GMC 2500 Diesel rpm's at cruising speed

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josh

GMC 2500 Diesel rpm's at cruising speed

Unread post by josh » Thu Apr 22, 2010 8:38 am

I have a '94 GMC 2500, 6.5l turbo diesel. With stock equipment, when traveling at 70 mph, it was turning 2100 rpm's. The average fuel economy was 15 mpg. I installed a k&n filter and 3" custom straight pipe exhaust. Now, when traveling at 70 mph, it is turning 2500 rpm's. What would cause this increase? If anything, I would have expected lower rpm's at cruising speeds. Also, the fuel economy has not changed, or possibly decreased slightly. Of everyone I have spoken to, nobody has any idea as to what is happening. Thank you for any help you may offer.

willwildy
Posts: 20
Joined: Fri Aug 13, 2010 10:59 pm

Re: rpm's at cruising speed

Unread post by willwildy » Sat Aug 14, 2010 12:20 am

The answer is it isn't, unless you have either a bad clutch on a stick shift or an auto transmission that isn't functioning properly.

On most cars your vehicle speed is read at the output from your transmission. the transmission ties the engine to that output in a more or less fixed relationship. Hence so many revolutions of the engine in a certain gear will always result in so many revolutions of the output shaft of the transmission and so many revolutions of the tires. What you did to the vehicle should not have affected this relationship nor should it be causing your tachometer to read incorrectly. However being a diesel it might. It all depends on how motor revolutions are sensed. The other option is that the modifications are causing your automatic trans (if you have an automatic) to shift later or to not engage the lockup torque converter function. This would also explain the fact that your mileage didn't increase or even went down. To exactly diagnose your problem I will need more info, such as type of transmission, how many forward gears it has etcetera. I can tell you however that you should feel one more "shift" in an automatic than is indicated on the gear selector. so for a 4 speed auto you should feel all four gears and then a fifth when the torque converter lock up engages. My opinion is that your not getting that and it is being caused by your modifications. Should be an easy fix however. In closing let me say my opinion is based on very limited information.
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