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Old 01-08-2020, 11:44 AM   #77 (permalink)
EcoModding Lurker
Join Date: Jun 2010
Location: Henry, VA
Posts: 54

The Little Car - '00 Chevrolet Metro
90 day: 91.08 mpg (US)

The Big Car - '94 Chevrolet Caprice Wagon
90 day: 44.9 mpg (US)
Thanks: 9
Thanked 103 Times in 27 Posts
94 Caprice Wagon project--now 50.55 MPG with "3/4 ton" drivetrain/suspension

I know it’s been awhile (over three years) since the last update on this project, but I thought I’d go ahead and report on the changes I’ve been slowly making.

In short: the car is now mechanically equivalent to a 3/4 ton or “2500 series” truck (transmission, rear axle, springs, brakes, tires, etc.), and I’ve given up on the “interchangeable axle and wheels” setup that I experimented with initially.

Best tank is now 50.55 MPG (14.837 gallons over 750 miles) with the car as currently configured.

There are a few more things I might change moving forward (are we ever truly finished with these projects?), but I feel like I’ve finally achieved my goal: a second, “big” car to complement our Metro—one that can carry 9 people, tow up to ~10,000 pounds, and achieve low to mid 40’s MPG under normal driving conditions.

As I did in my initial post, I’ll summarize the mods the car currently has, and then give some more details below. Obviously, since so much has changed I’m not going to cover everything in detail, but if you have any questions about something I haven’t covered, please don’t hesitate to ask.

__________________________________________________ ___

Mods Summary

Aero mods
-Roof rack delete (holes and voids have now been filled in and streamlined)
-Rear wiper delete
-Moon disc wheel covers (front only)
-Side skirts (lowered from first version)
-Rear fender skirts (cover entire rear wheel)
-Front air dam made from conveyor belt
-Factory air dam (under radiator) removed, and opening below front of radiator sealed off
-Impala SS grill with removable grill block inserts
-Internal baffle to direct air from grill into the radiator
-Rear kammback extension (redesigned, and fixed to/hinges with rear glass)
-1995-96 Caprice/Impala side mirrors
-Power antenna override switch, to allow CD listening with antenna down

-Manual steering conversion, using manual steering box from an S10 pickup
-Lightweight, 23% underdrive crank pulley (made by "ASP")
-A/C delete, using low-mount alternator relocation bracket
-Engine tuning, using "Tuner CAT" program (mainly moderate tweaks to AFR and ignition timing)
-Converted from dual to single exhaust (to clear rear sway bar)
-Manual transmission swap—NV4500 transmission from 1997+ GM 2500/3500 truck
-Custom 0.643 fifth gear ratio in transmission
-GM 9.5” (14-bolt) rear axle, with 4:10 gears and 11-5/32” x 2-3/4” brakes
-Eaton E-Locker differential (from junkyard Hummer H2)
-Aluminum rear brake drums (adapted from 1961-64 Lincoln front drums)
-16" aluminum wheels (from 2000 Chevy Tahoe)
-Nokian Rotiiva HT LT215/85R16 E/10PR (LRR) tires, aired to 90 PSI
-Cooper Discoverer STT PRO LT215/85R16 E/10PR tires (rear only), for driving off-road in soft/muddy conditions; only installed when needed

-Heavy duty "J55" front suspension/brakes from a commercial chassis Cadillac Fleetwood
-Moog 80090 front springs (710 lbs/in rate)
-Moog CC603 rear springs (393 lbs/in rate)
-Monroe "Severe Service" heavy duty shock absorbers
-Rear sway bar w/upgraded LCAs (from Impala SS)
-UMI 3628 rear upper control arms (to adjust pinion angle and increase stability)
-GM 15712438 bumpers on rear axle bump stops (to serve as overload springs)
-Front frame/sway bar brace (camaro,trans am chassis, 3 way sub frame connectors, 2nd gen frame)
-Class III/IV hitch (for towing, but also stiffens rear frame)

Instrumentation and Controls
-UTCOMP trip/MPG computer
-Innovate Motorsports LC-2 wideband oxygen sensor (connected to UTCOMP)
-Gauge cluster (with analog speedo and tach) from GM F-body
-Kill switch (interrupting the distributor sensor feed)
-Differential lock switch on dash
-Tekonsha Primus IQ trailer brake controller

-Amsoil full synthetic in engine, transmission, and axle
-Ethanol free premium gas

