LuLu Posted August 5, 2008 Report Share Posted August 5, 2008 I spent the past 10 days on a road trip with friends, from Chicago to Charleston SC. 988 miles one-way door to door. We took two cars, both Panther frames and very similar in many regards. I was suprised to see such a difference in MPG. Car 1 was Gary's 2003 Mercury Marauder. 4.6L-4V with a mild professional tune, 4R70W tranny and 3:55 gears and TractionLoc. Tires are BFG-KDWS, front is 235/50-18 (32 PSI) and rear is 255/50-18 (30 PSI), exactly what the factory suggests. The Marauder has a front air dam similar to the SGT. 40K miles on the car. Car 2 was my 2008 Mercury Gran Marquis, all factory stock. 4.6L-2V, 4R70W tranny and 2:73 gears, open end. Tires are the same BFG-KDWS, 225/60-16 (35 PSI) on all four corners. No air dam. 20K miles on the car. Gary's not a "hammer lane" lead foot, but I am. Thus he took the point most of the drive. Our cruising speed averaged 80 MPH set by the cruise control, but I took the point on ocassion. Passing through the Smokey Mountains, an interesting development unfolded. On graded inclines and without adjusting his cruise control, Gary would pull away from me to the point I had to click off my OD and add some throttle to stay with him. On the graded declines that followed, I had to click off the cruise control and add brake, or, risk bumping into him. When I was on point, Gary had to brake on the inclines to avoid bumping me and add throttle on the declines to stay with me. There is not a significant weight difference between these cars, less than 200 pounds, not including what the luggage of "significant others" added to the mix. We kicked this around over dinner and concluded that Gary's added HP and TQ pulled his car quicker and easier up the inclines, while my car didn't have such power on tap. Likewise, on the declines, Gary's wider footprint and soft PSI added greater drag than my smaller foot print and higher PSI. CAR 1 MPG...20 MPG overall trip average, no peak available due to older EEC electronics. Car 2 MPG...27 overall trip average, with a peak of 28.2 This suggests that tire size and PSI plays a much more important role in MPG than many suspect. Nice to have wider tires on the pavement when traveling through the mountains, but they come with an added tax in MPG. Before we started this, I suggested to Gary that he bump his PSI a tad, but he preferred to follow factory guidelines. BTW, the Marauder was EPA rated at 17 city and 23 highway MPG. My Gran Marquis was rated likewise. Frankly, I wasn't prepared for such a remarkable difference in MPG, and in my favor as well. Your thoughts? Link to comment Share on other sites More sharing options...
ilmor Posted August 5, 2008 Report Share Posted August 5, 2008 This is why Prius' have such skinny (and hard, I imagine) tires - to maximize fuel economy by reducing rolling resistance. Also, I read last night that there is a new single rear motorhome tire that has been designed to replace the dualies - again, for the same reason. Auto and tire manufacturers are also working with tire compounds and tread designs, and higher pressures, to achieve the same goal (high mpg). The skinnier the better for mileage, with the tradeoffs behind poorer handling and braking. Link to comment Share on other sites More sharing options...
Shelby Dodge Guy Posted August 6, 2008 Report Share Posted August 6, 2008 gears....3:55 vs. 2:73. My SGT Stang vs my friends GT with 3:31 facto gears gets close to the same difference as you and your friends, its about a 5 MPG difference. Air pressure helps in keeping less drag and an inline track on running conditions (more balanced) and will help in fuel mileage, but not that much. Link to comment Share on other sites More sharing options...
CH53Driver Posted August 6, 2008 Report Share Posted August 6, 2008 ...with the tradeoffs behind poorer handling and braking. Great, just what we need considering how the average driver drives these days! Link to comment Share on other sites More sharing options...
68fastback Posted August 6, 2008 Report Share Posted August 6, 2008 Very interesting. Imo, it's mostly about the gears with diff type, tires and pressure to a lesser degree. Also, differences in the 'attack' programming on the speed-control (if different) might affect it too and his alternative tune likely provides better throttle response at the expense of sucking more fuel on each and every transition of the speed-control. Thanks for sharing the results! Link to comment Share on other sites More sharing options...
ilmor Posted August 6, 2008 Report Share Posted August 6, 2008 Great, just what we need considering how the average driver drives these days! Isn't that the truth!! Link to comment Share on other sites More sharing options...
