Just thought I would ask if anyone else has experienced this after a pumpkin change out - 04 sedan, so changed the rear diff to 3.7 from 3.3, now cruising rpm is up about 12% but mpg increased by almost 10% highway and 15% city ???????? Definately not as hard on the throttle off a stop but unexpected.

 

I would imagine your speedometer/odometer is not accurate now. Did you check their accuracy with a GPS.

 

I know with my other cars changing the fluid is very beneficial. Noone thinks to change out the fluid from the factory which is usually the cheapest fluid they can get. Going aftermarktet the fluids were developed to coat better and not break down as quick. Therefore resulting in better gas milage.

 

Yes. It's common, however if you did not correct your speedometer after the gear change, then your numbers are skewed.

I changed my mustangs final drive from 2.7 to 3.7 and gained fuel economy as a result. The theory is that the mechanical advantage you gain from the gear swap places less load on the engine. So if you are easy on the gas, should result in better economy.

However, highway rpmsmdo climb and that can kill highway economy. I do notice I get 2 mpg better at 60mph than I do at 80 mph.

But your values seem high for such a small gearing bump. Did you correct the speedo?

 

I opted instead to run 245/55/17's (rears) at one point, which gives me about 2-3 mpg gain on the highway. My speedometer is off by 2.5-3.5mph from stock, but there's a no-brainer solution for this. While I have less power on take-off than most any other G (my 0-60 is slightly slower), most likely the 1/2 mile and beyond will be faster, and my cruising speed will be 25,000-3,000rpms slower at 70mph.

I get around 27-29 highway and 21-24 city, or 450-500 miles per tank with this set-up (plus some other things). It also looks good since there is less wheel gap, the tires are fairly wide, and I don't have to worry about the huge potholes/rocks/debris tearing up my undercarriage or bumpers b/c I'm not lowered.

Just my .02

 

Wait a minute, we have to reprogram our ecu's to correct for gear ratios after a gear swap? Why doesn't the ecu just compare the transmission output speed sensor and the rear abs sensor to figure out the gear ratio on its own? My 2000 dodge dakota could did that from 1998+. I didn't have to correct a thing when I swapped from 3.92 to 4.56 ratio in it.
These newer, smarter, more technologically advanced cars don't do that? what the hell

 

No, you do not need to reprogram the ECU to compensate for a gear change on these cars. Speed is calculated from the wheel sensors.

I have the 3.54 gear in my 03 sedan and I saw about a 1.5mpg drop in pure highway driving. Around town driving is basically unchanged. Higher cruise rpms obviously will increase full consumption. At 70mph, my 03 is spinning about 250rpms higher. I wouldn't change it though. I love having the extra torque multiplication and extra response. The VLSD doesn't hurt either.

I'm a bit perplexed as to why a car would gain mpg with a deeper gear unless your driving style has changed dramatically.

 

why doesnt someone try and drop the gearing way down and have the engine turn 2000rpms at 90 or so. my old TA would do that and get over 30mpg at that speed, im sure these cars could do better with smaller engines

 

^ I'm pretty sure the VQ doesn't make enough torque at low rpms to make that possible. Maybe if it had a LS3 swap.

 

Op was talking about his differential and gears not tires lol.

 

miltoda - good point, didnt think of it that way.

 

It was an option being introduced, no more no less-- something not everyone considers when looking at mpg, which was one of the pieces of info in the re-gearing of the differential according to the OP--- did you miss that logic in his post?

Quote:

mpg increased by almost 10% highway and 15% city

Even the title of the thread addesses MPG.

Your self aggrandizing critique of my well intentioned and aligned post, however, adds no value and detracts from the discussion. So say something meaningful or GTFO!

 

you know you always use these big words to describe the simplest things....just pointing out that op was talking about fluid and you redirecting back at me counteracts your point because now you took it back off subject jeez

 

I thought the OP was just talking about how he changed the gear 'ratio' and not the fluid?

 

Op said nothing about fluid. He changed his final drive ratio