Other mods
-Deep cycle battery
-LED headlights and exterior lights
-Auxiliary brake vacuum reservoir and electric vacuum pump, to allow for indefinite engine off coasting
-Roof painted with reflective white paint (for interior heat reduction)
-Llumar Air 80 "clear" window tint (for interior heat reduction)
-Engine coolant and oil heaters (for winter use)
-Auxiliary coolant pump in heater circuit (to provide heat during EOC in winter)
-Interior fan override switch (to allow passive ventilation without fan)
-Headrests in all three middle row positions (for comfort/safety)
-Child seat tethers in all middle/rear seating positions
-Locking seat belts (from Chevy Trailblazer) in the middle row, to make child seats easier to install
-Third row has seatbelts for 2 OR 3 passengers (bringing total to 9)
-Compact spare tire (less weight and more storage space)
-Rear interior compartments modified, to allow for more storage space

Planned future mods
-Diesel swap?
-Hydraulic hybrid?

__________________________________________________ ___

Mods Details

The following details are in no particular chronological or logical order, but simply describe some of the discoveries that I’ve made over the last three-plus years and provide some insight into the rationale behind several of the more substantial modifications.

The mishap that paid for it all

First, it’s worth mentioning that in late 2016 we had a minor accident…that ended up funding up the entire project.

Only the left front fender and door were affected, but the insurance company decided to “total” the car. We opted to keep it (of course), but we weren’t forced to get a salvage title for it, and the check that the insurance company wrote us was 175% of what we paid for the car itself in 2013. So basically, it paid for the car and pretty much all of the mods up to that point.

Initially, I got an ugly fender and door from the u-pull junkyard for cheap and put those on:

Then, a while later, I was able to find a color-matching “parts car” for basically scrap price:

This allowed me to have a door, fender, and hood that matched again, without having to paint them (the car already had a mismatched black hood when we bought it). It also allowed me to upgrade to the lower-drag 1995-96 side mirrors, since the parts car was a 1995 model. I sold the engine and transmission from the parts car (since they weren’t needed) which basically paid for the rest of it, so now I have free parts whenever I need them. It’s come in useful several times already.

Gearing discoveries

Probably the biggest change to the project, since the last time that I posted, is that I’ve moved away from my original idea of having interchangeable rear axles. Initially, the car had an NV3500 manual transmission swapped in, and two axles: a 7.5” 10-bolt with 2.14 gears, and a 9.5” 14-bolt with 4.10 gears and an Eaton E-Locker. (The 4.10 gears and E-Locker came from a junkyard Hummer H2; by the time I sold the extra parts from combining the Hummer axle and commercial-chassis Cadillac axle, the gears and locker were basically free.) I did have the car set up for interchangeable axles for some time (quick-disconnect fittings on the brake lines, custom e-brake cable bracket, etc.), and actually swapped them back and forth a few times, too. However, I quickly made an interesting discovery: I could get pretty much the same mileage with the heavy-duty 4.10 geared axle as I could with the light-duty 2.14 geared axle!

It turns out that with consistent P&G and EOC, gearing doesn’t really matter that much. When I first converted the car to stick-shift, I was still pretty new to EOC—so I was under the (conventional wisdom) assumption that the higher the gearing, and the lower the RPMs, the better. However, EOC turns that logic on its head. As long as the engine is kept in the “sweet spot” from a BSFC standpoint, gearing doesn’t really matter (and, surprisingly, there’s not that much difference in parasitic losses between a GM 7.5” and 9.5” rear end, either). Now—if I were to do a lot of highway cruising, the 2.14 rear would clearly have an advantage. But since I pretty much always P&G, I ended up leaving the heavy-duty 4.10 rear in the car all the time. With the NV3500’s 0.73:1 fifth gear, the RPMs would get pretty high in highway driving—which was a bit annoying at times as it made pulses shorter, noisier, and more frequent—but the overall impact on MPG was minimal.

Even with the 4.10 rear and the NV3500’s 4.02:1 first gear, though, I still found myself sometimes wanting a lower first gear (particularly pulling heavy loads, uphill, off road). So I started looking, and eventually found a recently-rebuilt NV4500 transmission from a junkyard’s clearance sale for cheap. This gave me a much lower first gear (5.61:1), and a much heavier-duty transmission overall for towing purposes (with minimal/unnoticeable increase in parasitic losses). I could still use the stock transmission crossmember with the NV4500, but since it’s much larger than the NV3500 the transmission tunnel had to be modified, and since it has a fixed yoke output, I had to have a custom driveshaft made.

