I'm sure this has been discussed before but I can't find it. I was on the highway in my Ford Taurus station wagon and noticed at 60 mph it was turning 2000 rpm exactly. So I hopped in my Volvo 940 Turbo wagon and hit 60 mph and saw it was turning 2500 rpm(in over drive). It's obvious why the Ford get 28 mpg and the Volvo gets 22 mpg. Although one's a 6 cylinder and the others a 4 the hp is almost the same - about 160 hp. So, I'm sure people have tried a lot of things to increase their mileage but to me the obvious thing is lower the rpm. And there's 2 ways of doing it - 1. Change the gearing in the rear end or 2. go to a taller tire. I prefer the easy way and here's where my questions start. How tall can you go before the engine starts to bog down? And I know it will throw the speedo off but a few mph I wouldn't mind. Can the speedo or sensor be recalculated to match the actual speed?

 

I wonder why no one ever thought of that before?
How "tall" can you go? May be another inch or two before the tires scrape the fenders and the handling characteristics are affected to the point where you don't have an acceptably street usable car. It takes a lot of considerations to arrive at a combination of decisions which in the end produce a car that does this and that, and yes, they are all compromises!

 

Any idea how much a 1 inch increase would equate to in rpm drop? Also, what I'd like to know is if anyone's tried this and how did it affect their car's drive ability?
And looking under my car there seems to be a lot of room, as long as I don't lower it.

 

don't forget to allow for the full suspension travel range when guessing what tire size fits.


the standard tires on my 1992 740T wagon have a OD of 24.2", or a radius of 12.1" ... if you add one inch to the radius, 13.2", then your difference in RPMs' will be exactly proportional to 12.1/13.1, so 2500 rpm * 12.1/13.1 == 2300 rpm, and your speedometer would read 55 MPH (60 MPH * 12.1/13.1) when you're actually going 60, or if your speedo is reading 60, you'll really be going 60*13.1/12.1 = 65 MPH.

 

Here's a novel idea. Your numbers show an 8% reduction in rpms by going 2" taller. I don't know if that would equate to an 8% increase in mpg but what about this: 1993 960's had a 3.31 rear end ratio compared to my car's 3.73 and that would equate to an 11% drop in rpms. Is that a possible swap - 960 rear end gears in an 940? And just to consider the absurd - add that to 2" taller tires. The question is would the engine have enough torque to move the car out of it's own way?

 

the 960's are IRS except the 92-93 (or was it 94?) wagons were live axle. they also had a different transmission, I don't know what the ratios were.

ok... 1993 940 owners manual says...
and 960 says...

 

Yeah I saw the numbers earlier and the only real difference is first gear. I guess it's lower with the 960's to compensate for the higher rear axle ratio. But once the car's rolling 2nd, 3rd and 4th are pretty close. So in theory it would work, but you probably couldn't burn rubber in first. I read a post on Turbobricks and they say the easiest way is just swap the whole rear axle and since 960 wagons are a solid axle I don't see why it wouldn't work?

 

note that gear ratios are inverse, as they are reduction ratios. so a smaller number is a higher gear. the final drive is a multiplier, so the 960 above, 1st gear is 2.8*3.31 = 9.628, eg, for every 9.628 engine turns, the rear wheels turn once. the 940T, that was 1:9.14

anyways, lowering RPM at a given speed doesn't automatically increase gas mileage, there's another factor, as it will take more throttle at that lower RPM to maintain the same HP output required to maintain the same constant speed, and the same HP at lower RPM requires more torque. so what really counts is the engine efficiency at the actual RPM.

 

You know what seems strange, the non-turbo Gl which only puts out 110 hp has a lower ratio(higher number) rear end yet it gets better mileage than the Turbo? I can seen that they would have to lower the gearing with a weaker engine but, like you said, I guess the power band and when it kicks in determines mpg. Also, I read that the UK version of the Turbo gets 3 to 4 mpg better than the US version. Weaker smog standards?


A RELATED MATTER: My new car runs and drives great but has a sooty tailpipe(too rich?) which I'm sure is affecting mpg. Off the top of your head, o2 sensor?

 

UK uses imperial gallons.

 

Oh, that explains that.