mph, gearing and rpm

Newbie · Post by **Newbie** » Wed Oct 02, 2002 3:29 pm

Question: What can I expect to be turning rpm wise at 60mph in 4th gear? I have a swing axle but not sure of the rear ratio? (They came with 2 ratios right? 4.175 and something else depending whether it was a 1200/1300cc or the later 1500cc.) Anyways, this is assuming 165/15 tires or something like 185s.

Also, second question, what safe maximum rpm can I wind an almost stock (only mod is free flowing exhaust) 1600DP to? (I realize it depends in part on the torque/hp curve since there's no point in winding it when those figures start to fall off, but aside from that...)

Post by **Marc** » Wed Oct 02, 2002 5:25 pm

What year is the trans? US-market cars came with 4.375:1 R&P through `66, 4.125 (6% taller) from `67 through ~`72. The latter ones will have a trans number that starts with "AH".
How tall are your tires? (Distance from the center of the axle to the ground X 2)

Newbie · Post by **Newbie** » Wed Oct 02, 2002 6:29 pm

Since I'm building/restoring the car from the ground up I'm not sure of either. I have 3 swingaxle trannies stored at a friend's barn, and I'll use the best one of the bunch but don't have the time to get the numbers to i.d. them. As for the tires I haven't even gotten around to buying those but they'll most likely be stock 165s or maybe 185s. If I could do the measurements and i.d. the trans I'd do the calculations myself. I was hoping someone out there would be able to give me an idea based on their tire/trans gearing combo (4.125 or 4.375).

Post by **Marc** » Wed Oct 02, 2002 6:49 pm

For the sake of argument, then, let's assume that the effective tire diameter is 24.5" and the trans is an "AH" (4.125 R&P). Your RPM at 60 MPH would be 3022. Not a bad place to be, the engine can run here all day long without even breathing hard. A stock engine is capable of turning over 5500RPM, at which point you need to be concerned about crankshaft flex with a non-counterweighted crank, but you're well past the HP peak by then and due for valve float soon (at around 5700). This resolves to a safe maximum steady speed of about 90 MPH @ 4750 RPM. Figure 6% less (85 MPH) with a 4.375 R&P.
"Freeway Flyer" transmissions are pretty much bunk - unless you really need to exceed ~90 MPH a stock trans is all you need, and if you can't get there it's a HP problem not a gearing problem.

vw62rag · Post by **vw62rag** » Wed Oct 02, 2002 8:57 pm

3022 rpms?i have a 1600 with 40 dells,1.25s,merged header,msd 6al and a 4.12 trans and on the freeway im scared to hold it at 3000 rpms i usually try to stay around 2500(usually puts me at 55mph)cuz im scared im gona over rev it, should i be scared?o and my tires are 165/15s.i mean my motor is in real good shape but it jus gets so loud i feel im streesing it out around 3000 steady.or am i just over reacting?

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67type3 · Post by **67type3** » Thu Oct 03, 2002 2:46 am

don't worry. my t3 with 195/50 slim tires revv just under 3500rpm at 65mph. i never pass 4000rom on the highway, since i need to work on my tie rods and suspensin before i can go faster. the engine fan works at optimum best around 3000-3500 rpm.

JoeyB · Post by **JoeyB** » Thu Oct 03, 2002 5:29 am

Scared to rev it above 3000 rpms??!!!!
Try riding a Harley Davidson at 70+ mph on the highway with the engine right under your you know what screaming at about 4000+ rpm's with the wind blowing and tractor trailers 4 feet away from you in rush hour traffic on a monday morning. Then you'll start to get a little nervous.

Newbie · Post by **Newbie** » Sat Oct 05, 2002 12:15 am

Thanks. (BTW, that is slow....meaning low mph at high rpms...will probably have big time trouble breaking 95mph with a stock 1600DP assume...) Oh well, still gotta love the aircooled.