martinwalsh Posted March 1, 2011 Share Posted March 1, 2011 (edited) Hello, I just got back from a trip to Salt Lake City from Portland, OR in my '78 Chinook. My gas mileage was 10-11 mpg each way. I believe I should have gotten much better mileage, as there wasn't much weight in it and I did no exceed 55 mph all that much. This Chinook has huge tires on the back (275/60/R15). I want to ditch these and get 185R14 D rated tires on 14" rims in the rear. I know this will change my final drive gearing, but will it change my gas mileage? If so will the improved gas mileage be significant enough to warrant the effort? Thanks! Marty Edited March 1, 2011 by martinwalsh Quote Link to comment Share on other sites More sharing options...
Derek up North Posted March 1, 2011 Share Posted March 1, 2011 It'll certainly reduce your rolling resistance.It'll change your gearing by about 8%-9%. It seems from what I've read, you should be getting 15-20mpg in a Chinook under those conditions. Quote Link to comment Share on other sites More sharing options...
Lee & Joan Posted March 2, 2011 Share Posted March 2, 2011 Wouldn't changeing to a smaller diameter tire increase the engine RPM at any given speed? This would be something like turning off the overdrive, lessening the lugging on the engine, but increaseing revs which should decrease MPG? Quote Link to comment Share on other sites More sharing options...
Derek up North Posted March 2, 2011 Share Posted March 2, 2011 Assuming we're talking about an automatic, it's usually advised not to use overdrive because of the 'long' gearing. Having those big tires on the rear just makes it that much longer and less usable. Quote Link to comment Share on other sites More sharing options...
flatspin Posted March 2, 2011 Share Posted March 2, 2011 wondering what sizes you're running for tires all around, and what kinds. chunky off-roaders are going to slow you down, and pushing that camper up further off the ground has to put a cramp on mileage too. a tuneup and running regular gas are good ideas too. i get 13+ in a 90 v6 sunrader that is 21 feet. i'm thinking 10mpg is low for you! -andrew Hello, I just got back from a trip to Salt Lake City from Portland, OR in my '78 Chinook. My gas mileage was 10-11 mpg each way. I believe I should have gotten much better mileage, as there wasn't much weight in it and I did no exceed 55 mph all that much. This Chinook has huge tires on the back (275/60/R15). I want to ditch these and get 185R14 D rated tires on 14" rims in the rear. I know this will change my final drive gearing, but will it change my gas mileage? If so will the improved gas mileage be significant enough to warrant the effort? Thanks! Marty Quote Link to comment Share on other sites More sharing options...
86rader Posted March 4, 2011 Share Posted March 4, 2011 running higher rpm may or may not decrease mileage. if the engine is lugging, it is not running efficiently and will get crappy mileage. and crappy is what i would call 10-11 mpg for a chinook. those little guys ought to get close to 20 driven conservatively. sounds to me like the chinook is not running very well and the big mudder tires are also taking a toll. Quote Link to comment Share on other sites More sharing options...
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