due to that. I would expect the mileage to be good or the power to be good and the fact that both are horrible (and were before the engine was replaced) has be stumped.

Any thoughts on this? Is there something else that I'm not thinking of or am I on the right track with the gear ratio, wheel size and RPMs?

[TaNK_Em]

For your application he probably had 3:73s put in. there would be no point of putting in 4:10s.

- Kevin

[2000CAYukon]

A crude way is to count the number of revolutions the driveshaft makes for each revolution the rear tire makes. Mark the drive shaft and mark the tire (chalk works). You should be able to tell if you have 3.08, 3.42, 3.73 or 4.11s

//2000CAYukon

[JaVeRo]

The most certain way of checking the gear ratio is to take the cover off and count the teeth.
41 on the ring gear and 10 on the pinion would be 4.10 (41/10)
41 on the ring gear and 11 on the pinion would be 3.73 (41/11=3.72727...)
41 on the ring gear and 12 on the pinion would be 3.42 (41/12=3.41666...)

An easier way of checking it but more prone to mistake would be to jack up one wheel and put the truck in neutral. Since the other tire is still on the ground your ratio is doubled due to the spider gears turning. put a mark on the driveshaft and have someone turn the wheel while you count the number of rotations of the driveshaft. If 20 turns of the wheel give you 41 turns of the driveshaft then you have a 4.10; 22 turns to get 41 on the driveshaft would be 3.73; and 24 turns to get 41 on the driveshaft would be 3.42.

If it is a G80 locker you will need to turn it slow to keep the locker from kicking in. I'm not sure how it will react with a posi, I guess it depend on what kind.

Hope this helps,
James

edit: I'm a slow typer, Mr. Yukon's way sounds easier. Slack in the rearend may throw it off a little.

[mcbowers]

If the 2100 rpm @ 65 mph is correct you have a very high rear ratio[maybe 3.08] & that is not causing your mpg problem!

[muff34]

Maybe the Cat is plugged. what engine 350 in it or 305. 2100 Rpm At 65 Mph sounds good to me