Before I put the NV4500 in the car, however, I made an internal change in the transmission that helped lower the highway speed RPMs a bit…

Custom NV4500 fifth gear

When I was researching information about the NV4500 transmission, I noticed that there were two versions of the transmission made: an earlier one, with a lower first gear and slightly wider ratio spacing, and a later one, with a higher first gear and slightly closer ratio spacing. However, I also noticed that both versions had basically the same fifth gear ratio (~0.73:1). This got me wondering if the gearsets internally could be mixed and matched to create different ratios.

Since the NV4500 is a very popular transmission in the off-roading community, I scoured discussion boards looking for folks who had tried making custom, mixed and matched ratios—but couldn’t find anyone who had. I then carefully studied parts listings for these transmissions online, and surmised that custom combinations of gears was possible. In particular, according to the tooth count of the gear sets involved, a fifth gear set from an early NV4500 transmission, installed in a later edition transmission, would produce a fifth gear ratio of 0.643:1! Again, I couldn’t find an example of anyone who had done this, but I thought it was worth a shot. So, I ordered an early fifth-gear set to put in my later-edition transmission, and I can confirm—it works! My NV4500 has an “extra tall” fifth gear ratio.

This “extra tall” fifth gear, compared to the stock ratio, is like going from a 4.10 rear to a 3.55 rear in fifth gear (while still keeping the gearing advantage of the 4.10 rear in all of the lower gears). The engine now spins about 2100 rpm at 70 mph in fifth gear. This helped to mellow out P&G on the highway, and means that, practically speaking, I never have to rev above 2000 RPM in normal driving. I highly recommend this modification for anyone who has a vehicle with a 5.61 first-gear NV4500 transmission.

Tire experimentation

Another area in which I’ve made some interesting discoveries is in tires (rolling resistance). I acquired a number of inexpensive used tires and wheels in various sizes; in fact, at one point I had 7-8 pairs of different matched tires, mounted on rims, that I could swap on and off. I did some coast down tests, as well as more extended road tests, with these different tires. Unfortunately, I think it would make an already (too) long post way longer to try to detail all of these tests. Instead, I’ll just summarize the two biggest takeaways I got from that process.

First, the design of the tire itself is far more important than the size and weight of the tire. There’s a member on here (CapriRacer) who has argued this for years, and from what I’ve seen, I’d have to say he’s right. I’ve tried Bridgestone Ecopia tires in three different sizes that all coasted down about the same distance; I’ve also compared LRR all-terrain tires (Nokian Rotiiva AT) to smaller, lighter, off-brand P-metric highway tires, and the bigger, LRR all-terrain tires coasted better and used measurably less fuel. So, instead of worrying about which size of tire to use on your vehicle, it seems that trying to find a model of tire with low rolling resistance will yield better results.

That begs the question—how do you compare the rolling resistance of various tire models? Unfortunately, in the US there isn’t a standardized test or rating that can be used for comparison. But in the EU, however, there is. I’ve used this site ( to search for tires and sort them by their RR rating. Not all of the tires sold in Europe are available in the US, but many are, and at the very least you can see trends of how the different tire manufacturers compare with each other with regard to fuel economy.

I ended up settling on Nokian Rotiiva HT tires on the wagon. I wanted LT tires for towing capacity and stability, and these tires have an excellent RR rating in the EU test. They have a “C” rating (far better than any competitor in the same size/class), which, if you read the technical details of how the EU label test is conducted, is actually the same RR coefficient as a P-metric tire with a “B” rating. I can confirm that these tires roll very easily, despite the fact that they are heavier, LT metric tires—they coast just about the same as the Bridgestone Ecopia tires that I compared them with. Of course, being LT tires with a sidewall max of 80 PSI doesn’t hurt, either.

The second major takeaway I’ve discovered is that tread depth (wear) makes a big difference in rolling resistance. The same tires (make, model, and size), worn to the wear bars, will roll significantly farther in a coast-down test than new tires. I’ve noticed this for years on my Metro—every time I put on a new set of tires (even of the same make and model), my MPG would drop noticeably, and then slowly improve again in the coming months as the tires wear. However, I was able to demonstrate this even more clearly in a back-to-back coastdown test with this wagon. When I bought the Nokian Rotiiva HTs, I found a set of four of them used on eBay for less than the cost of two new ones. The listing led me to believe that all four had a tread depth of ~8/32nds; however, when they arrived I discovered that two of them were 8/32nds, but the other two were new—with a depth of 15/32nds! After I had them mounted, I was able to test them back to back on the rear of the car with a different set of tires staying on the front. The two used tires coasted significantly farther in the coastdown test than the two new ones did—and that was with only swapping between sets of two! If I would have had two sets of four tires (one used, and one new), the difference would have been even more pronounced. Given that we don’t drive this car very much, and that the two new tires would probably dry-rot before they were completely worn, I went ahead and had the two new tires “shaved” by a local shop to match the tread depth of the two used ones.