Cobramach1 Posted August 6, 2008 Report Share Posted August 6, 2008 gears....3:55 vs. 2:73. My SGT Stang vs my friends GT with 3:31 facto gears gets close to the same difference as you and your friends, its about a 5 MPG difference. Air pressure helps in keeping less drag and an inline track on running conditions (more balanced) and will help in fuel mileage, but not that much. It's all about the gears in this case. A good example for me personally was that my '70 Mach 1 was getting ~2-3mpg with 4.30 gears in it on the street. However, I had to drive across country with the car, so put in a different third member with 3.00 gears in it and maintained about 12-15mpg with it the entire trip. It's really not that hard to understand why this is considering there is a major difference in rpm's the engine is running. So long as the engine has enough hp, the high gearing allows a much lower rpm to be run. As an interesting side note, with the 4.30 gears in my '70 Mach 1 when I was on the dragstrip, my mileage turned from miles per gallon to gallons per mile! Link to comment Share on other sites More sharing options...
LuLu Posted August 9, 2008 Author Report Share Posted August 9, 2008 It's all about the gears in this case... I understand your point and agree in a usual (and common) way. But, this scenario was a bit different. We were traveling through the Smokey mountains, and while I expected to have to add some throttle to keep up with Gary on the inclines due to his gearing and native power, I wasn't expecting Gary to have to add throttle to keep up with me on the declines. This works against the theory that it's all gears, yes? Tire size and PSI has something to do with "coasting"? Gary and I both "coasted" down the inclines, and Gary fell back. Had to use throttle to keep up, and I find that remarkable. I'm remembering my '61 VW bug and my '65 Corvair. Both rear engine/transaxle, yes. But, also both with very skinny tires, and they plowed through Chicago snow like there was no tomorrow. Tire size and inflation does matter, IMHO. Oh well, it's not that important. Just an unusual observation... Happy motoring, gents. Link to comment Share on other sites More sharing options...
mciarochi Posted August 9, 2008 Report Share Posted August 9, 2008 Very interesting. Imo, it's mostly about the gears with diff type, tires and pressure to a lesser degree. Also, differences in the 'attack' programming on the speed-control (if different) might affect it too and his alternative tune likely provides better throttle response at the expense of sucking more fuel on each and every transition of the speed-control. Thanks for sharing the results! Agreed. All other things being equal (engine, weight, drag, speed), the more times the engine turns over, the more gas it eats. If you drive the same 5-speed car over the same road at the same speed in fourth gear, you will burn more gas than if you drove the car in fifth gear. When you change the final gear ratio, you are having the same effect on mileage. That said, I will agree that the tire psi will make a significant difference. Not 8mpg at those pressures, but maybe 1 or 2 mpg. I bump the tires up a few for rain (hydroplaning) and highway (mpg) driving. Link to comment Share on other sites More sharing options...
Super Snake Posted August 9, 2008 Report Share Posted August 9, 2008 Great, just what we need considering how the average driver drives these days! Hey How's the Gas Milage on that Echo? :happy feet: Link to comment Share on other sites More sharing options...
ndwhitehorse Posted August 10, 2008 Report Share Posted August 10, 2008 Just for reference, my SGT/SC (paxton supercharger kit) with stock 3.55 gears got 26.1 mpg at 80 mph on a 220 mile interstate run with nearly calm winds. light footed city driving is about 19 to 20 mpg, with more aggresive driving yielding about 18 mpg. This is about 1 mpg better than my wife's Honda Accord v6, with 207 more hp! Link to comment Share on other sites More sharing options...
