Haven't been on here for awhile Root/Dan - but you've re-inspired me on mine to keep cranking away, although I've realized I need to do some other "building projects" to fully free up my garage to keep mine on stands/rotisserie full time without rolling it out/jack-up & jack-down/roll back every friggin' time!
On to your tires & mpg ....
No don't swap in a 1.7 or 1.8, nor trade down for one. If anything & if you don't need to smog test it for DMV up there, then back date your 76 2L to 74 spec (since 73 has some hard to find NOS parts that were 1 MY only) - with D-jet, 2.0 SSIs you have & a OEM 2.0 Banana Muffler (quieter than Bursch) & tune it to factory specs, get stock 165R15 tires on there, then go drive it "easy" for best milage, and you should get close to 39 or 40 mpg on the flat highway (city & mixed & hills will be less due to more work.
The biggest input by far affecting your mpg is attached to the end of your right leg - waaay down there!
Yup - that foot on the gas pedal, along with gear selection - not changes, but which one you stick it into to cruise!
Third may be the wheels, as the Fuchs 2L lightweight forged alloys were 10.1 lbs. IIRC, while the Rivieras you're running are probably 13-14+/- lbs. each - so that affects the overall weight, and also the rolling resistance & inertia which the motor needs to overcome to spin the wheels (also affected by tire size below).
Second may be the tires - & there were several questions posted here on sizes, speedo/odo errors & mpg, so I'll offer my 2 cents.
Going to 165(/80)R15 - BTW 2.0's were stock spec'd for 165
HR15 & 1.7's/1.8s for 165SR15, but you can drive 2L's with SR's if you're not testing the limits over their 112 mph speed rating. HRs go to 130 mph & TRs go to 118 mph, but latter weren't available back in the day, so not spec'd. The 73-74 2.0 was spec'd as 115 or 118 mph max., but I've had my totally stock 73 2L up to 126 mph. The 75-76 was 112+/- mph IIRC. So what are your choices now?
There are Kumho & several other 165SR15 tires around at reasonable under $100 prices, as well as the Vredestein Sport & Michelin XZX are over $100 (Michi well over!).
Bridgestone makes a nice 165TR15 for $70-90+/- that looks nice & grips well according to some local 70's 911 & 912 guys who run them.
Only Vredestein Sport+ ($105+/-) & Michi XAS (tube type @ $280 w/ tube) are currently available in 165HR15, but I've got a poll post topic to try to get Dunlop to bring back their SP57 165HR15s, in the garage (vote there for price & number of tires if you're interested.
All stock spec tires were tubeless because the steel & alloy "J" rims were for tubeless, & I'd been told those can pinch & pop tubes if you use tube-type tires in them (either at installation or while driving & flexing the sidewalls - per tire experts back in the day.
The 914 Speedometers & Odometer were not inaccurate - especially as compared to other cars of that day, and particularly with American makes. Perhaps a fraction or 1% off at most. However, they were geared specifically for the stock OE tire size of 165R15, and not the "low-n-wide fatties" that many like to run today for max grip!
The OE spec 165R15 tires (/80 height to width ratio) will give a
zero speedo/odo error - or maybe a fraction of a percent if gears are worn, since that's the size the speedo & odo are geared for. They'll also seem quicker to turn since the smaller contact patch is less resistance on non-power steering - not better gripping, but the price for more grip is less mpg on larger & wider/lower profile tires like yours.
Michelin 185/70VR15 XWX tubeless tires are 0.8% too fast & 195/65R15 are double that at 1.6% - while your 185/65R15 are 3.8% off fast (so you're over correcting at 7%), & 185/75R15 are 2.1% too slow, & 175/75R15 is only 0.2% off fast - but I've not seen any recently. Those 185/70VR15 XWXs were the optional upgrade on the M470 & M471 factory options for the 914/4 & 914-6 respectively,and many AXers & racers back in the day used them cuz they were sticky but drops your mpg. Today they're north of $400 apiece!
I'd also guess that your 75 2.0L would get better mileage if fitted out with the stock D-jet (74 or 75-76 MY as noted above), and I don't know if L-jet is better or worse, but the factory didn't convert the 2.0s to L-jet when it came out in the 74 MY on the USA 1.8Ls, so I presume there was a reason, since the L-jet was supposed to be less expensive to build. Ditto for the back-dated exhaust, but with the stock Banana (Bursch was tuned for HP, not mpg - whereas OE balanced the two). Of course, you'd want to pull off any smog equipment for the 75-76 set-up if you don't need to smog test it up there, since the 73-74 set-up produced 91/95 hp vs. 81/86 hp for 75-76 set-up, as well as getting better mpg.
I know for a fact that it is possible to eek a stock 914 2L up to or over 40 mpg with careful driving - most/all in 5th. Back in the late 70's I took my stock 73 2L from LA to SF & filled up near Bakersfield & again up at the north end of the San Joaquin Valley - so I had mostly flat land (a stock 914 can easily do LA to SF with gas to spare), get my right foot "soft" on the gas, stayed mostly in 5th (even on those small grades near Coalinga), kept it mostly to the 55-65 mph range (55 was the limit back then
) and I got about 41 mpg - but was bored stiff!!
BTW - I had the same time & money conflicts with getting stick time while raising my kids. Now that they're grown & gone, I hope to do a kit plane or restore an older Bonanza after my 914 is done!
Cheers!
Tom
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