So the point is: if you have a vehicle that doesn’t see a significant amount of miles each year, it might be best in terms of both financial savings and efficiency gains to purchase used, LRR tires. If it’s a vehicle that gets driven more often, however, the lower price of used tires may be offset by the expense of more frequent mounting, and new LRR tires might make more sense financially.

One final tire discovery I made: highway tires are terrible off-road. Even if it’s only been because of wet grass, there have several times that I have gotten “stuck” off-road with this car—despite having a lockable rear differential. Buying (and using) mud-terrain tires on the rear of the car, when I know I’m going to need to drive off road, pulling a trailer, in wet conditions, has been a very practical investment. The mud-terrains, combined with the E-locker, give this wagon 4x4-like abilities off-road.

Front-end aero tweaks

One final area of improvements I’ll describe is aerodynamic tweaks to the front of the vehicle. When I initially posted about this car, I had already done the standard aero mods: roof rack delete, rear wiper delete, Kammback, rear wheel skirts, smooth wheel covers, side skirts, lowered factory front air dam, and a basic grill block. However, I had a suspicion that the entire front of the car could use a complete overhaul. From the factory, this car has a small air dam under the radiator, with a large opening between the bottom rear of the bumper cover and the radiator. This creates low pressure behind the radiator while the car is moving, drawing air up behind the bumper and through the radiator. It does have a small grill from the factory, too, but I was able to completely seal it off without any noticeable decrease in engine cooling. It seemed as if the majority of the airflow in the OEM design was intended to come from underneath, behind the bumper.

This design works great for cooling purposes—even towing on hot summer days, I never noticed elevated engine temperatures. But, I had a feeling that it also increases overall drag and airflow under the car. It dawned on me that the extension that I had put on the factory air dam probably didn’t decrease airflow under the car at all—it just increased airflow through the radiator.

So, I decided to do a complete overhaul of the design. I removed the factory air dam, and made a new, lower one with a piece of a used conveyor belt that wraps around the front of the bumper. I trimmed the factory bumper cover and bolted the conveyor belt directly to the steel bumper support. Then, to keep the conveyor belt from billowing rearward while driving, I mounted a second bumper support (from the parts car) below the factory one, using very stiff springs. This allows it to “give” if the front end bottoms out or bumps an obstruction. I then built a baffle around the front of the radiator that prevents any air from entering from under the car, and that keeps air from the grill opening from going anywhere but into the radiator. Finally, I replaced the stock Caprice grill with an Impala SS grill, so that I could make easily removable grill block inserts that fit into the grill openings.

Right now, I’ve got the entire upper half of the Impala SS grill covered over, so the only air entering the radiator is from the bottom half of the grill. That seems like a very small opening in comparison with the stock design; but, because of the internal baffle all of the air gets directed into the radiator, and so far I haven’t noticed any elevated engine temps, even while towing. This new design also greatly decreases both the amount of air flowing through the engine compartment and the amount of air flowing under the car. Obviously, I wasn’t able to do an A/B/A test with the original design, but there did seem to be an increase of a few MPGs once it was completed. And my kids think it looks like a racecar now.

Loads I’ve towed

Obviously, this car has been extensively modified for greater fuel economy. But it’s been modified for greater towing capacity, too. I thought I’d share pictures of some of the loads I’ve hauled. The trailer in the pictures below is a 16 ft. 10,000 lb. gross capacity dovetail trailer; as you can see, I also built removable sides and a tailgate for it.

This is a load of composted cow manure for our garden. It was very wet when it was loaded, as it had been raining for several days prior. I don’t know the exact weight, but I’m guessing that it was around 8000-9000 lbs. gross. I typically get 23-25 MPG with the trailer empty; with this load it was about 19 MPG on the way home.

This second load is eight large rectangular square straw bales weighing about 500 lbs. each. The trailer weighs about 2200 lbs. empty, so this one is about 6000 lbs. gross. Even though it was lighter than the load of manure, I only averaged 16 MPG on the way home due to the significantly increased drag.

Comparison to GM C20/C2500 series trucks

Finally, I thought I’d list all of the upgrades that I’ve made to the car that are equivalent to a GM C20/C2500 truck. As you can see, it really can lay claim to being a “3/4 ton” station wagon now.

Given that this post is already too long…that’s all I’m sharing for now. I welcome any questions or observations!


Last edited by funkhoss; 01-08-2020 at 06:22 PM..
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