LuLu Posted August 10, 2008 Author Report Share Posted August 10, 2008 Just for reference, my SGT/SC (paxton supercharger kit) with stock 3.55 gears got 26.1 mpg at 80 mph on a 220 mile interstate run with nearly calm winds. light footed city driving is about 19 to 20 mpg, with more aggresive driving yielding about 18 mpg. This is about 1 mpg better than my wife's Honda Accord v6, with 207 more hp! Agreed. I get likewise MPG with LuLu, and I've seen it as high as 29. Average highway is 26-27, and average cuty is 20-22, depending on my mood. I wouldn't expect a blower car to suffer much impact on MPG under normal driving conditions. Blowers are relatively passive until you summon the boost. Link to comment Share on other sites More sharing options...
Cobramach1 Posted August 10, 2008 Report Share Posted August 10, 2008 I understand your point and agree in a usual (and common) way. But, this scenario was a bit different. We were traveling through the Smokey mountains, and while I expected to have to add some throttle to keep up with Gary on the inclines due to his gearing and native power, I wasn't expecting Gary to have to add throttle to keep up with me on the declines. This works against the theory that it's all gears, yes? Tire size and PSI has something to do with "coasting"? Gary and I both "coasted" down the inclines, and Gary fell back. Had to use throttle to keep up, and I find that remarkable. No, it's still in the gears... Look at what you said and your observations and then think about the gearing again. The lower the gearing the more it is going to slow down if you're coasting because the engine rpm's will be climbing more rapidly. When you don't have your foot in the gas and are just coasting, the engine will be fighting back the rpm's on its own and only allow itself to reach a certain rpm without you giving it more throttle. With the lower gears in Gary's car he has to apply gas to keep up with you because the engine is holding back his car from continuing to gain speed, whereas your car is going much faster for a lot less rpm's so your engine isn't holding you back just yet allowing you to go faster down the declines before your car will also be slowed by the rpm's. In essence your high gear ratio will allow you to reach a much greater speed on the decline without gas than Gary's car. I tried to explain it the best I could. Kinda tough by typing, but hope you understand the reasoning from my explanation. Just trying to educate you on gearing and how they affect a vehicle in this situation... Link to comment Share on other sites More sharing options...
LuLu Posted August 10, 2008 Author Report Share Posted August 10, 2008 No, it's still in the gears... Look at what you said and your observations and then think about the gearing again. The lower the gearing the more it is going to slow down if you're coasting because the engine rpm's will be climbing more rapidly. When you don't have your foot in the gas and are just coasting, the engine will be fighting back the rpm's on its own and only allow itself to reach a certain rpm without you giving it more throttle. With the lower gears in Gary's car he has to apply gas to keep up with you because the engine is holding back his car from continuing to gain speed, whereas your car is going much faster for a lot less rpm's so your engine isn't holding you back just yet allowing you to go faster down the declines before your car will also be slowed by the rpm's. In essence your high gear ratio will allow you to reach a much greater speed on the decline without gas than Gary's car. I tried to explain it the best I could. Kinda tough by typing, but hope you understand the reasoning from my explanation. Just trying to educate you on gearing and how they affect a vehicle in this situation... Thank you for your time in reply, but pinning this down to just gearing alone doesn't explain it very well. Tire size and PSI are factors in play. Gary's Marauder has approximately 30mm more of contact patch per tire (225 vs, 255), and IMHO, his tires were underinflated. That's added friction, rolling resistance, and it would take more throttle to overcome this. Link to comment Share on other sites More sharing options...
ndwhitehorse Posted August 11, 2008 Report Share Posted August 11, 2008 For the record my wife's Honda has the exact same tire size as the stock Shelby GT...don't know her final drive ratio, although she does have the 6 speed tranny so she doesn't haven't "active cylinder management" or whatever Honda brand calls it. Link to comment Share on other sites More sharing options...
68fastback Posted August 11, 2008 Report Share Posted August 11, 2008 For the record my wife's Honda has the exact same tire size as the stock Shelby GT...don't know her final drive ratio, although she does have the 6 speed tranny so she doesn't haven't "active cylinder management" or whatever Honda brand calls it. Wou;dn't compression braking account for the downhill difference. The 3.55s should offer considerably more than the 2.79s<?> I think. 3.55s are almost 30% more gear. I agree TP will afffect it also, but I think to a lesser degree. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.